Post on 07-Jul-2018
8182019 Polymeric Surfactants in Disperse System
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Polymeric surfactants in disperse systems
Tharwat Tadros
89 Nash Grove Lane Wokingham Berkshire RG40 4HE UK
a b s t r a c ta r t i c l e i n f o
Available online 5 November 2008
Keywords
Homopolymers
Block and graft copolymers
Solution properties
Adsorption and conformation
Steric stabilization
Emulsions
Suspensions
Latexes
Nano-emulsions
This overview starts with a section on general classi1047297cation of polymeric surfactants Both homopolymers block
and graft copolymers are described The solution properties of polymeric surfactants is described by using the
FloryndashHuggins theory Particularattention is given to theeffect of solvency of themedium forthe polymer chains
The adsorption and conformation of homopolymers block and graft copolymers at the solidliquid interface isdescribed The theories of polymer adsorption and their predictions are brie1047298y described This is followed by a
description of the experimental techniques that can be applied to study polymeric surfactant adsorption
Examples of adsorption isotherms of non-ionic polymeric surfactants are given The effect of solvency on the
adsorption amount is also described Results for the adsorbed layer thickness of polymeric surfactants are given
with particular attention to the effectof molecular weight The interactionbetweenparticles containing adsorbed
layers is described in terms of the unfavorable mixing of the stabilizing chains when these are in good solvent
conditions The entropic volume restriction or elastic interaction that occurs on considerable overlap is also
described Combination of these two effects forms the basis of the theory of steric stabilization The energyndash
distance curveproducedwith thesesterically stabilized systems is describedwith particular attention of the effect
of the ratio of adsorbed layer thickness to droplet radiusExamples of oil-in-water (OW) and water-in-oil (WO)
emulsions stabilized with polymeric surfactants are given Of particular interest is the OW emulsions stabilized
using hydrophobically modi1047297ed inulin (INUTECregSP1) The emulsions produced are highly stable against
coalescenceboth in water and high electrolyte concentrations This is accountedfor by the multipoint attachment
of the polymeric surfactantto the oil droplets withseveral alkyl groups and the strongly hydrated loops and tails
of linear polyfructose Evidence of this high stability was obtained from disjoining pressure measurements
Stabilization of suspensions using INUTECregSP1 was described with particular reference to latexes that wereprepared using emulsion polymerization Thehigh stability of thelatexes is attributedto thestrongadsorptionof
the polymeric surfactant on the particle surfaces and the enhanced steric stabilization produced by the strongly
hydrated polyfructose loops and tails Evidence for such high stability was obtained using Atomic Force
Microscopy (AFM) measurements The last part of the overview described the preparation and stabilization of
nano-emulsions using INUTECregSP1 In particular thepolymericsurfactant wasveryeffectivein reducing Ostwald
ripening as a result of its strong adsorption and the Gibbs elasticity produced by the polymeric surfactant
copy 2008 Elsevier BV All rights reserved
Contents
1 Introduction 282
11 General classi1047297cation of polymeric surfactants 282
12 Solution properties of polymeric surfactants 283
13 Adsorption and conformation of polymeric surfactants at interfaces 285
2 Theories of polymer adsorption 285
3 Experimental techni ques for studying polymeric surfactant adsorption 286
31 Measurement of the adsorption isotherm 287
32 Measurement of the fraction of segments p 287
33 Determination of the segment density distribution ρ(z) and adsorbed layer thickness δh 288
4 Examples of the adsorption isotherms of nonionic polymeric sur factants 289
5 Interaction between particles containing adsorbed polymeric surfactant layers steric stabilization 289
51 Mixing interaction Gmix 290
52 Elastic interaction Gel 290
Advances in Colloid and Interface Science 147ndash148 (2009) 281ndash299
E-mail address tharwattadrosfsnetcouk
288
289
290
290
0001-8686$ ndash see front matter copy 2008 Elsevier BV All rights reserved
doi101016jcis200810005
Contents lists available at ScienceDirect
Advances in Colloid and Interface Science
j o u r n a l h o m e p a g e w w w e l s e v i e r c o m l o c a t e c i s
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httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 219
6 Emulsions stabilized by polymeric surfactants 292
61 WO emulsions stabilized with PHSndashPEOndashPHS block copolymer 293
7 Suspensions stabilised using polymeric surfactants 295
71 Polymeric surfactants in emulsion polymerization 296
711 Dispersion polymerization 299
72 Polymeric surfactants for stabilisation of preformed latex dispersions
73 Use of polymeric surfactants for preparation and stabilisation of nano-emulsions
References
1 Introduction
Polymeric surfactants are essential materials for preparation of
many disperse systems of which we mention dyestuffs paper
coatings inks agrochemicals pharmaceuticals personal care pro-
ducts ceramics and detergents [1] One of the most important
applications of polymeric surfactants is in the preparation of oil-in-
water (OW) and water-in-oil (WO) emulsions as well as solidliquid
dispersions [23] In this case the hydrophobic portion of the
surfactant molecule should adsorb ldquostronglyrdquo at the OW or becomes
dissolved in the oil phase leaving the hydrophilic components in the
aqueous medium whereby they become strongly solvated by the
water molecules
The other major application of surfactants is for the preparation of
solidliquid dispersions (usually referred to as suspensions) There are
generally two methods for preparation of suspensions referred to as
condensation and dispersions methods In the 1047297rst case one starts
with molecular units and build-up the particles by a process of
nucleation and growth [4] A typical example is the preparation of
polymer lattices In this case the monomer (such as styrene or
methylmethacrylate) is emulsi1047297ed in water using an anionic or non-
ionic surfactant (such as sodium dodecyl sulphate or alcohol
ethoxylate) An initiator such as potassium persulphate is added and
when the temperature of the system is increased initiation occurs
resulting in the formation of the latex (polystyrene or polymethyl-
methacrylate) In the dispersion methods preformed particles
(usually powders) are dispersed in an aqueous solution containing asurfactant The latter is essential for adequate wetting of the powder
(both external and internal surfaces of the powder aggregates and
agglomerates must be wetted) [1] This is followed by dispersion of
the powder using high speed stirrers and 1047297nally the dispersion is
ldquomilledrdquo to reduce the particle size to the appropriate range
For stabilization of emulsionsand suspensions against 1047298occulation
coalescence and Ostwald ripening the following criteria must be
satis1047297ed (i) Complete coverage of the droplets or particles by the
surfactant Any bare patches may result in 1047298occulation as a result of
van der Waals attraction or bridging (ii) Strong adsorption (or
lsquorsquoanchoringrsquorsquo) of the surfactant molecule to the surface of droplet or
particle (iii) Strong solvation (hydration) of the stabilizing chain to
provide effective steric stabilization (iv) Reasonably thick adsorbed
layer to prevent weak 1047298occulation [1]Most of the above criteria for stability are best served by using a
polymeric surfactant In particular molecules of the AndashB AndashBndashA
blocks and BAn (or ABn) grafts (see below) are the most ef 1047297cient for
stabilisation of emulsions and suspensions In this case the B chain
(referred to as the ldquoanchoringrdquo chain) is chosen to be highly insoluble
in the medium and with a high af 1047297nity to the surface in the case of
suspensions or soluble in the oil in the case of emulsions The A chain
is chosen to be highly soluble in the medium and strongly solvated by
its molecules These block and graft copolymers are ideal for
preparation of concentrated emulsions and suspensions which are
needed in many industrial applications
In this overview I will start with a section on the general
classi1047297cation of polymeric surfactants This is followed by a discussion
of their solution properties The next section will be devoted to the
adsorption of polymeric surfactants at the solidliquid interface
whereby a summary will be given to some of the theoretical
treatments and the methods that may be applied for studying
polymeric surfactant adsorption and its conformation at the SL
interface The same principles may be applied to the adsorption of
polymeric surfactants at the LL interface although theoretical
treatments of this problem are not as developed as those for the SL
interface Thenext section will be devoted to theprinciples involved in
stabilisation of emulsions and suspensions using polymeric surfac-
tants ie the general theoryof steric stabilisation Two sections will be
devoted to describe the use of polymeric surfactants for stabilisation
of suspensions and emulsions The use of polymeric surfactants for
stabilization of nano-emulsions against Ostwald ripening will also be
described
11 General classi 1047297cation of polymeric surfactants
Perhaps the simplest type of a polymeric surfactant is a homo-
polymer that is formed from the same repeating units such as poly
(ethylene oxide) or poly(vinyl pyrrolidone) These homopolymers
have little surface activity at the OW interface since the homo-
polymer segments (ethylene oxide or vinylpyrrolidone) are highly
water soluble and have little af 1047297nity to the interface However such
homopolymers may adsorb signi1047297cantly at the SL interface Even if
the adsorption energy per monomer segment to the surface is small
(fraction of kT where k is the Boltzmann constant and T is the
absolute temperature) the total adsorption energy per molecule maybe suf 1047297cient to overcome the unfavourable entropy loss of the
molecule at the SL interface
Clearly homopolymers are not the most suitable emulsi1047297ers or
dispersants A small variant is to use polymers that contain speci1047297c
groups that have high af 1047297nity to the surface This is exempli1047297ed by
partially hydrolysed poly(vinyl acetate) (PVAc) technically referred to
as poly(vinyl alcohol) (PVA) The polymer is prepared by partial
hydrolysis of PVAc leaving some residual vinyl acetate groups Most
commercially available PVA molecules contain 4ndash12 acetate groups
These acetate groups which are hydrophobic give the molecule its
amphipathic character On a hydrophobic surface such as polystyrene
the polymer adsorbs with preferential attachment of the acetate
groups on the surface leaving the more hydrophilic vinyl alcohol
segments dangling in the aqueous medium These partially hydro-lysed PVA molecules also exhibit surface activity at the OW interface
[5]
The most convenient polymeric surfactants are those of the block
and graft copolymer type A block copolymer is a linear arrangement
of blocks of variable monomer composition The nomenclature for a
diblock is poly-A-block-poly-B and for a triblock is poly-A-block-poly-
B-poly-A One of the most widely used triblock polymeric surfactants
are the ldquoPluronicsrdquo (BASF Germany) or ldquoSynperonic PErdquo (ICI UK)
which consists of two poly-A blocks of poly(ethylene oxide) (PEO) and
one block of poly(propylene oxide) (PPO) Several chain lengths of PEO
and PPO are available More recently triblocks of PPOndashPEOndashPPO
(inverse Pluronics) became available for some speci1047297c applications
The above polymeric triblocks can be applied as emulsi1047297ers or
dispersants whereby the assumption is made that the hydrophobicPPO
291
292
293
294
296
296
297
300
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Clearly when χb12 B2 is positive and mixing is non-ideal leading to
positive deviation (repulsion) this occurs when the polymer chains
are in ldquogoodrdquo solvent conditions In contrast when χN12 B2 is
negative and mixing is non-ideal leading to negative deviation
(attraction) this occurs when the polymer chains are in ldquopoorrdquo
solvent conditions (precipitation of the polymer may occur under
these conditions) Since the polymer solvency depends on tempera-
ture one can also de1047297ne a theta temperature θ at which χ=12
The function [(12)minus
χ] can also be expressed in terms of twomixing parameters an enthalpy parameter κ 1 and an entropy
parameter ψ1 ie
1
2minusχ
= κ 1minusψ1 eth8THORN
The θ-temperature can also be de1047297ned in terms of κ 1 and ψ1
θ = κ 1T
ψ1
eth9THORN
Alternatively one can write
1
2
minusχ = ψ1 1minusθ
T eth10THORN
Although the FloryndashHuggins theory is sound in principle several
experimental results cannot be accounted for For example it was
found that the χ parameter depends on the polymer concentration in
solution Most serious is the fact that many polymer solutions (such as
PEO) show phase separation on heating when the theory predictsthat
it should happen only on cooling
The solution properties of copolymers are much more complicated
This is due to the fact that the two copolymer components A and B
behave differently in different solvents Only when the two compo-
nents are both soluble in the same solvent then they exhibit similar
solution properties This is the case for example for a non-polar
copolymer in a non-polar solvent It should also be emphasised that
the FloryndashHuggins theory was developed for ideal linear polymers
Indeed with branched polymers consisting of high monomer density
(eg star branched polymers) the θ-temperature depends on the
lengthof the armsand isin general lower thanthatof a linearpolymer
with the same molecular weight
Another complication arises from speci1047297c interaction with the
solvent eg hydrogen bonding between polymer and solvent
molecules (eg with PEO and PVA in water) Also aggregation insolution (lack of complete dissolution) may present another problem
One of the most useful parameters for characterising the
conformation of a polymer in solution is the root mean-square (rms)
end to end length br 2N12 which represents a con1047297guration character r
as the distance from one end group to the other of a chain molecule
Another useful parameter is the radius of gyration bs2N
12 which is a
measure of the effective size of a polymer molecule (it is the root
mean-square distance of the elements of the chain from its centre of
gravity)
For linear polymers
bs2N
1=2 = br 2N1=2
61=2 eth11THORN
The radius of gyration of a polymer in solution can be determined
from light scattering measurements
As mentioned above dilute solutions of copolymers is solvents that
are good for both components exhibit similar behaviour to hompo-
lymer chains However in a selective solvent whereby the medium is
a good solvent for one component say A and a poor solvent for the
second component B the very different solvent af 1047297nities to the two
components will have a large effect on the conformation of the
isolated chain This results in formation of aggregates involving
several macromolecules in dilute solutions (low Critical Aggregation
Concentration CAC) It is believed that the polymeric aggregates are
Fig 3 Various conformations of macromolecules on a plane surface
284 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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spherical [5] The CAC of these block and graft copolymers is usually
very low
Several methods may be applied to obtain the aggregate size and
shape of block and graft copolymers of which light scattering small
angle X-ray and neutron scattering are probably the most direct
Dynamic light scattering (photon correlation spectroscopy) can also
be applied to obtain the hydrodynamic radius of the aggregate This
technique is relatively easy to perform when compared with static
light scattering since it does not require special preparation of thesample
13 Adsorption and conformation of polymeric surfactants at interfaces
Understanding the adsorption and conformation of polymeric
surfactants at interfaces is key to knowing how these molecules act as
stabilizers Most basic ideas on adsorption and conformation of
polymers have been developed for the solidliquid interface [10ndash17]
The process of polymer adsorption is fairly complicated mdash it
involves PolymerSurface Interaction PolymerSolvent Interaction
SurfaceSolvent Interaction The con1047297guration (conformation) of the
polymer at the solidliquid interface The polymersurface interaction
is described in terms of adsorption energy per segment χs The
polymersolvent interaction is described in terms of the Floryndash
Huggins interaction parameter χ The polymer con1047297guration is
described by sequences of Trains segments in direct contact with
the surface Loops segments in between the trains that extend into
solution Tails ends of the molecules that also extend into solution A
schematic representation of the various polymer con1047297gurations is
given in Fig 3
For homopolymers eg poly(ethylene oxide) (PEO) or polyvinyl
pyrrolidone (PVP) a train-loop-tail con1047297guration is the case For
adsorption to occur a minimum energy of adsorption per segment χs
is required When a polymer molecule adsorbs on a surface it looses
con1047297gurational entropy and this must be compensated by an
adsorption energy χs per segment This is schematically shown in
Fig 4 where the adsorbed amount Γ is plotted versus χs The
minimum value of χs can be very small (b01kT ) since a large number
of segments per molecule are adsorbed For a polymer with say 100segments and 10 of these are in trains the adsorption energy per
molecule now reaches 1kT (with χs =01kT ) For 1000 segments the
adsorption energy per molecule is now 10kT
Homopolymers are not the most suitable for stabilisation of
dispersions For strong adsorption one needs the molecule to be
ldquoinsolublerdquo in the medium and has strong af 1047297nity (ldquoanchoringrdquo) to the
surface For stabilisation one needs the molecule to be highly soluble
in themedium andstronglysolvated by itsmolecules mdash This requires a
FloryndashHuggins interaction parameter less than 05 The above
opposing effects can be resolved by introducing ldquoshortrdquo blocks in the
moleculewhichare insolublein themedium andhavea strong af 1047297nity
to the surface as illustrated in Fig 3b Example Partially hydrolysed
polyvinyl acetate (88 hydrolysed ie with 12 acetate groups)
usually referred to as polyvinyl alcohol (PVA)
The above requirements are better satis1047297ed using AndashB AndashBndashA and
BAn graft copolymers B is chosento be highly insolublein themedium
and it should have high af 1047297nity to the surface This is essential to
ensure strong ldquoanchoringrdquo to the surface (irreversible adsorption) A is
chosen to be highly soluble in the medium and strongly solvated by
its molecules The FloryndashHuggins χ parameter can be applied in this
case For a polymer in a good solvent χ has to be lower than 05 the
smaller the χ value the better the solvent for the polymer chains
Examples of B for a hydrophobic particles in aqueous media are
polystyrene polymethylmethacrylate Examples of A in aqueous
media are polyethylene oxide polyacrylic acid polyvinyl pyrollidone
and polysaccharides For non-aqueous media such as hydrocarbons
the A chain(s) could be poly(12-hydroxystearic acid)
For full description of polymer adsorption one needs to obtain
information on the following (i) The amount of polymer adsorbed Γ (in mg or moles) per unit area of the particles It is essential to know
thesurface area of theparticles in thesuspensionNitrogenadsorption
on the powder surface may give such information (by application of
the BET equation) provided there will be no change in area on
dispersing the particles in the medium For many practical systems a
change in surface area may occur on dispersing the powder in which
case one has to use dye adsorption to measure the surface area (some
assumptions have to be made in this case) (ii)The fraction of segments
in direct contact with thesurface ie the fraction of segmentsin trains
p ( p = (Number of segments in direct contact with the surface) Total
Number) (iii)The distribution of segments in loops and tails ρ(z)
which extend in several layersfrom the surface ρ(z) is usually dif 1047297cult
to obtain experimentally although recently application of small angle
neutron scattering could obtain such information An alternative anduseful parameter for assessing ldquosteric stabilisationrdquo is the hydro-
dynamic thickness δh (thickness of the adsorbed or grafted polymer
layer plus any contribution from the hydration layer) Several methods
can be applied to measure δh as will be discussed below
2 Theories of polymer adsorption
Two main approaches have been developed to treat the problem of
polymer adsorption (i) Random Walk approach This is based on
Florys treatment of the polymer chain in solution the surface was
considered as a re1047298ecting barrier (ii) Statistical mechanical approach
Thepolymer con1047297gurationwas treated as being made of three types of
structures trains loops and tails each having a different energy state
The random walk approach is an unrealistic model to the problemof polymer adsorption since the polymer interacts in a speci1047297c
manner with the surface and the solvent The statistical mechanical
approach is a more realistic model for the problem of polymer
adsorption since it takes into account the various interactions
involved A useful model for treating polymer adsorption and
con1047297guration was suggested by Scheutjens and Fleer (SF theory)
[14ndash16] that is referred to as the step weighted random walk approach
which is summarized below
The SF theory starts without any assumption for the segment
density distribution The partition functions were derived for the
mixture of free and adsorbed polymer molecules as well as for the
solvent molecules All chain conformations were described as step
weighted random walks on a quasi-crystalline lattice which extends in
parallel layers from the surface mdash
this is schematically shown in Fig 5Fig 4 Variation of adsorption amount Γ with adsorption energy per segment χs
285T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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The partition function is written in terms of a number of con1047297gura-
tions These were treated as connectedsequencesof segments In each
layer random mixing between segments and solvent molecules was
assumed (mean 1047297eld approximation) Each step in the random walk
was assigned a weighting factor pi
pi consists of three contributions (i) An adsorption energyχs (which
exists only for the segments that are near the surface) (ii) Con1047297gura-
tional entropy of mixing (that exists in each layer) (iii) Segment-solvent
interaction parameterχ (the FloryndashHuggins interaction parameter note
that χ=0 for an athermal solvent χ=05 for a θ-solvent)
From the weighting factors the statistical weight of any chain
conformation can be calculated for a given segment concentration
pro1047297le using a matrix formulation Fig 6 shows typical adsorption
isotherms plotted as surface coverage (in equivalent monolayers)
versus polymer volume fraction ϕ in bulk solution (ϕ was taken to
vary between 0 and 10minus3 which is the normal experimental range)
The results in Fig 6 show the effect of increasing the chain length r
and effect of solvency (athermal solvent with χ=0 and theta solvent
with χ=05) The adsorption energy χs was taken to be the same and
equal to 1kT When r = 1 θ is very small and the adsorption increases linearly
with increase of ϕ (Henrys type isotherm) On the other hand when
r = 10 the isotherm deviates much from a straight line and approaches
a Langmuirian type However when r ge20 high af 1047297nity isotherms are
obtained This implies that the 1047297rst added polymer chains are
completely adsorbed resulting in extremely low polymer concentra-
tion in solution (approaching zero) This explains the irreversibility of
adsorption of polymeric surfactants with r N100 The adsorption
isotherms with r =100 and above are typical of those observed
experimentally for most polymers that are not too polydisperse ie
showing a steep rise followed by a nearly horizontal plateau (which
only increases few percent per decade increase of ϕ) In these dilute
solutions the effect of solvency is most clearly seen with poor
solvents giving the highest adsorbed amounts In good solvents θ is
much smaller and levels off for long chains to attain an adsorption
plateau which is essentially independent of molecular weight
Another point that emerges from the SF theory is the difference in
shape between the experimental and theoretical adsorption isotherms
in thelow concentration region Theexperimentalisotherms areusuallyrounded whereas those predicted fromtheory are1047298at This is accounted
for in terms of the molecular weight distribution (polydispersity) which
is encountered with most practical polymers This effect has been
explained by CohenndashStuart et al [17] With polydisperse polymers the
larger molecular weight fractions adsorb preferentially over the smaller
ones At low polymer concentrations nearly all polymer fractions are
adsorbed leaving a small fraction of the polymer with the lowest
molecular weights in solution As the polymer concentration is
increased the higher molecular weight fractions displace the lower
ones on thesurfacewhichare nowreleasedin solution thusshiftingthe
molecular weight distribution in solution to lower values This process
continues with further increase in polymer concentration leading to
fractionation whereby the higher molecular weight fractions are
adsorbed at the expense of the lower fractions which are released to
the bulk solution However in very concentrated solutions monomers
adsorb preferentially with respect to polymers and short chains with
respect to larger ones This is due to the fact that in this region the
conformational entropy term predominates the free energy disfavour-
ing the adsorption of long chains
According to the SF theory the bound fraction p is high at low
concentration and relatively independent of molecular weight when
r N20 However with increase in surface coverage andor molecular
weight p tends to decrease indicating the formation of larger loops
and tails
The structure of the adsorbed layer is described in terms of the
segment density distribution As an illustration Fig 7 shows some
calculations using the SF theory for loops and tails with r =1000
ϕ= 10minus6 and χ=05 In this example 38 of the segments are in
trains 555 in loops and 65 in tails This theory demonstrates theimportance of tails which dominate the total distribution in the outer
region
3 Experimental techniques for studying polymeric surfactant
adsorption
As mentioned above for full characterization of polymeric sur-
factant adsorption one needs to determine three parameters (i) The
adsorbed amount Γ (mg mminus2 or mol mminus2) as a function of equilibrium
concentration C eq ie the adsorption isotherm (ii) The fraction of
segments in direct contact with the surface p (number of segments in
Fig 5 Schematic representation of a polymer molecule adsorbing on a 1047298at surface mdash
quasi-crystalline lattice with segments 1047297lling layers that are parallel to the surface
(random mixing of segments and solvent molecules in each layer is assumed)
Fig 6 Adsorption isotherms for oligomers and polymers in the dilute region based on
the SF theory Fig 7 Loop tail and total segment pro1047297le according to the SF theory
286 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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The microelectrophoresis technique is based on measurement of
the electrophoretic mobility u of the particles in the presence and
absence of the polymer layer From u one can calculate the zeta
potential ζ using the Huckel equation (which is applicable for small
particles and extended double layers ie κ Rbb1 where κ is the
DebyendashHuckel parameter that is related to the salt concentration
u = 23eeon η
eth15THORN
where ε is the relative permittivity of the medium and ε o is the
permittivity of free space
By measuring ζ of the particles with and without the adsorbed
polymer layer one can obtain the hydrodynamic thickness δh For
accurate measurements one should carry the measurements at various
electrolyte concentrations and extrapolate the results to the plateau
value Several automatic instruments are available for measurement of
the electrophoretic mobility Malvern zeta sizerndashCoulter Delsa sizer and
Broekahven instrument All these instruments are easy to use and the
measurement can be carried out within few minutes
4 Examples of the adsorption isotherms of nonionic polymericsurfactants
Fig 9 shows the adsorption isotherms for PEO with different
molecular weights on PS (at room temperature It can be seen that the
amount adsorbed in mgm-2 increases with increase in the polymer
molecular weight Fig 10 shows the variation of the hydrodynamic
thickness δh with molecular weight M δh shows a linear increase with
log M δh increases with n the number of segments in the chain
according to
δhasympn08 eth16THORN
Fig 11 shows the adsorption isotherms of PVA with various
molecular weights on PS latex (at 25 degC) [19] The polymers were
obtained by fractionation of a commercial sample of PVA with an
average molecular weight of 45000 The polymer also contained 12
vinyl acetate groups As with PEO the amount of adsorption increases
with increase in M The isotherms are also of the high af 1047297nity type Γ at the plateau increases linearly with M 12
The hydrodynamic thickness was determined using PCS and theresults are given below
M 67000 43000 28000 17000 8000
δhnm 255 197 140 98 33
δh seems to increase linearly with increase in the molecular weight
The effect of solvency on adsorption was investigated by increasing
the temperature (the PVA molecules are less soluble at higher
temperature) or addition of electrolyte (KCl) [20] The results are
shown in Figs 12 and 13 for M =65100 As can be seen from Fig 12
increase of temperature results in reduction of solvency of the
mediumfor the chain (due to break down of hydrogen bonds) and this
results in an increase in the amount adsorbed Addition of KCl (which
reduces the solvency of the medium for the chain) results in anincrease in adsorption (as predicted by theory)
The adsorption of block and graft copolymers is more complex
since the intimate structure of the chain determines the extent of
adsorption [18] Randomcopolymers adsorbin an intermediate way to
that of the corresponding homopolymers Blockcopolymers retain the
adsorption preference of the individual blocks The hydrophilic block
(eg PEO) the buoy (previously referred to as the A chain) extends
away from the particle surface into the bulk solution whereas the
hydrophobic anchor block (previously referred to as the B chain) (eg
PS or PPO) provides 1047297rm attachment to the surface Fig 14 shows the
theoretical prediction of diblock copolymer adsorption according to
Fig 9 Adsorption isotherms for PEO on PS
Fig 10 Hydrodynamic thickness of PEO on PS as a function of the molecular weight
Fig 11 Adsorption isotherms of PVA with different molecular weights on polystyrene
latex at 25 degC
Fig 12 In1047298uence of temperature on adsorption
288 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
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the Scheutjens and Fleer theory The surface density σ is plotted
versus the fraction of anchor segmentsν A The adsorption depends on
the anchorbuoy composition
The amount of adsorption is higher than for homopolymers and
the adsorbed layer thickness is more extended and dense Fig 15
shows the theoretical prediction for the adsorbed layer thickness δwhich is plotted as a function of ν A For a triblock copolymer AndashBndashA
with two buoy chains (A) and one anchor chain (B) the behaviour is
similar to that of diblock copolymers This is shown in Fig16 for PEOndash
PPOndashPEO block (Pluronic)
5 Interaction between particles containing adsorbed polymeric
surfactant layers steric stabilization
When two particles each with a radius R and containing an
adsorbed polymer layer with a hydrodynamic thickness δh approach
each other to a surfacendashsurface separation distance h that is smaller
than 2 δh the polymer layers interact with each other resulting in two
main situations [21] (i) The polymer chains may overlap with each
other (ii) The polymer layer may undergo some compression In both
cases there will be an increase in the local segment density of the
polymer chains in the interaction region This is schematicallyillustrated in Fig 17 The real situation is perhaps in between the
above two cases ie the polymer chains may undergo some
interpenetration and some compression
Provided the dangling chains (the A chains in AndashB AndashBndashA block or
BAn graft copolymers) are in a good solvent this local increase in
segment density in the interaction zone will result in strong repulsion
as a result of two main effects (i) Increase in the osmotic pressure in
the overlap region as a result of the unfavourable mixing of the
polymer chains when these are in good solvent conditions This is
referred to as osmotic repulsion or mixing interaction and it is
described by a free energy of interaction Gmix (ii) Reduction of the
con1047297gurational entropy of the chains in the interaction zone this
entropy reduction results from the decrease in the volume available
for thechains when these areeither overlapped or compressed This is
referred to as volume restriction interaction entropic or elastic
interaction and it is described by a free energy of interaction Gel
Combination of Gmix and Gel is usually referred to as the steric
interaction free energy Gs ie
Gs = Gmix + Gel eth17THORN
The sign of Gmix depends on the solvency of the medium for the
chains If in a good solvent ie the FloryndashHuggins interaction
parameter χ is less than 05 then Gmix is positive and the mixing
interaction leads to repulsion (see below) In contrast if χN05 (ie the
chains are in a poor solvent condition) Gmix is negative and the
mixing interaction becomes attractive Gel is always positive and
hence in some cases one can produce stable dispersions in a relatively
poor solvent (enhanced steric stabilisation)
51 Mixing interaction Gmix
This results from the unfavourable mixing of the polymer chainswhen these are in a good solvent conditions This is schematically
shown in Fig 18
Consider two spherical particles with the same radius and each
containing an adsorbed polymer layer with thickness δ Before
overlap one can de1047297ne in each polymer layer a chemical potential
for the solvent μ iα and a volume fraction for the polymer in the
layerϕ2 In the overlap region (volume element dV ) the chemical
potential of the solvent is reduced to μ i β This results from the increase
in polymer segment concentration in this overlap region
In the overlap region the chemical potential of the polymer chains
is now higher than in the rest of the layer (with no overlap) This
Fig 13 In1047298uence of addition of KCl on adsorption
Fig 14 Prediction of Adsorption of diblock copolymer
Fig15 Theoretical predictions of the adsorbed layer thickness for a diblock copolymer
Fig 16 Adsorbed amount (mg mminus2) versus fraction of anchor segment for an AndashBndashA
triblock copolymer (PEOndash
PPOndash
PEO)
289T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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amounts to an increase in the osmotic pressure in the overlap region
as a result solvent will diffuse from thebulk to the overlap region thus
separating the particles and hence a strong repulsive energy arises
from this effect The above repulsive energy can be calculated by
considering the free energy of mixing of two polymer solutions as for
example treated by Flory and Krigbaum [22] The free energy of
mixing is given by two terms (i) An entropy term that depends on the
volume fraction of polymer and solvent (ii)An energy term that is
determined by the FloryndashHuggins interaction parameter χ
Using the above theory one can derive an expression for the free
energy of mixing of two polymer layers (assuming a uniform segment
density distribution in each layer) surrounding two spherical particles
as a function of the separation distance h between the particles The
expression for Gmix is
Gmix
kT =
2V 22V 1
m2
2
1
2minusχ
δminus
h
2
2
3R + 2δ + h
2
eth18THORN
k is the Boltzmann constant T is the absolute temperature V 2 is the
molar volume of polymer V 1 is the molar volume of solvent and ν 2 is
the number of polymer chains per unit area
The sign of Gmix depends on the value of the FloryndashHuggins
interaction parameter χ if χb05 Gmix is positive and the interaction
is repulsive if χN05 Gmix is negative and the interaction is attractiveif χ=05 Gmix =0 and this de1047297nes the θ-condition
52 Elastic interaction Gel
Thisarises fromthe lossin con1047297gurational entropy of thechains on
the approach of a second particle As a result of this approach the
volume available for the chains becomes restricted resulting in loss of
the number of con1047297gurations This can be illustrated by considering a
simple molecule represented by a rod that rotates freely in a
hemisphere across a surface (Fig 19) When the two surfaces are
separated byan in1047297nitedistanceinfin thenumber of con1047297gurations of the
rod is Ω(infin) which is proportional to the volume of the hemisphere
When a secondparticleapproaches to a distance h suchthatit cuts the
hemisphere (loosing some volume) the volume available to the chains
is reduced and the number of con1047297gurations become Ω(h) which is
less than Ω(infin) For two 1047298at plates Gel is given by the following
expression
Gel
kT = 2m2 ln
X heth THORN
X infineth THORN
= 2m2Rel heth THORN eth19THORN
where Rel
(h) is a geometric function whose form depends on the
segment density distribution It should be stressed that Gel is always
positive and could play a major role in steric stabilisation It becomes
very strong when the separation distance between the particles
becomes comparable to the adsorbed layer thickness δ
Combination of Gmix and Gel with GA (the van der Waals attractive
energy)gives the total free energy of interaction GT (assuming there is
no contribution from any residual electrostatic interaction) ie
GT = Gmix + Gel + GA eth20THORN
A schematic representation of the variation of Gmix Gel GA and GT
with surfacendashsurface separation distance h is shown in Fig 20
Gmix increases very sharply with decrease of h when hb2δ Gel
increases very sharply with decrease of h when hbδ GT versus h
shows a minimum Gmin at separation distances comparable to 2δ
When h b2δ GT shows a rapid increase with decrease in h
The depth of the minimum depends on the Hamaker constant A
the particle radius R and adsorbed layer thickness δ Gmin increases
with increase of A and R At a given A and R Gmin increases with
decrease in δ (ie with decrease of the molecular weight M w of the
stabiliser This is illustrated in Fig 21 which shows the energyndash
distance curves as a function of δR The larger the value of δR the
smaller the value of Gmin In this case the system may approach
thermodynamic stability as is the case with nano-dispersions
6 Emulsions stabilized by polymeric surfactants
The most effective method for emulsion stabilization is to use
polymeric surfactants that stronglyadsorb at the OWor WO interface
Fig 17 Schematic representation of the interaction between particles containing adsorbed polymer layers
Fig 18 Schematic representation of polymer layer overlap
Fig 19 Schematic representation of con1047297gurational entropy loss on approach of a
second particle
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and produce effective steric stabilization against strong 1047298occulation
coalescence and Ostwald ripening [23]
As mentioned above a graft copolymer of the ABn type was
synthesized by grafting several alkyl groups on an inulin (polyfruc-
tose) chain The polymeric surfactant (INUTECregSP1) consists of a
linear polyfructose chain (the stabilizing A chain) and several alkyl
groups (the B chains) that provide multi-anchor attachment to the oil
droplets This polymeric surfactant produces enhanced steric stabili-
zation both in water and high electrolyte concentrations as will be
discussed later
For water-in-oil (WO) emulsions an AndashBndashA block copolymer of
poly (12-hydroxystearic acid) (PHS) (the A chains) and poly (ethylene
oxide) (PEO) (the B chain) PHSndashPEOndashPHS is commercially available
(Arlacel P135 UNIQEMA) The PEO chain (that is soluble in the water
droplets) forms the anchor chain whereas the PHS chains form the
stabilizing chains PHS is highly soluble in most hydrocarbon solvents
and is strongly solvated by its molecules The structure of the PHSndash
PEOndashPHS block copolymer is schematically shown in Fig 22The conformation of the polymeric surfactant at the WO interface
is schematically shown in Fig 23
Emulsions of Isopar Mwater and cyclomethiconewater were
prepared using INUTECregSP1 5050 (vv) OW emulsions were
prepared and the emulsi1047297er concentration was varied from 025 to 2
(wv) based on the oil phase 05 (wv) emulsi1047297erwas suf 1047297cient for
stabilization of these 5050 (vv) emulsions [23]
The emulsions were stored at room temperature and 50 degC and
optical micrographs were taken at intervals of time (for a year) in
order to check the stability Emulsions prepared in water were very
stableshowingno change in droplet size distributionover more than a
year period and this indicated absence of coalescence Any weak
1047298occulation that occurred was reversible and the emulsion could be
redispersed by gentle shaking Fig 24 shows an optical micrograph fora dilute 5050 (vv) emulsion that was stored for 15 and 14 weeks at
50 degC
No change in droplet size was observed after storage for more than
1 year at 50 degC indicating absence of coalescence The same result was
obtained when using different oils Emulsions were also stable against
coalescence in the presence of high electrolyte concentrations (up to
4 mol dmminus3 or ~25 NaCl
The above stability in high electrolyte concentrations is not
observed with polymeric surfactants based on polethylene oxide
The high stability observed using INUTECregSP1 is related to its
strong hydration both in water and in electrolyte solutions The
hydration of inulin (the backbone of HMI) could be assessed using
cloud point measurements A comparison was also made with PEO
with two molecular weights namely 4000 and 20000
Solutionsof PEO 4000 and 20000 showeda systematic decrease of
cloud point with increase in NaCl or MgSO4 concentration In contrast
inulin showed no cloud point up to 4 mol dmminus3 NaCl and up to 1 mol
dmminus3 MgSO4
The above results explain the difference between PEO and inulin
With PEO the chains show dehydration when the NaCl concentration
is increased above 2 mol dmminus3 or 05 mol dmminus3 MgSO4 The inulin
chains remain hydrated at much higher electrolyte concentrations It
seems that the linear polyfructose chains remain strongly hydrated athigh temperature and high electrolyte concentrations
The high emulsion stability obtained whenusingINUTECregSP1 can be
accounted for by the following factors (i) The multi-point attachment
of the polymer by several alkyl chains that are grafted on the backbone
(ii) The strong hydration of the polyfructose ldquoloopsrdquo both in water and
high electrolyte concentrations (χ remains below 05 under these
conditions) (iii) Thehigh volumefraction (concentration) of the loops at
the interface (iv) Enhanced steric stabilization this is the case with
multi-point attachment which produces strong elastic interaction
Evidence for the high stability of the liquid 1047297lm between emulsion
droplets when using INUTECregSP1 was obtained by Exerowa et al [24]
using disjoining pressure measurements This is illustrated in Fig 25
which shows a plot of disjoining pressure versus separation distance
between two emulsion droplets at various electrolyte concentrations
The results show that by increasing the capillary pressure a stable
Newton Black Film (NBF) is obtained at a 1047297lm thickness of sim7 nm
The lack of rupture of the 1047297lm at the highest pressure applied of
45times104 Pa indicate the high stability of the 1047297lm in water and in high
electrolyte concentrations (up to 20 mol dmminus3 NaCl)
The lack of rupture of the NBF up to the highest pressure applied
namely 45times104 Pa clearly indicatesthe high stability of the liquid1047297lm
in the presence of high NaCl concentrations (up to 2 mol dm minus3) This
result is consistent with the high emulsion stability obtained at high
electrolyte concentrations and high temperature Emulsions of Isopar
M-in-water are very stable under such conditions and this could be
accounted for by the high stability of the NBF The droplet size of
5050 OW emulsions prepared using 2 INUTEC regSP1 is in the region
of 1ndash10 μ m This corresponds to a capillary pressure of ~3times104 Pa for
the 1 μ m drops and ~3times103 Pa for the 10 μ m drops These capillarypressures are lower than those to which the NBF have been sub-
jected to and this clearly indicates the high stability obtained against
coalescence in these emulsions
61 WO emulsions stabilized with PHS ndashPEOndashPHS block copolymer
WO emulsions (the oil being Isopar M) were prepared using PHSndash
PEOndashPHS block copolymer at high water volume fractions (N07) The
emulsions have a narrow droplet size distribution with a z -average
radius of 183 nm [25] They also remained 1047298uid up to high water
volume fractions (N06) This could be illustrated from viscosityndash
volume fraction curves as is shown in Fig 26
Fig 20 Energyndashdistance curves for sterically stabilized systems
Fig 21 Variation of Gmin with δR
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The effective volume fraction ϕeff of the emulsions (the core
droplets plus the adsorbed layer) could be calculated from the relative
viscosity and using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth21THORN
Where η r is the relative viscosity ϕp is the maximum packing fraction(sim07) and [ η ] is the intrinsic viscosity that is equal to 25 for hard-
spheres
The calculations based on Eq (22) are shown in Fig 27 (square
symbols) From the effective volume fraction ϕeff and the core volume
fraction ϕ the adsorbed layer thickness could be calculated This was
found to be in the region of 10 nm at ϕ =04 and it decreased with
increase in ϕ
The WO emulsions prepared using the PHSndashPEOndashPHS block
copolymer remained stable both at room temperature and 50 degC
This is consistent with the structure of the block copolymer the B
chain (PEO) is soluble in water and it forms a very strong anchor at the
WO interface The PHS chains (the A chains) provide effective steric
stabilization since the chains are highly soluble in Isopar M and are
strongly solvated by its molecules
7 Suspensions stabilised using polymeric surfactants
There are generally two procedures for preparation of solidliquid
dispersions
(i) Condensation methods build-up of particles from mole-
cular units ie nucleation and growth A special procedure
is the preparation of latexes by emulsion or dispersion
polymerization
(ii) Dispersion methods in this case one starts with preformed
large particles or crystals which are dispersed in the liquid by
using a surfactant (wetting agent) with subsequent breaking up
of the large particles by milling (comminution) to achieve the
desirable particle size distribution A dispersing agent (usuallya
polymeric surfactant) is used for the dispersion process and
subsequent stabilization of the resulting suspension
There are generally two procedures for preparation of latexes
(i) Emulsion polymerization the monomers that are essentially
insoluble in the aqueous medium are emulsi1047297ed using a
surfactant and an initiator is added while heating the system
to produce the polymer particles that are stabilized electro-
statically (when using ionic surfactants) or sterically (when
using non-ionic surfactants)
(ii) Dispersion polymerization the monomers are dissolved in a
solvent in which the resulting polymer particlesare insoluble A
protective colloid (normally a block or graft copolymer) is
added to prevent 1047298occulation of the resulting polymers
particles that are produced on addition of an initiator This
method is usually applied for the production of non-aqueous
latex dispersions and is sometimes referred to as Non-Aqueous
Dispersion Polymerization (NAD)
Surfactants play a crucial role in the process of latex preparation
since they determine the stabilizing ef 1047297ciency and the effectiveness of
the surface active agent ultimately determines the number of particles
Fig 22 Schematic representation of the structure of PHSndashPEOndashPHS block copolymer
Fig 23 Conformation of PHSndash
PEOndash
PHS polymeric surfactant at the WO interface
292 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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and their size The effectiveness of any surface active agent in
stabilizing the particles is the dominant factor and the number of
micelles formed is relatively unimportant In the NAD process themonomer normally an acrylic is dissolved in a non-aqueous solvent
normally an aliphatic hydrocarbon and an oil soluble initiator and a
stabilizer (to protect the resulting particles from1047298occulation) is added
to the reaction mixture The most successful stabilizers used in NAD
are block and graft copolymers Preformed graft stabilizers based
on poly(12-hydroxy stearic acid) (PHS) are simple to prepare and
effective in NAD polymerization
Dispersion methods are used for the preparation of suspensions of
preformed particles The role of surfactants (or polymers) in the
dispersion process can be analyzed in terms of the three processesinvolved (i) Wetting of the powder by the liquid (ii) Breaking of the
aggregates and agglomerates (iii) Comminution of the resulting
particles and their subsequent stabilization All these processes are
affected by surfactants or polymers which adsorb on the powder
surface thus aiding the wetting of the powder break-up of the
aggregates and agglomerates and subsequent reduction of particle
size by wet milling
71 Polymeric surfactants in emulsion polymerization
Recently the graft copolymer of hydrophobically modi1047297ed inulin
(INUTECreg SP1) has been used in emulsion polymerization of styrene
methyl methacrylate butyl acrylate and several other monomers [26]All lattices were prepared by emulsion polymerisation using potas-
sium persulphate as initiator The z -average particle size was
determined by photon correlation spectroscopy (PCS) and electron
micrographs were also taken
Fig 24 Optical micrographs of OW emulsions stabilized with INUTEC regSP1 stored at
50 degC for 15 weeks (a) and 14 weeks (b)
Fig 25 Variation of disjoining pressure with equivalent 1047297lm thickness at various NaCl concentrations
Fig 26 Viscosityndashvolume fraction for WO emulsion stabilized with PHSndashPEOndashPHS
block copolymer experimental data calculated using Eq (22)
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Emulsion polymerization of styrene or methylmethacrylate showed
an optimum weight ratio of (INUTEC)Monomer of 00033 for PS and
0001 for PMMA particles The (initiator)(Monomer) ratio was
kept constant at 000125 The monomer conversion was higher than
85 in all cases Latex dispersions of PS reaching 50 and of PMMA
reaching 40 could be obtained using such low concentration of
INUTECregSP1 Fig 27 shows the variation of particle diameter with
monomer concentration
The stability of the latexes was determined by determining the
critical coagulation concentration (CCC) using CaCl2 The CCC was low
(00175ndash005 mol dmminus3) but this was higher than that for the latex
prepared without surfactant Post addition of INUTEC regSP1 resulted in
a large increase in the CCC as is illustrated in Fig 28 which shows log
W minus log C curves (where W is the ratio between the fast 1047298occulation
rate constant to the slow 1047298occulation rate constant referred to as the
stability ratio) at various additions of INUTECregSP1
As with the emulsions the high stability of the latex when using
INUTECregSP1 is due to the strong adsorption of the polymeric
surfactant on the latex particles and formation of strongly hydrated
loops and tails of polyfructose that provide effective steric stabiliza-
tion Evidence for the strong repulsion produced when using
INUTECregSP1 was obtained from Atomic Force Microscopy investiga-
tions [27] whereby the force between hydrophobic glass spheres and
hydrophobic glass plate both containing an adsorbed layer of
INUTECregSP1 was measured as a function of distance of separation
both in water and in the presence of various Na2SO4 concentrations
The results are shown in Figs 29 and 30
Fig 27 Electron micrographs of the latexes
Fig 28 In1047298uence of post addition of INUTECreg
SP1 on the latex stability
Fig 29 Forcendashdistance curves between hydrophobised glass surfaces containing
adsorbed INUTECreg
SP1 in water
294 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
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layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
297T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1819
radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1919
With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 219
6 Emulsions stabilized by polymeric surfactants 292
61 WO emulsions stabilized with PHSndashPEOndashPHS block copolymer 293
7 Suspensions stabilised using polymeric surfactants 295
71 Polymeric surfactants in emulsion polymerization 296
711 Dispersion polymerization 299
72 Polymeric surfactants for stabilisation of preformed latex dispersions
73 Use of polymeric surfactants for preparation and stabilisation of nano-emulsions
References
1 Introduction
Polymeric surfactants are essential materials for preparation of
many disperse systems of which we mention dyestuffs paper
coatings inks agrochemicals pharmaceuticals personal care pro-
ducts ceramics and detergents [1] One of the most important
applications of polymeric surfactants is in the preparation of oil-in-
water (OW) and water-in-oil (WO) emulsions as well as solidliquid
dispersions [23] In this case the hydrophobic portion of the
surfactant molecule should adsorb ldquostronglyrdquo at the OW or becomes
dissolved in the oil phase leaving the hydrophilic components in the
aqueous medium whereby they become strongly solvated by the
water molecules
The other major application of surfactants is for the preparation of
solidliquid dispersions (usually referred to as suspensions) There are
generally two methods for preparation of suspensions referred to as
condensation and dispersions methods In the 1047297rst case one starts
with molecular units and build-up the particles by a process of
nucleation and growth [4] A typical example is the preparation of
polymer lattices In this case the monomer (such as styrene or
methylmethacrylate) is emulsi1047297ed in water using an anionic or non-
ionic surfactant (such as sodium dodecyl sulphate or alcohol
ethoxylate) An initiator such as potassium persulphate is added and
when the temperature of the system is increased initiation occurs
resulting in the formation of the latex (polystyrene or polymethyl-
methacrylate) In the dispersion methods preformed particles
(usually powders) are dispersed in an aqueous solution containing asurfactant The latter is essential for adequate wetting of the powder
(both external and internal surfaces of the powder aggregates and
agglomerates must be wetted) [1] This is followed by dispersion of
the powder using high speed stirrers and 1047297nally the dispersion is
ldquomilledrdquo to reduce the particle size to the appropriate range
For stabilization of emulsionsand suspensions against 1047298occulation
coalescence and Ostwald ripening the following criteria must be
satis1047297ed (i) Complete coverage of the droplets or particles by the
surfactant Any bare patches may result in 1047298occulation as a result of
van der Waals attraction or bridging (ii) Strong adsorption (or
lsquorsquoanchoringrsquorsquo) of the surfactant molecule to the surface of droplet or
particle (iii) Strong solvation (hydration) of the stabilizing chain to
provide effective steric stabilization (iv) Reasonably thick adsorbed
layer to prevent weak 1047298occulation [1]Most of the above criteria for stability are best served by using a
polymeric surfactant In particular molecules of the AndashB AndashBndashA
blocks and BAn (or ABn) grafts (see below) are the most ef 1047297cient for
stabilisation of emulsions and suspensions In this case the B chain
(referred to as the ldquoanchoringrdquo chain) is chosen to be highly insoluble
in the medium and with a high af 1047297nity to the surface in the case of
suspensions or soluble in the oil in the case of emulsions The A chain
is chosen to be highly soluble in the medium and strongly solvated by
its molecules These block and graft copolymers are ideal for
preparation of concentrated emulsions and suspensions which are
needed in many industrial applications
In this overview I will start with a section on the general
classi1047297cation of polymeric surfactants This is followed by a discussion
of their solution properties The next section will be devoted to the
adsorption of polymeric surfactants at the solidliquid interface
whereby a summary will be given to some of the theoretical
treatments and the methods that may be applied for studying
polymeric surfactant adsorption and its conformation at the SL
interface The same principles may be applied to the adsorption of
polymeric surfactants at the LL interface although theoretical
treatments of this problem are not as developed as those for the SL
interface Thenext section will be devoted to theprinciples involved in
stabilisation of emulsions and suspensions using polymeric surfac-
tants ie the general theoryof steric stabilisation Two sections will be
devoted to describe the use of polymeric surfactants for stabilisation
of suspensions and emulsions The use of polymeric surfactants for
stabilization of nano-emulsions against Ostwald ripening will also be
described
11 General classi 1047297cation of polymeric surfactants
Perhaps the simplest type of a polymeric surfactant is a homo-
polymer that is formed from the same repeating units such as poly
(ethylene oxide) or poly(vinyl pyrrolidone) These homopolymers
have little surface activity at the OW interface since the homo-
polymer segments (ethylene oxide or vinylpyrrolidone) are highly
water soluble and have little af 1047297nity to the interface However such
homopolymers may adsorb signi1047297cantly at the SL interface Even if
the adsorption energy per monomer segment to the surface is small
(fraction of kT where k is the Boltzmann constant and T is the
absolute temperature) the total adsorption energy per molecule maybe suf 1047297cient to overcome the unfavourable entropy loss of the
molecule at the SL interface
Clearly homopolymers are not the most suitable emulsi1047297ers or
dispersants A small variant is to use polymers that contain speci1047297c
groups that have high af 1047297nity to the surface This is exempli1047297ed by
partially hydrolysed poly(vinyl acetate) (PVAc) technically referred to
as poly(vinyl alcohol) (PVA) The polymer is prepared by partial
hydrolysis of PVAc leaving some residual vinyl acetate groups Most
commercially available PVA molecules contain 4ndash12 acetate groups
These acetate groups which are hydrophobic give the molecule its
amphipathic character On a hydrophobic surface such as polystyrene
the polymer adsorbs with preferential attachment of the acetate
groups on the surface leaving the more hydrophilic vinyl alcohol
segments dangling in the aqueous medium These partially hydro-lysed PVA molecules also exhibit surface activity at the OW interface
[5]
The most convenient polymeric surfactants are those of the block
and graft copolymer type A block copolymer is a linear arrangement
of blocks of variable monomer composition The nomenclature for a
diblock is poly-A-block-poly-B and for a triblock is poly-A-block-poly-
B-poly-A One of the most widely used triblock polymeric surfactants
are the ldquoPluronicsrdquo (BASF Germany) or ldquoSynperonic PErdquo (ICI UK)
which consists of two poly-A blocks of poly(ethylene oxide) (PEO) and
one block of poly(propylene oxide) (PPO) Several chain lengths of PEO
and PPO are available More recently triblocks of PPOndashPEOndashPPO
(inverse Pluronics) became available for some speci1047297c applications
The above polymeric triblocks can be applied as emulsi1047297ers or
dispersants whereby the assumption is made that the hydrophobicPPO
291
292
293
294
296
296
297
300
282 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 319
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Clearly when χb12 B2 is positive and mixing is non-ideal leading to
positive deviation (repulsion) this occurs when the polymer chains
are in ldquogoodrdquo solvent conditions In contrast when χN12 B2 is
negative and mixing is non-ideal leading to negative deviation
(attraction) this occurs when the polymer chains are in ldquopoorrdquo
solvent conditions (precipitation of the polymer may occur under
these conditions) Since the polymer solvency depends on tempera-
ture one can also de1047297ne a theta temperature θ at which χ=12
The function [(12)minus
χ] can also be expressed in terms of twomixing parameters an enthalpy parameter κ 1 and an entropy
parameter ψ1 ie
1
2minusχ
= κ 1minusψ1 eth8THORN
The θ-temperature can also be de1047297ned in terms of κ 1 and ψ1
θ = κ 1T
ψ1
eth9THORN
Alternatively one can write
1
2
minusχ = ψ1 1minusθ
T eth10THORN
Although the FloryndashHuggins theory is sound in principle several
experimental results cannot be accounted for For example it was
found that the χ parameter depends on the polymer concentration in
solution Most serious is the fact that many polymer solutions (such as
PEO) show phase separation on heating when the theory predictsthat
it should happen only on cooling
The solution properties of copolymers are much more complicated
This is due to the fact that the two copolymer components A and B
behave differently in different solvents Only when the two compo-
nents are both soluble in the same solvent then they exhibit similar
solution properties This is the case for example for a non-polar
copolymer in a non-polar solvent It should also be emphasised that
the FloryndashHuggins theory was developed for ideal linear polymers
Indeed with branched polymers consisting of high monomer density
(eg star branched polymers) the θ-temperature depends on the
lengthof the armsand isin general lower thanthatof a linearpolymer
with the same molecular weight
Another complication arises from speci1047297c interaction with the
solvent eg hydrogen bonding between polymer and solvent
molecules (eg with PEO and PVA in water) Also aggregation insolution (lack of complete dissolution) may present another problem
One of the most useful parameters for characterising the
conformation of a polymer in solution is the root mean-square (rms)
end to end length br 2N12 which represents a con1047297guration character r
as the distance from one end group to the other of a chain molecule
Another useful parameter is the radius of gyration bs2N
12 which is a
measure of the effective size of a polymer molecule (it is the root
mean-square distance of the elements of the chain from its centre of
gravity)
For linear polymers
bs2N
1=2 = br 2N1=2
61=2 eth11THORN
The radius of gyration of a polymer in solution can be determined
from light scattering measurements
As mentioned above dilute solutions of copolymers is solvents that
are good for both components exhibit similar behaviour to hompo-
lymer chains However in a selective solvent whereby the medium is
a good solvent for one component say A and a poor solvent for the
second component B the very different solvent af 1047297nities to the two
components will have a large effect on the conformation of the
isolated chain This results in formation of aggregates involving
several macromolecules in dilute solutions (low Critical Aggregation
Concentration CAC) It is believed that the polymeric aggregates are
Fig 3 Various conformations of macromolecules on a plane surface
284 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 519
spherical [5] The CAC of these block and graft copolymers is usually
very low
Several methods may be applied to obtain the aggregate size and
shape of block and graft copolymers of which light scattering small
angle X-ray and neutron scattering are probably the most direct
Dynamic light scattering (photon correlation spectroscopy) can also
be applied to obtain the hydrodynamic radius of the aggregate This
technique is relatively easy to perform when compared with static
light scattering since it does not require special preparation of thesample
13 Adsorption and conformation of polymeric surfactants at interfaces
Understanding the adsorption and conformation of polymeric
surfactants at interfaces is key to knowing how these molecules act as
stabilizers Most basic ideas on adsorption and conformation of
polymers have been developed for the solidliquid interface [10ndash17]
The process of polymer adsorption is fairly complicated mdash it
involves PolymerSurface Interaction PolymerSolvent Interaction
SurfaceSolvent Interaction The con1047297guration (conformation) of the
polymer at the solidliquid interface The polymersurface interaction
is described in terms of adsorption energy per segment χs The
polymersolvent interaction is described in terms of the Floryndash
Huggins interaction parameter χ The polymer con1047297guration is
described by sequences of Trains segments in direct contact with
the surface Loops segments in between the trains that extend into
solution Tails ends of the molecules that also extend into solution A
schematic representation of the various polymer con1047297gurations is
given in Fig 3
For homopolymers eg poly(ethylene oxide) (PEO) or polyvinyl
pyrrolidone (PVP) a train-loop-tail con1047297guration is the case For
adsorption to occur a minimum energy of adsorption per segment χs
is required When a polymer molecule adsorbs on a surface it looses
con1047297gurational entropy and this must be compensated by an
adsorption energy χs per segment This is schematically shown in
Fig 4 where the adsorbed amount Γ is plotted versus χs The
minimum value of χs can be very small (b01kT ) since a large number
of segments per molecule are adsorbed For a polymer with say 100segments and 10 of these are in trains the adsorption energy per
molecule now reaches 1kT (with χs =01kT ) For 1000 segments the
adsorption energy per molecule is now 10kT
Homopolymers are not the most suitable for stabilisation of
dispersions For strong adsorption one needs the molecule to be
ldquoinsolublerdquo in the medium and has strong af 1047297nity (ldquoanchoringrdquo) to the
surface For stabilisation one needs the molecule to be highly soluble
in themedium andstronglysolvated by itsmolecules mdash This requires a
FloryndashHuggins interaction parameter less than 05 The above
opposing effects can be resolved by introducing ldquoshortrdquo blocks in the
moleculewhichare insolublein themedium andhavea strong af 1047297nity
to the surface as illustrated in Fig 3b Example Partially hydrolysed
polyvinyl acetate (88 hydrolysed ie with 12 acetate groups)
usually referred to as polyvinyl alcohol (PVA)
The above requirements are better satis1047297ed using AndashB AndashBndashA and
BAn graft copolymers B is chosento be highly insolublein themedium
and it should have high af 1047297nity to the surface This is essential to
ensure strong ldquoanchoringrdquo to the surface (irreversible adsorption) A is
chosen to be highly soluble in the medium and strongly solvated by
its molecules The FloryndashHuggins χ parameter can be applied in this
case For a polymer in a good solvent χ has to be lower than 05 the
smaller the χ value the better the solvent for the polymer chains
Examples of B for a hydrophobic particles in aqueous media are
polystyrene polymethylmethacrylate Examples of A in aqueous
media are polyethylene oxide polyacrylic acid polyvinyl pyrollidone
and polysaccharides For non-aqueous media such as hydrocarbons
the A chain(s) could be poly(12-hydroxystearic acid)
For full description of polymer adsorption one needs to obtain
information on the following (i) The amount of polymer adsorbed Γ (in mg or moles) per unit area of the particles It is essential to know
thesurface area of theparticles in thesuspensionNitrogenadsorption
on the powder surface may give such information (by application of
the BET equation) provided there will be no change in area on
dispersing the particles in the medium For many practical systems a
change in surface area may occur on dispersing the powder in which
case one has to use dye adsorption to measure the surface area (some
assumptions have to be made in this case) (ii)The fraction of segments
in direct contact with thesurface ie the fraction of segmentsin trains
p ( p = (Number of segments in direct contact with the surface) Total
Number) (iii)The distribution of segments in loops and tails ρ(z)
which extend in several layersfrom the surface ρ(z) is usually dif 1047297cult
to obtain experimentally although recently application of small angle
neutron scattering could obtain such information An alternative anduseful parameter for assessing ldquosteric stabilisationrdquo is the hydro-
dynamic thickness δh (thickness of the adsorbed or grafted polymer
layer plus any contribution from the hydration layer) Several methods
can be applied to measure δh as will be discussed below
2 Theories of polymer adsorption
Two main approaches have been developed to treat the problem of
polymer adsorption (i) Random Walk approach This is based on
Florys treatment of the polymer chain in solution the surface was
considered as a re1047298ecting barrier (ii) Statistical mechanical approach
Thepolymer con1047297gurationwas treated as being made of three types of
structures trains loops and tails each having a different energy state
The random walk approach is an unrealistic model to the problemof polymer adsorption since the polymer interacts in a speci1047297c
manner with the surface and the solvent The statistical mechanical
approach is a more realistic model for the problem of polymer
adsorption since it takes into account the various interactions
involved A useful model for treating polymer adsorption and
con1047297guration was suggested by Scheutjens and Fleer (SF theory)
[14ndash16] that is referred to as the step weighted random walk approach
which is summarized below
The SF theory starts without any assumption for the segment
density distribution The partition functions were derived for the
mixture of free and adsorbed polymer molecules as well as for the
solvent molecules All chain conformations were described as step
weighted random walks on a quasi-crystalline lattice which extends in
parallel layers from the surface mdash
this is schematically shown in Fig 5Fig 4 Variation of adsorption amount Γ with adsorption energy per segment χs
285T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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The partition function is written in terms of a number of con1047297gura-
tions These were treated as connectedsequencesof segments In each
layer random mixing between segments and solvent molecules was
assumed (mean 1047297eld approximation) Each step in the random walk
was assigned a weighting factor pi
pi consists of three contributions (i) An adsorption energyχs (which
exists only for the segments that are near the surface) (ii) Con1047297gura-
tional entropy of mixing (that exists in each layer) (iii) Segment-solvent
interaction parameterχ (the FloryndashHuggins interaction parameter note
that χ=0 for an athermal solvent χ=05 for a θ-solvent)
From the weighting factors the statistical weight of any chain
conformation can be calculated for a given segment concentration
pro1047297le using a matrix formulation Fig 6 shows typical adsorption
isotherms plotted as surface coverage (in equivalent monolayers)
versus polymer volume fraction ϕ in bulk solution (ϕ was taken to
vary between 0 and 10minus3 which is the normal experimental range)
The results in Fig 6 show the effect of increasing the chain length r
and effect of solvency (athermal solvent with χ=0 and theta solvent
with χ=05) The adsorption energy χs was taken to be the same and
equal to 1kT When r = 1 θ is very small and the adsorption increases linearly
with increase of ϕ (Henrys type isotherm) On the other hand when
r = 10 the isotherm deviates much from a straight line and approaches
a Langmuirian type However when r ge20 high af 1047297nity isotherms are
obtained This implies that the 1047297rst added polymer chains are
completely adsorbed resulting in extremely low polymer concentra-
tion in solution (approaching zero) This explains the irreversibility of
adsorption of polymeric surfactants with r N100 The adsorption
isotherms with r =100 and above are typical of those observed
experimentally for most polymers that are not too polydisperse ie
showing a steep rise followed by a nearly horizontal plateau (which
only increases few percent per decade increase of ϕ) In these dilute
solutions the effect of solvency is most clearly seen with poor
solvents giving the highest adsorbed amounts In good solvents θ is
much smaller and levels off for long chains to attain an adsorption
plateau which is essentially independent of molecular weight
Another point that emerges from the SF theory is the difference in
shape between the experimental and theoretical adsorption isotherms
in thelow concentration region Theexperimentalisotherms areusuallyrounded whereas those predicted fromtheory are1047298at This is accounted
for in terms of the molecular weight distribution (polydispersity) which
is encountered with most practical polymers This effect has been
explained by CohenndashStuart et al [17] With polydisperse polymers the
larger molecular weight fractions adsorb preferentially over the smaller
ones At low polymer concentrations nearly all polymer fractions are
adsorbed leaving a small fraction of the polymer with the lowest
molecular weights in solution As the polymer concentration is
increased the higher molecular weight fractions displace the lower
ones on thesurfacewhichare nowreleasedin solution thusshiftingthe
molecular weight distribution in solution to lower values This process
continues with further increase in polymer concentration leading to
fractionation whereby the higher molecular weight fractions are
adsorbed at the expense of the lower fractions which are released to
the bulk solution However in very concentrated solutions monomers
adsorb preferentially with respect to polymers and short chains with
respect to larger ones This is due to the fact that in this region the
conformational entropy term predominates the free energy disfavour-
ing the adsorption of long chains
According to the SF theory the bound fraction p is high at low
concentration and relatively independent of molecular weight when
r N20 However with increase in surface coverage andor molecular
weight p tends to decrease indicating the formation of larger loops
and tails
The structure of the adsorbed layer is described in terms of the
segment density distribution As an illustration Fig 7 shows some
calculations using the SF theory for loops and tails with r =1000
ϕ= 10minus6 and χ=05 In this example 38 of the segments are in
trains 555 in loops and 65 in tails This theory demonstrates theimportance of tails which dominate the total distribution in the outer
region
3 Experimental techniques for studying polymeric surfactant
adsorption
As mentioned above for full characterization of polymeric sur-
factant adsorption one needs to determine three parameters (i) The
adsorbed amount Γ (mg mminus2 or mol mminus2) as a function of equilibrium
concentration C eq ie the adsorption isotherm (ii) The fraction of
segments in direct contact with the surface p (number of segments in
Fig 5 Schematic representation of a polymer molecule adsorbing on a 1047298at surface mdash
quasi-crystalline lattice with segments 1047297lling layers that are parallel to the surface
(random mixing of segments and solvent molecules in each layer is assumed)
Fig 6 Adsorption isotherms for oligomers and polymers in the dilute region based on
the SF theory Fig 7 Loop tail and total segment pro1047297le according to the SF theory
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The microelectrophoresis technique is based on measurement of
the electrophoretic mobility u of the particles in the presence and
absence of the polymer layer From u one can calculate the zeta
potential ζ using the Huckel equation (which is applicable for small
particles and extended double layers ie κ Rbb1 where κ is the
DebyendashHuckel parameter that is related to the salt concentration
u = 23eeon η
eth15THORN
where ε is the relative permittivity of the medium and ε o is the
permittivity of free space
By measuring ζ of the particles with and without the adsorbed
polymer layer one can obtain the hydrodynamic thickness δh For
accurate measurements one should carry the measurements at various
electrolyte concentrations and extrapolate the results to the plateau
value Several automatic instruments are available for measurement of
the electrophoretic mobility Malvern zeta sizerndashCoulter Delsa sizer and
Broekahven instrument All these instruments are easy to use and the
measurement can be carried out within few minutes
4 Examples of the adsorption isotherms of nonionic polymericsurfactants
Fig 9 shows the adsorption isotherms for PEO with different
molecular weights on PS (at room temperature It can be seen that the
amount adsorbed in mgm-2 increases with increase in the polymer
molecular weight Fig 10 shows the variation of the hydrodynamic
thickness δh with molecular weight M δh shows a linear increase with
log M δh increases with n the number of segments in the chain
according to
δhasympn08 eth16THORN
Fig 11 shows the adsorption isotherms of PVA with various
molecular weights on PS latex (at 25 degC) [19] The polymers were
obtained by fractionation of a commercial sample of PVA with an
average molecular weight of 45000 The polymer also contained 12
vinyl acetate groups As with PEO the amount of adsorption increases
with increase in M The isotherms are also of the high af 1047297nity type Γ at the plateau increases linearly with M 12
The hydrodynamic thickness was determined using PCS and theresults are given below
M 67000 43000 28000 17000 8000
δhnm 255 197 140 98 33
δh seems to increase linearly with increase in the molecular weight
The effect of solvency on adsorption was investigated by increasing
the temperature (the PVA molecules are less soluble at higher
temperature) or addition of electrolyte (KCl) [20] The results are
shown in Figs 12 and 13 for M =65100 As can be seen from Fig 12
increase of temperature results in reduction of solvency of the
mediumfor the chain (due to break down of hydrogen bonds) and this
results in an increase in the amount adsorbed Addition of KCl (which
reduces the solvency of the medium for the chain) results in anincrease in adsorption (as predicted by theory)
The adsorption of block and graft copolymers is more complex
since the intimate structure of the chain determines the extent of
adsorption [18] Randomcopolymers adsorbin an intermediate way to
that of the corresponding homopolymers Blockcopolymers retain the
adsorption preference of the individual blocks The hydrophilic block
(eg PEO) the buoy (previously referred to as the A chain) extends
away from the particle surface into the bulk solution whereas the
hydrophobic anchor block (previously referred to as the B chain) (eg
PS or PPO) provides 1047297rm attachment to the surface Fig 14 shows the
theoretical prediction of diblock copolymer adsorption according to
Fig 9 Adsorption isotherms for PEO on PS
Fig 10 Hydrodynamic thickness of PEO on PS as a function of the molecular weight
Fig 11 Adsorption isotherms of PVA with different molecular weights on polystyrene
latex at 25 degC
Fig 12 In1047298uence of temperature on adsorption
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the Scheutjens and Fleer theory The surface density σ is plotted
versus the fraction of anchor segmentsν A The adsorption depends on
the anchorbuoy composition
The amount of adsorption is higher than for homopolymers and
the adsorbed layer thickness is more extended and dense Fig 15
shows the theoretical prediction for the adsorbed layer thickness δwhich is plotted as a function of ν A For a triblock copolymer AndashBndashA
with two buoy chains (A) and one anchor chain (B) the behaviour is
similar to that of diblock copolymers This is shown in Fig16 for PEOndash
PPOndashPEO block (Pluronic)
5 Interaction between particles containing adsorbed polymeric
surfactant layers steric stabilization
When two particles each with a radius R and containing an
adsorbed polymer layer with a hydrodynamic thickness δh approach
each other to a surfacendashsurface separation distance h that is smaller
than 2 δh the polymer layers interact with each other resulting in two
main situations [21] (i) The polymer chains may overlap with each
other (ii) The polymer layer may undergo some compression In both
cases there will be an increase in the local segment density of the
polymer chains in the interaction region This is schematicallyillustrated in Fig 17 The real situation is perhaps in between the
above two cases ie the polymer chains may undergo some
interpenetration and some compression
Provided the dangling chains (the A chains in AndashB AndashBndashA block or
BAn graft copolymers) are in a good solvent this local increase in
segment density in the interaction zone will result in strong repulsion
as a result of two main effects (i) Increase in the osmotic pressure in
the overlap region as a result of the unfavourable mixing of the
polymer chains when these are in good solvent conditions This is
referred to as osmotic repulsion or mixing interaction and it is
described by a free energy of interaction Gmix (ii) Reduction of the
con1047297gurational entropy of the chains in the interaction zone this
entropy reduction results from the decrease in the volume available
for thechains when these areeither overlapped or compressed This is
referred to as volume restriction interaction entropic or elastic
interaction and it is described by a free energy of interaction Gel
Combination of Gmix and Gel is usually referred to as the steric
interaction free energy Gs ie
Gs = Gmix + Gel eth17THORN
The sign of Gmix depends on the solvency of the medium for the
chains If in a good solvent ie the FloryndashHuggins interaction
parameter χ is less than 05 then Gmix is positive and the mixing
interaction leads to repulsion (see below) In contrast if χN05 (ie the
chains are in a poor solvent condition) Gmix is negative and the
mixing interaction becomes attractive Gel is always positive and
hence in some cases one can produce stable dispersions in a relatively
poor solvent (enhanced steric stabilisation)
51 Mixing interaction Gmix
This results from the unfavourable mixing of the polymer chainswhen these are in a good solvent conditions This is schematically
shown in Fig 18
Consider two spherical particles with the same radius and each
containing an adsorbed polymer layer with thickness δ Before
overlap one can de1047297ne in each polymer layer a chemical potential
for the solvent μ iα and a volume fraction for the polymer in the
layerϕ2 In the overlap region (volume element dV ) the chemical
potential of the solvent is reduced to μ i β This results from the increase
in polymer segment concentration in this overlap region
In the overlap region the chemical potential of the polymer chains
is now higher than in the rest of the layer (with no overlap) This
Fig 13 In1047298uence of addition of KCl on adsorption
Fig 14 Prediction of Adsorption of diblock copolymer
Fig15 Theoretical predictions of the adsorbed layer thickness for a diblock copolymer
Fig 16 Adsorbed amount (mg mminus2) versus fraction of anchor segment for an AndashBndashA
triblock copolymer (PEOndash
PPOndash
PEO)
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amounts to an increase in the osmotic pressure in the overlap region
as a result solvent will diffuse from thebulk to the overlap region thus
separating the particles and hence a strong repulsive energy arises
from this effect The above repulsive energy can be calculated by
considering the free energy of mixing of two polymer solutions as for
example treated by Flory and Krigbaum [22] The free energy of
mixing is given by two terms (i) An entropy term that depends on the
volume fraction of polymer and solvent (ii)An energy term that is
determined by the FloryndashHuggins interaction parameter χ
Using the above theory one can derive an expression for the free
energy of mixing of two polymer layers (assuming a uniform segment
density distribution in each layer) surrounding two spherical particles
as a function of the separation distance h between the particles The
expression for Gmix is
Gmix
kT =
2V 22V 1
m2
2
1
2minusχ
δminus
h
2
2
3R + 2δ + h
2
eth18THORN
k is the Boltzmann constant T is the absolute temperature V 2 is the
molar volume of polymer V 1 is the molar volume of solvent and ν 2 is
the number of polymer chains per unit area
The sign of Gmix depends on the value of the FloryndashHuggins
interaction parameter χ if χb05 Gmix is positive and the interaction
is repulsive if χN05 Gmix is negative and the interaction is attractiveif χ=05 Gmix =0 and this de1047297nes the θ-condition
52 Elastic interaction Gel
Thisarises fromthe lossin con1047297gurational entropy of thechains on
the approach of a second particle As a result of this approach the
volume available for the chains becomes restricted resulting in loss of
the number of con1047297gurations This can be illustrated by considering a
simple molecule represented by a rod that rotates freely in a
hemisphere across a surface (Fig 19) When the two surfaces are
separated byan in1047297nitedistanceinfin thenumber of con1047297gurations of the
rod is Ω(infin) which is proportional to the volume of the hemisphere
When a secondparticleapproaches to a distance h suchthatit cuts the
hemisphere (loosing some volume) the volume available to the chains
is reduced and the number of con1047297gurations become Ω(h) which is
less than Ω(infin) For two 1047298at plates Gel is given by the following
expression
Gel
kT = 2m2 ln
X heth THORN
X infineth THORN
= 2m2Rel heth THORN eth19THORN
where Rel
(h) is a geometric function whose form depends on the
segment density distribution It should be stressed that Gel is always
positive and could play a major role in steric stabilisation It becomes
very strong when the separation distance between the particles
becomes comparable to the adsorbed layer thickness δ
Combination of Gmix and Gel with GA (the van der Waals attractive
energy)gives the total free energy of interaction GT (assuming there is
no contribution from any residual electrostatic interaction) ie
GT = Gmix + Gel + GA eth20THORN
A schematic representation of the variation of Gmix Gel GA and GT
with surfacendashsurface separation distance h is shown in Fig 20
Gmix increases very sharply with decrease of h when hb2δ Gel
increases very sharply with decrease of h when hbδ GT versus h
shows a minimum Gmin at separation distances comparable to 2δ
When h b2δ GT shows a rapid increase with decrease in h
The depth of the minimum depends on the Hamaker constant A
the particle radius R and adsorbed layer thickness δ Gmin increases
with increase of A and R At a given A and R Gmin increases with
decrease in δ (ie with decrease of the molecular weight M w of the
stabiliser This is illustrated in Fig 21 which shows the energyndash
distance curves as a function of δR The larger the value of δR the
smaller the value of Gmin In this case the system may approach
thermodynamic stability as is the case with nano-dispersions
6 Emulsions stabilized by polymeric surfactants
The most effective method for emulsion stabilization is to use
polymeric surfactants that stronglyadsorb at the OWor WO interface
Fig 17 Schematic representation of the interaction between particles containing adsorbed polymer layers
Fig 18 Schematic representation of polymer layer overlap
Fig 19 Schematic representation of con1047297gurational entropy loss on approach of a
second particle
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and produce effective steric stabilization against strong 1047298occulation
coalescence and Ostwald ripening [23]
As mentioned above a graft copolymer of the ABn type was
synthesized by grafting several alkyl groups on an inulin (polyfruc-
tose) chain The polymeric surfactant (INUTECregSP1) consists of a
linear polyfructose chain (the stabilizing A chain) and several alkyl
groups (the B chains) that provide multi-anchor attachment to the oil
droplets This polymeric surfactant produces enhanced steric stabili-
zation both in water and high electrolyte concentrations as will be
discussed later
For water-in-oil (WO) emulsions an AndashBndashA block copolymer of
poly (12-hydroxystearic acid) (PHS) (the A chains) and poly (ethylene
oxide) (PEO) (the B chain) PHSndashPEOndashPHS is commercially available
(Arlacel P135 UNIQEMA) The PEO chain (that is soluble in the water
droplets) forms the anchor chain whereas the PHS chains form the
stabilizing chains PHS is highly soluble in most hydrocarbon solvents
and is strongly solvated by its molecules The structure of the PHSndash
PEOndashPHS block copolymer is schematically shown in Fig 22The conformation of the polymeric surfactant at the WO interface
is schematically shown in Fig 23
Emulsions of Isopar Mwater and cyclomethiconewater were
prepared using INUTECregSP1 5050 (vv) OW emulsions were
prepared and the emulsi1047297er concentration was varied from 025 to 2
(wv) based on the oil phase 05 (wv) emulsi1047297erwas suf 1047297cient for
stabilization of these 5050 (vv) emulsions [23]
The emulsions were stored at room temperature and 50 degC and
optical micrographs were taken at intervals of time (for a year) in
order to check the stability Emulsions prepared in water were very
stableshowingno change in droplet size distributionover more than a
year period and this indicated absence of coalescence Any weak
1047298occulation that occurred was reversible and the emulsion could be
redispersed by gentle shaking Fig 24 shows an optical micrograph fora dilute 5050 (vv) emulsion that was stored for 15 and 14 weeks at
50 degC
No change in droplet size was observed after storage for more than
1 year at 50 degC indicating absence of coalescence The same result was
obtained when using different oils Emulsions were also stable against
coalescence in the presence of high electrolyte concentrations (up to
4 mol dmminus3 or ~25 NaCl
The above stability in high electrolyte concentrations is not
observed with polymeric surfactants based on polethylene oxide
The high stability observed using INUTECregSP1 is related to its
strong hydration both in water and in electrolyte solutions The
hydration of inulin (the backbone of HMI) could be assessed using
cloud point measurements A comparison was also made with PEO
with two molecular weights namely 4000 and 20000
Solutionsof PEO 4000 and 20000 showeda systematic decrease of
cloud point with increase in NaCl or MgSO4 concentration In contrast
inulin showed no cloud point up to 4 mol dmminus3 NaCl and up to 1 mol
dmminus3 MgSO4
The above results explain the difference between PEO and inulin
With PEO the chains show dehydration when the NaCl concentration
is increased above 2 mol dmminus3 or 05 mol dmminus3 MgSO4 The inulin
chains remain hydrated at much higher electrolyte concentrations It
seems that the linear polyfructose chains remain strongly hydrated athigh temperature and high electrolyte concentrations
The high emulsion stability obtained whenusingINUTECregSP1 can be
accounted for by the following factors (i) The multi-point attachment
of the polymer by several alkyl chains that are grafted on the backbone
(ii) The strong hydration of the polyfructose ldquoloopsrdquo both in water and
high electrolyte concentrations (χ remains below 05 under these
conditions) (iii) Thehigh volumefraction (concentration) of the loops at
the interface (iv) Enhanced steric stabilization this is the case with
multi-point attachment which produces strong elastic interaction
Evidence for the high stability of the liquid 1047297lm between emulsion
droplets when using INUTECregSP1 was obtained by Exerowa et al [24]
using disjoining pressure measurements This is illustrated in Fig 25
which shows a plot of disjoining pressure versus separation distance
between two emulsion droplets at various electrolyte concentrations
The results show that by increasing the capillary pressure a stable
Newton Black Film (NBF) is obtained at a 1047297lm thickness of sim7 nm
The lack of rupture of the 1047297lm at the highest pressure applied of
45times104 Pa indicate the high stability of the 1047297lm in water and in high
electrolyte concentrations (up to 20 mol dmminus3 NaCl)
The lack of rupture of the NBF up to the highest pressure applied
namely 45times104 Pa clearly indicatesthe high stability of the liquid1047297lm
in the presence of high NaCl concentrations (up to 2 mol dm minus3) This
result is consistent with the high emulsion stability obtained at high
electrolyte concentrations and high temperature Emulsions of Isopar
M-in-water are very stable under such conditions and this could be
accounted for by the high stability of the NBF The droplet size of
5050 OW emulsions prepared using 2 INUTEC regSP1 is in the region
of 1ndash10 μ m This corresponds to a capillary pressure of ~3times104 Pa for
the 1 μ m drops and ~3times103 Pa for the 10 μ m drops These capillarypressures are lower than those to which the NBF have been sub-
jected to and this clearly indicates the high stability obtained against
coalescence in these emulsions
61 WO emulsions stabilized with PHS ndashPEOndashPHS block copolymer
WO emulsions (the oil being Isopar M) were prepared using PHSndash
PEOndashPHS block copolymer at high water volume fractions (N07) The
emulsions have a narrow droplet size distribution with a z -average
radius of 183 nm [25] They also remained 1047298uid up to high water
volume fractions (N06) This could be illustrated from viscosityndash
volume fraction curves as is shown in Fig 26
Fig 20 Energyndashdistance curves for sterically stabilized systems
Fig 21 Variation of Gmin with δR
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The effective volume fraction ϕeff of the emulsions (the core
droplets plus the adsorbed layer) could be calculated from the relative
viscosity and using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth21THORN
Where η r is the relative viscosity ϕp is the maximum packing fraction(sim07) and [ η ] is the intrinsic viscosity that is equal to 25 for hard-
spheres
The calculations based on Eq (22) are shown in Fig 27 (square
symbols) From the effective volume fraction ϕeff and the core volume
fraction ϕ the adsorbed layer thickness could be calculated This was
found to be in the region of 10 nm at ϕ =04 and it decreased with
increase in ϕ
The WO emulsions prepared using the PHSndashPEOndashPHS block
copolymer remained stable both at room temperature and 50 degC
This is consistent with the structure of the block copolymer the B
chain (PEO) is soluble in water and it forms a very strong anchor at the
WO interface The PHS chains (the A chains) provide effective steric
stabilization since the chains are highly soluble in Isopar M and are
strongly solvated by its molecules
7 Suspensions stabilised using polymeric surfactants
There are generally two procedures for preparation of solidliquid
dispersions
(i) Condensation methods build-up of particles from mole-
cular units ie nucleation and growth A special procedure
is the preparation of latexes by emulsion or dispersion
polymerization
(ii) Dispersion methods in this case one starts with preformed
large particles or crystals which are dispersed in the liquid by
using a surfactant (wetting agent) with subsequent breaking up
of the large particles by milling (comminution) to achieve the
desirable particle size distribution A dispersing agent (usuallya
polymeric surfactant) is used for the dispersion process and
subsequent stabilization of the resulting suspension
There are generally two procedures for preparation of latexes
(i) Emulsion polymerization the monomers that are essentially
insoluble in the aqueous medium are emulsi1047297ed using a
surfactant and an initiator is added while heating the system
to produce the polymer particles that are stabilized electro-
statically (when using ionic surfactants) or sterically (when
using non-ionic surfactants)
(ii) Dispersion polymerization the monomers are dissolved in a
solvent in which the resulting polymer particlesare insoluble A
protective colloid (normally a block or graft copolymer) is
added to prevent 1047298occulation of the resulting polymers
particles that are produced on addition of an initiator This
method is usually applied for the production of non-aqueous
latex dispersions and is sometimes referred to as Non-Aqueous
Dispersion Polymerization (NAD)
Surfactants play a crucial role in the process of latex preparation
since they determine the stabilizing ef 1047297ciency and the effectiveness of
the surface active agent ultimately determines the number of particles
Fig 22 Schematic representation of the structure of PHSndashPEOndashPHS block copolymer
Fig 23 Conformation of PHSndash
PEOndash
PHS polymeric surfactant at the WO interface
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and their size The effectiveness of any surface active agent in
stabilizing the particles is the dominant factor and the number of
micelles formed is relatively unimportant In the NAD process themonomer normally an acrylic is dissolved in a non-aqueous solvent
normally an aliphatic hydrocarbon and an oil soluble initiator and a
stabilizer (to protect the resulting particles from1047298occulation) is added
to the reaction mixture The most successful stabilizers used in NAD
are block and graft copolymers Preformed graft stabilizers based
on poly(12-hydroxy stearic acid) (PHS) are simple to prepare and
effective in NAD polymerization
Dispersion methods are used for the preparation of suspensions of
preformed particles The role of surfactants (or polymers) in the
dispersion process can be analyzed in terms of the three processesinvolved (i) Wetting of the powder by the liquid (ii) Breaking of the
aggregates and agglomerates (iii) Comminution of the resulting
particles and their subsequent stabilization All these processes are
affected by surfactants or polymers which adsorb on the powder
surface thus aiding the wetting of the powder break-up of the
aggregates and agglomerates and subsequent reduction of particle
size by wet milling
71 Polymeric surfactants in emulsion polymerization
Recently the graft copolymer of hydrophobically modi1047297ed inulin
(INUTECreg SP1) has been used in emulsion polymerization of styrene
methyl methacrylate butyl acrylate and several other monomers [26]All lattices were prepared by emulsion polymerisation using potas-
sium persulphate as initiator The z -average particle size was
determined by photon correlation spectroscopy (PCS) and electron
micrographs were also taken
Fig 24 Optical micrographs of OW emulsions stabilized with INUTEC regSP1 stored at
50 degC for 15 weeks (a) and 14 weeks (b)
Fig 25 Variation of disjoining pressure with equivalent 1047297lm thickness at various NaCl concentrations
Fig 26 Viscosityndashvolume fraction for WO emulsion stabilized with PHSndashPEOndashPHS
block copolymer experimental data calculated using Eq (22)
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Emulsion polymerization of styrene or methylmethacrylate showed
an optimum weight ratio of (INUTEC)Monomer of 00033 for PS and
0001 for PMMA particles The (initiator)(Monomer) ratio was
kept constant at 000125 The monomer conversion was higher than
85 in all cases Latex dispersions of PS reaching 50 and of PMMA
reaching 40 could be obtained using such low concentration of
INUTECregSP1 Fig 27 shows the variation of particle diameter with
monomer concentration
The stability of the latexes was determined by determining the
critical coagulation concentration (CCC) using CaCl2 The CCC was low
(00175ndash005 mol dmminus3) but this was higher than that for the latex
prepared without surfactant Post addition of INUTEC regSP1 resulted in
a large increase in the CCC as is illustrated in Fig 28 which shows log
W minus log C curves (where W is the ratio between the fast 1047298occulation
rate constant to the slow 1047298occulation rate constant referred to as the
stability ratio) at various additions of INUTECregSP1
As with the emulsions the high stability of the latex when using
INUTECregSP1 is due to the strong adsorption of the polymeric
surfactant on the latex particles and formation of strongly hydrated
loops and tails of polyfructose that provide effective steric stabiliza-
tion Evidence for the strong repulsion produced when using
INUTECregSP1 was obtained from Atomic Force Microscopy investiga-
tions [27] whereby the force between hydrophobic glass spheres and
hydrophobic glass plate both containing an adsorbed layer of
INUTECregSP1 was measured as a function of distance of separation
both in water and in the presence of various Na2SO4 concentrations
The results are shown in Figs 29 and 30
Fig 27 Electron micrographs of the latexes
Fig 28 In1047298uence of post addition of INUTECreg
SP1 on the latex stability
Fig 29 Forcendashdistance curves between hydrophobised glass surfaces containing
adsorbed INUTECreg
SP1 in water
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711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
295T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
297T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1919
With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 319
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 419
Clearly when χb12 B2 is positive and mixing is non-ideal leading to
positive deviation (repulsion) this occurs when the polymer chains
are in ldquogoodrdquo solvent conditions In contrast when χN12 B2 is
negative and mixing is non-ideal leading to negative deviation
(attraction) this occurs when the polymer chains are in ldquopoorrdquo
solvent conditions (precipitation of the polymer may occur under
these conditions) Since the polymer solvency depends on tempera-
ture one can also de1047297ne a theta temperature θ at which χ=12
The function [(12)minus
χ] can also be expressed in terms of twomixing parameters an enthalpy parameter κ 1 and an entropy
parameter ψ1 ie
1
2minusχ
= κ 1minusψ1 eth8THORN
The θ-temperature can also be de1047297ned in terms of κ 1 and ψ1
θ = κ 1T
ψ1
eth9THORN
Alternatively one can write
1
2
minusχ = ψ1 1minusθ
T eth10THORN
Although the FloryndashHuggins theory is sound in principle several
experimental results cannot be accounted for For example it was
found that the χ parameter depends on the polymer concentration in
solution Most serious is the fact that many polymer solutions (such as
PEO) show phase separation on heating when the theory predictsthat
it should happen only on cooling
The solution properties of copolymers are much more complicated
This is due to the fact that the two copolymer components A and B
behave differently in different solvents Only when the two compo-
nents are both soluble in the same solvent then they exhibit similar
solution properties This is the case for example for a non-polar
copolymer in a non-polar solvent It should also be emphasised that
the FloryndashHuggins theory was developed for ideal linear polymers
Indeed with branched polymers consisting of high monomer density
(eg star branched polymers) the θ-temperature depends on the
lengthof the armsand isin general lower thanthatof a linearpolymer
with the same molecular weight
Another complication arises from speci1047297c interaction with the
solvent eg hydrogen bonding between polymer and solvent
molecules (eg with PEO and PVA in water) Also aggregation insolution (lack of complete dissolution) may present another problem
One of the most useful parameters for characterising the
conformation of a polymer in solution is the root mean-square (rms)
end to end length br 2N12 which represents a con1047297guration character r
as the distance from one end group to the other of a chain molecule
Another useful parameter is the radius of gyration bs2N
12 which is a
measure of the effective size of a polymer molecule (it is the root
mean-square distance of the elements of the chain from its centre of
gravity)
For linear polymers
bs2N
1=2 = br 2N1=2
61=2 eth11THORN
The radius of gyration of a polymer in solution can be determined
from light scattering measurements
As mentioned above dilute solutions of copolymers is solvents that
are good for both components exhibit similar behaviour to hompo-
lymer chains However in a selective solvent whereby the medium is
a good solvent for one component say A and a poor solvent for the
second component B the very different solvent af 1047297nities to the two
components will have a large effect on the conformation of the
isolated chain This results in formation of aggregates involving
several macromolecules in dilute solutions (low Critical Aggregation
Concentration CAC) It is believed that the polymeric aggregates are
Fig 3 Various conformations of macromolecules on a plane surface
284 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 519
spherical [5] The CAC of these block and graft copolymers is usually
very low
Several methods may be applied to obtain the aggregate size and
shape of block and graft copolymers of which light scattering small
angle X-ray and neutron scattering are probably the most direct
Dynamic light scattering (photon correlation spectroscopy) can also
be applied to obtain the hydrodynamic radius of the aggregate This
technique is relatively easy to perform when compared with static
light scattering since it does not require special preparation of thesample
13 Adsorption and conformation of polymeric surfactants at interfaces
Understanding the adsorption and conformation of polymeric
surfactants at interfaces is key to knowing how these molecules act as
stabilizers Most basic ideas on adsorption and conformation of
polymers have been developed for the solidliquid interface [10ndash17]
The process of polymer adsorption is fairly complicated mdash it
involves PolymerSurface Interaction PolymerSolvent Interaction
SurfaceSolvent Interaction The con1047297guration (conformation) of the
polymer at the solidliquid interface The polymersurface interaction
is described in terms of adsorption energy per segment χs The
polymersolvent interaction is described in terms of the Floryndash
Huggins interaction parameter χ The polymer con1047297guration is
described by sequences of Trains segments in direct contact with
the surface Loops segments in between the trains that extend into
solution Tails ends of the molecules that also extend into solution A
schematic representation of the various polymer con1047297gurations is
given in Fig 3
For homopolymers eg poly(ethylene oxide) (PEO) or polyvinyl
pyrrolidone (PVP) a train-loop-tail con1047297guration is the case For
adsorption to occur a minimum energy of adsorption per segment χs
is required When a polymer molecule adsorbs on a surface it looses
con1047297gurational entropy and this must be compensated by an
adsorption energy χs per segment This is schematically shown in
Fig 4 where the adsorbed amount Γ is plotted versus χs The
minimum value of χs can be very small (b01kT ) since a large number
of segments per molecule are adsorbed For a polymer with say 100segments and 10 of these are in trains the adsorption energy per
molecule now reaches 1kT (with χs =01kT ) For 1000 segments the
adsorption energy per molecule is now 10kT
Homopolymers are not the most suitable for stabilisation of
dispersions For strong adsorption one needs the molecule to be
ldquoinsolublerdquo in the medium and has strong af 1047297nity (ldquoanchoringrdquo) to the
surface For stabilisation one needs the molecule to be highly soluble
in themedium andstronglysolvated by itsmolecules mdash This requires a
FloryndashHuggins interaction parameter less than 05 The above
opposing effects can be resolved by introducing ldquoshortrdquo blocks in the
moleculewhichare insolublein themedium andhavea strong af 1047297nity
to the surface as illustrated in Fig 3b Example Partially hydrolysed
polyvinyl acetate (88 hydrolysed ie with 12 acetate groups)
usually referred to as polyvinyl alcohol (PVA)
The above requirements are better satis1047297ed using AndashB AndashBndashA and
BAn graft copolymers B is chosento be highly insolublein themedium
and it should have high af 1047297nity to the surface This is essential to
ensure strong ldquoanchoringrdquo to the surface (irreversible adsorption) A is
chosen to be highly soluble in the medium and strongly solvated by
its molecules The FloryndashHuggins χ parameter can be applied in this
case For a polymer in a good solvent χ has to be lower than 05 the
smaller the χ value the better the solvent for the polymer chains
Examples of B for a hydrophobic particles in aqueous media are
polystyrene polymethylmethacrylate Examples of A in aqueous
media are polyethylene oxide polyacrylic acid polyvinyl pyrollidone
and polysaccharides For non-aqueous media such as hydrocarbons
the A chain(s) could be poly(12-hydroxystearic acid)
For full description of polymer adsorption one needs to obtain
information on the following (i) The amount of polymer adsorbed Γ (in mg or moles) per unit area of the particles It is essential to know
thesurface area of theparticles in thesuspensionNitrogenadsorption
on the powder surface may give such information (by application of
the BET equation) provided there will be no change in area on
dispersing the particles in the medium For many practical systems a
change in surface area may occur on dispersing the powder in which
case one has to use dye adsorption to measure the surface area (some
assumptions have to be made in this case) (ii)The fraction of segments
in direct contact with thesurface ie the fraction of segmentsin trains
p ( p = (Number of segments in direct contact with the surface) Total
Number) (iii)The distribution of segments in loops and tails ρ(z)
which extend in several layersfrom the surface ρ(z) is usually dif 1047297cult
to obtain experimentally although recently application of small angle
neutron scattering could obtain such information An alternative anduseful parameter for assessing ldquosteric stabilisationrdquo is the hydro-
dynamic thickness δh (thickness of the adsorbed or grafted polymer
layer plus any contribution from the hydration layer) Several methods
can be applied to measure δh as will be discussed below
2 Theories of polymer adsorption
Two main approaches have been developed to treat the problem of
polymer adsorption (i) Random Walk approach This is based on
Florys treatment of the polymer chain in solution the surface was
considered as a re1047298ecting barrier (ii) Statistical mechanical approach
Thepolymer con1047297gurationwas treated as being made of three types of
structures trains loops and tails each having a different energy state
The random walk approach is an unrealistic model to the problemof polymer adsorption since the polymer interacts in a speci1047297c
manner with the surface and the solvent The statistical mechanical
approach is a more realistic model for the problem of polymer
adsorption since it takes into account the various interactions
involved A useful model for treating polymer adsorption and
con1047297guration was suggested by Scheutjens and Fleer (SF theory)
[14ndash16] that is referred to as the step weighted random walk approach
which is summarized below
The SF theory starts without any assumption for the segment
density distribution The partition functions were derived for the
mixture of free and adsorbed polymer molecules as well as for the
solvent molecules All chain conformations were described as step
weighted random walks on a quasi-crystalline lattice which extends in
parallel layers from the surface mdash
this is schematically shown in Fig 5Fig 4 Variation of adsorption amount Γ with adsorption energy per segment χs
285T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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The partition function is written in terms of a number of con1047297gura-
tions These were treated as connectedsequencesof segments In each
layer random mixing between segments and solvent molecules was
assumed (mean 1047297eld approximation) Each step in the random walk
was assigned a weighting factor pi
pi consists of three contributions (i) An adsorption energyχs (which
exists only for the segments that are near the surface) (ii) Con1047297gura-
tional entropy of mixing (that exists in each layer) (iii) Segment-solvent
interaction parameterχ (the FloryndashHuggins interaction parameter note
that χ=0 for an athermal solvent χ=05 for a θ-solvent)
From the weighting factors the statistical weight of any chain
conformation can be calculated for a given segment concentration
pro1047297le using a matrix formulation Fig 6 shows typical adsorption
isotherms plotted as surface coverage (in equivalent monolayers)
versus polymer volume fraction ϕ in bulk solution (ϕ was taken to
vary between 0 and 10minus3 which is the normal experimental range)
The results in Fig 6 show the effect of increasing the chain length r
and effect of solvency (athermal solvent with χ=0 and theta solvent
with χ=05) The adsorption energy χs was taken to be the same and
equal to 1kT When r = 1 θ is very small and the adsorption increases linearly
with increase of ϕ (Henrys type isotherm) On the other hand when
r = 10 the isotherm deviates much from a straight line and approaches
a Langmuirian type However when r ge20 high af 1047297nity isotherms are
obtained This implies that the 1047297rst added polymer chains are
completely adsorbed resulting in extremely low polymer concentra-
tion in solution (approaching zero) This explains the irreversibility of
adsorption of polymeric surfactants with r N100 The adsorption
isotherms with r =100 and above are typical of those observed
experimentally for most polymers that are not too polydisperse ie
showing a steep rise followed by a nearly horizontal plateau (which
only increases few percent per decade increase of ϕ) In these dilute
solutions the effect of solvency is most clearly seen with poor
solvents giving the highest adsorbed amounts In good solvents θ is
much smaller and levels off for long chains to attain an adsorption
plateau which is essentially independent of molecular weight
Another point that emerges from the SF theory is the difference in
shape between the experimental and theoretical adsorption isotherms
in thelow concentration region Theexperimentalisotherms areusuallyrounded whereas those predicted fromtheory are1047298at This is accounted
for in terms of the molecular weight distribution (polydispersity) which
is encountered with most practical polymers This effect has been
explained by CohenndashStuart et al [17] With polydisperse polymers the
larger molecular weight fractions adsorb preferentially over the smaller
ones At low polymer concentrations nearly all polymer fractions are
adsorbed leaving a small fraction of the polymer with the lowest
molecular weights in solution As the polymer concentration is
increased the higher molecular weight fractions displace the lower
ones on thesurfacewhichare nowreleasedin solution thusshiftingthe
molecular weight distribution in solution to lower values This process
continues with further increase in polymer concentration leading to
fractionation whereby the higher molecular weight fractions are
adsorbed at the expense of the lower fractions which are released to
the bulk solution However in very concentrated solutions monomers
adsorb preferentially with respect to polymers and short chains with
respect to larger ones This is due to the fact that in this region the
conformational entropy term predominates the free energy disfavour-
ing the adsorption of long chains
According to the SF theory the bound fraction p is high at low
concentration and relatively independent of molecular weight when
r N20 However with increase in surface coverage andor molecular
weight p tends to decrease indicating the formation of larger loops
and tails
The structure of the adsorbed layer is described in terms of the
segment density distribution As an illustration Fig 7 shows some
calculations using the SF theory for loops and tails with r =1000
ϕ= 10minus6 and χ=05 In this example 38 of the segments are in
trains 555 in loops and 65 in tails This theory demonstrates theimportance of tails which dominate the total distribution in the outer
region
3 Experimental techniques for studying polymeric surfactant
adsorption
As mentioned above for full characterization of polymeric sur-
factant adsorption one needs to determine three parameters (i) The
adsorbed amount Γ (mg mminus2 or mol mminus2) as a function of equilibrium
concentration C eq ie the adsorption isotherm (ii) The fraction of
segments in direct contact with the surface p (number of segments in
Fig 5 Schematic representation of a polymer molecule adsorbing on a 1047298at surface mdash
quasi-crystalline lattice with segments 1047297lling layers that are parallel to the surface
(random mixing of segments and solvent molecules in each layer is assumed)
Fig 6 Adsorption isotherms for oligomers and polymers in the dilute region based on
the SF theory Fig 7 Loop tail and total segment pro1047297le according to the SF theory
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The microelectrophoresis technique is based on measurement of
the electrophoretic mobility u of the particles in the presence and
absence of the polymer layer From u one can calculate the zeta
potential ζ using the Huckel equation (which is applicable for small
particles and extended double layers ie κ Rbb1 where κ is the
DebyendashHuckel parameter that is related to the salt concentration
u = 23eeon η
eth15THORN
where ε is the relative permittivity of the medium and ε o is the
permittivity of free space
By measuring ζ of the particles with and without the adsorbed
polymer layer one can obtain the hydrodynamic thickness δh For
accurate measurements one should carry the measurements at various
electrolyte concentrations and extrapolate the results to the plateau
value Several automatic instruments are available for measurement of
the electrophoretic mobility Malvern zeta sizerndashCoulter Delsa sizer and
Broekahven instrument All these instruments are easy to use and the
measurement can be carried out within few minutes
4 Examples of the adsorption isotherms of nonionic polymericsurfactants
Fig 9 shows the adsorption isotherms for PEO with different
molecular weights on PS (at room temperature It can be seen that the
amount adsorbed in mgm-2 increases with increase in the polymer
molecular weight Fig 10 shows the variation of the hydrodynamic
thickness δh with molecular weight M δh shows a linear increase with
log M δh increases with n the number of segments in the chain
according to
δhasympn08 eth16THORN
Fig 11 shows the adsorption isotherms of PVA with various
molecular weights on PS latex (at 25 degC) [19] The polymers were
obtained by fractionation of a commercial sample of PVA with an
average molecular weight of 45000 The polymer also contained 12
vinyl acetate groups As with PEO the amount of adsorption increases
with increase in M The isotherms are also of the high af 1047297nity type Γ at the plateau increases linearly with M 12
The hydrodynamic thickness was determined using PCS and theresults are given below
M 67000 43000 28000 17000 8000
δhnm 255 197 140 98 33
δh seems to increase linearly with increase in the molecular weight
The effect of solvency on adsorption was investigated by increasing
the temperature (the PVA molecules are less soluble at higher
temperature) or addition of electrolyte (KCl) [20] The results are
shown in Figs 12 and 13 for M =65100 As can be seen from Fig 12
increase of temperature results in reduction of solvency of the
mediumfor the chain (due to break down of hydrogen bonds) and this
results in an increase in the amount adsorbed Addition of KCl (which
reduces the solvency of the medium for the chain) results in anincrease in adsorption (as predicted by theory)
The adsorption of block and graft copolymers is more complex
since the intimate structure of the chain determines the extent of
adsorption [18] Randomcopolymers adsorbin an intermediate way to
that of the corresponding homopolymers Blockcopolymers retain the
adsorption preference of the individual blocks The hydrophilic block
(eg PEO) the buoy (previously referred to as the A chain) extends
away from the particle surface into the bulk solution whereas the
hydrophobic anchor block (previously referred to as the B chain) (eg
PS or PPO) provides 1047297rm attachment to the surface Fig 14 shows the
theoretical prediction of diblock copolymer adsorption according to
Fig 9 Adsorption isotherms for PEO on PS
Fig 10 Hydrodynamic thickness of PEO on PS as a function of the molecular weight
Fig 11 Adsorption isotherms of PVA with different molecular weights on polystyrene
latex at 25 degC
Fig 12 In1047298uence of temperature on adsorption
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the Scheutjens and Fleer theory The surface density σ is plotted
versus the fraction of anchor segmentsν A The adsorption depends on
the anchorbuoy composition
The amount of adsorption is higher than for homopolymers and
the adsorbed layer thickness is more extended and dense Fig 15
shows the theoretical prediction for the adsorbed layer thickness δwhich is plotted as a function of ν A For a triblock copolymer AndashBndashA
with two buoy chains (A) and one anchor chain (B) the behaviour is
similar to that of diblock copolymers This is shown in Fig16 for PEOndash
PPOndashPEO block (Pluronic)
5 Interaction between particles containing adsorbed polymeric
surfactant layers steric stabilization
When two particles each with a radius R and containing an
adsorbed polymer layer with a hydrodynamic thickness δh approach
each other to a surfacendashsurface separation distance h that is smaller
than 2 δh the polymer layers interact with each other resulting in two
main situations [21] (i) The polymer chains may overlap with each
other (ii) The polymer layer may undergo some compression In both
cases there will be an increase in the local segment density of the
polymer chains in the interaction region This is schematicallyillustrated in Fig 17 The real situation is perhaps in between the
above two cases ie the polymer chains may undergo some
interpenetration and some compression
Provided the dangling chains (the A chains in AndashB AndashBndashA block or
BAn graft copolymers) are in a good solvent this local increase in
segment density in the interaction zone will result in strong repulsion
as a result of two main effects (i) Increase in the osmotic pressure in
the overlap region as a result of the unfavourable mixing of the
polymer chains when these are in good solvent conditions This is
referred to as osmotic repulsion or mixing interaction and it is
described by a free energy of interaction Gmix (ii) Reduction of the
con1047297gurational entropy of the chains in the interaction zone this
entropy reduction results from the decrease in the volume available
for thechains when these areeither overlapped or compressed This is
referred to as volume restriction interaction entropic or elastic
interaction and it is described by a free energy of interaction Gel
Combination of Gmix and Gel is usually referred to as the steric
interaction free energy Gs ie
Gs = Gmix + Gel eth17THORN
The sign of Gmix depends on the solvency of the medium for the
chains If in a good solvent ie the FloryndashHuggins interaction
parameter χ is less than 05 then Gmix is positive and the mixing
interaction leads to repulsion (see below) In contrast if χN05 (ie the
chains are in a poor solvent condition) Gmix is negative and the
mixing interaction becomes attractive Gel is always positive and
hence in some cases one can produce stable dispersions in a relatively
poor solvent (enhanced steric stabilisation)
51 Mixing interaction Gmix
This results from the unfavourable mixing of the polymer chainswhen these are in a good solvent conditions This is schematically
shown in Fig 18
Consider two spherical particles with the same radius and each
containing an adsorbed polymer layer with thickness δ Before
overlap one can de1047297ne in each polymer layer a chemical potential
for the solvent μ iα and a volume fraction for the polymer in the
layerϕ2 In the overlap region (volume element dV ) the chemical
potential of the solvent is reduced to μ i β This results from the increase
in polymer segment concentration in this overlap region
In the overlap region the chemical potential of the polymer chains
is now higher than in the rest of the layer (with no overlap) This
Fig 13 In1047298uence of addition of KCl on adsorption
Fig 14 Prediction of Adsorption of diblock copolymer
Fig15 Theoretical predictions of the adsorbed layer thickness for a diblock copolymer
Fig 16 Adsorbed amount (mg mminus2) versus fraction of anchor segment for an AndashBndashA
triblock copolymer (PEOndash
PPOndash
PEO)
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amounts to an increase in the osmotic pressure in the overlap region
as a result solvent will diffuse from thebulk to the overlap region thus
separating the particles and hence a strong repulsive energy arises
from this effect The above repulsive energy can be calculated by
considering the free energy of mixing of two polymer solutions as for
example treated by Flory and Krigbaum [22] The free energy of
mixing is given by two terms (i) An entropy term that depends on the
volume fraction of polymer and solvent (ii)An energy term that is
determined by the FloryndashHuggins interaction parameter χ
Using the above theory one can derive an expression for the free
energy of mixing of two polymer layers (assuming a uniform segment
density distribution in each layer) surrounding two spherical particles
as a function of the separation distance h between the particles The
expression for Gmix is
Gmix
kT =
2V 22V 1
m2
2
1
2minusχ
δminus
h
2
2
3R + 2δ + h
2
eth18THORN
k is the Boltzmann constant T is the absolute temperature V 2 is the
molar volume of polymer V 1 is the molar volume of solvent and ν 2 is
the number of polymer chains per unit area
The sign of Gmix depends on the value of the FloryndashHuggins
interaction parameter χ if χb05 Gmix is positive and the interaction
is repulsive if χN05 Gmix is negative and the interaction is attractiveif χ=05 Gmix =0 and this de1047297nes the θ-condition
52 Elastic interaction Gel
Thisarises fromthe lossin con1047297gurational entropy of thechains on
the approach of a second particle As a result of this approach the
volume available for the chains becomes restricted resulting in loss of
the number of con1047297gurations This can be illustrated by considering a
simple molecule represented by a rod that rotates freely in a
hemisphere across a surface (Fig 19) When the two surfaces are
separated byan in1047297nitedistanceinfin thenumber of con1047297gurations of the
rod is Ω(infin) which is proportional to the volume of the hemisphere
When a secondparticleapproaches to a distance h suchthatit cuts the
hemisphere (loosing some volume) the volume available to the chains
is reduced and the number of con1047297gurations become Ω(h) which is
less than Ω(infin) For two 1047298at plates Gel is given by the following
expression
Gel
kT = 2m2 ln
X heth THORN
X infineth THORN
= 2m2Rel heth THORN eth19THORN
where Rel
(h) is a geometric function whose form depends on the
segment density distribution It should be stressed that Gel is always
positive and could play a major role in steric stabilisation It becomes
very strong when the separation distance between the particles
becomes comparable to the adsorbed layer thickness δ
Combination of Gmix and Gel with GA (the van der Waals attractive
energy)gives the total free energy of interaction GT (assuming there is
no contribution from any residual electrostatic interaction) ie
GT = Gmix + Gel + GA eth20THORN
A schematic representation of the variation of Gmix Gel GA and GT
with surfacendashsurface separation distance h is shown in Fig 20
Gmix increases very sharply with decrease of h when hb2δ Gel
increases very sharply with decrease of h when hbδ GT versus h
shows a minimum Gmin at separation distances comparable to 2δ
When h b2δ GT shows a rapid increase with decrease in h
The depth of the minimum depends on the Hamaker constant A
the particle radius R and adsorbed layer thickness δ Gmin increases
with increase of A and R At a given A and R Gmin increases with
decrease in δ (ie with decrease of the molecular weight M w of the
stabiliser This is illustrated in Fig 21 which shows the energyndash
distance curves as a function of δR The larger the value of δR the
smaller the value of Gmin In this case the system may approach
thermodynamic stability as is the case with nano-dispersions
6 Emulsions stabilized by polymeric surfactants
The most effective method for emulsion stabilization is to use
polymeric surfactants that stronglyadsorb at the OWor WO interface
Fig 17 Schematic representation of the interaction between particles containing adsorbed polymer layers
Fig 18 Schematic representation of polymer layer overlap
Fig 19 Schematic representation of con1047297gurational entropy loss on approach of a
second particle
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and produce effective steric stabilization against strong 1047298occulation
coalescence and Ostwald ripening [23]
As mentioned above a graft copolymer of the ABn type was
synthesized by grafting several alkyl groups on an inulin (polyfruc-
tose) chain The polymeric surfactant (INUTECregSP1) consists of a
linear polyfructose chain (the stabilizing A chain) and several alkyl
groups (the B chains) that provide multi-anchor attachment to the oil
droplets This polymeric surfactant produces enhanced steric stabili-
zation both in water and high electrolyte concentrations as will be
discussed later
For water-in-oil (WO) emulsions an AndashBndashA block copolymer of
poly (12-hydroxystearic acid) (PHS) (the A chains) and poly (ethylene
oxide) (PEO) (the B chain) PHSndashPEOndashPHS is commercially available
(Arlacel P135 UNIQEMA) The PEO chain (that is soluble in the water
droplets) forms the anchor chain whereas the PHS chains form the
stabilizing chains PHS is highly soluble in most hydrocarbon solvents
and is strongly solvated by its molecules The structure of the PHSndash
PEOndashPHS block copolymer is schematically shown in Fig 22The conformation of the polymeric surfactant at the WO interface
is schematically shown in Fig 23
Emulsions of Isopar Mwater and cyclomethiconewater were
prepared using INUTECregSP1 5050 (vv) OW emulsions were
prepared and the emulsi1047297er concentration was varied from 025 to 2
(wv) based on the oil phase 05 (wv) emulsi1047297erwas suf 1047297cient for
stabilization of these 5050 (vv) emulsions [23]
The emulsions were stored at room temperature and 50 degC and
optical micrographs were taken at intervals of time (for a year) in
order to check the stability Emulsions prepared in water were very
stableshowingno change in droplet size distributionover more than a
year period and this indicated absence of coalescence Any weak
1047298occulation that occurred was reversible and the emulsion could be
redispersed by gentle shaking Fig 24 shows an optical micrograph fora dilute 5050 (vv) emulsion that was stored for 15 and 14 weeks at
50 degC
No change in droplet size was observed after storage for more than
1 year at 50 degC indicating absence of coalescence The same result was
obtained when using different oils Emulsions were also stable against
coalescence in the presence of high electrolyte concentrations (up to
4 mol dmminus3 or ~25 NaCl
The above stability in high electrolyte concentrations is not
observed with polymeric surfactants based on polethylene oxide
The high stability observed using INUTECregSP1 is related to its
strong hydration both in water and in electrolyte solutions The
hydration of inulin (the backbone of HMI) could be assessed using
cloud point measurements A comparison was also made with PEO
with two molecular weights namely 4000 and 20000
Solutionsof PEO 4000 and 20000 showeda systematic decrease of
cloud point with increase in NaCl or MgSO4 concentration In contrast
inulin showed no cloud point up to 4 mol dmminus3 NaCl and up to 1 mol
dmminus3 MgSO4
The above results explain the difference between PEO and inulin
With PEO the chains show dehydration when the NaCl concentration
is increased above 2 mol dmminus3 or 05 mol dmminus3 MgSO4 The inulin
chains remain hydrated at much higher electrolyte concentrations It
seems that the linear polyfructose chains remain strongly hydrated athigh temperature and high electrolyte concentrations
The high emulsion stability obtained whenusingINUTECregSP1 can be
accounted for by the following factors (i) The multi-point attachment
of the polymer by several alkyl chains that are grafted on the backbone
(ii) The strong hydration of the polyfructose ldquoloopsrdquo both in water and
high electrolyte concentrations (χ remains below 05 under these
conditions) (iii) Thehigh volumefraction (concentration) of the loops at
the interface (iv) Enhanced steric stabilization this is the case with
multi-point attachment which produces strong elastic interaction
Evidence for the high stability of the liquid 1047297lm between emulsion
droplets when using INUTECregSP1 was obtained by Exerowa et al [24]
using disjoining pressure measurements This is illustrated in Fig 25
which shows a plot of disjoining pressure versus separation distance
between two emulsion droplets at various electrolyte concentrations
The results show that by increasing the capillary pressure a stable
Newton Black Film (NBF) is obtained at a 1047297lm thickness of sim7 nm
The lack of rupture of the 1047297lm at the highest pressure applied of
45times104 Pa indicate the high stability of the 1047297lm in water and in high
electrolyte concentrations (up to 20 mol dmminus3 NaCl)
The lack of rupture of the NBF up to the highest pressure applied
namely 45times104 Pa clearly indicatesthe high stability of the liquid1047297lm
in the presence of high NaCl concentrations (up to 2 mol dm minus3) This
result is consistent with the high emulsion stability obtained at high
electrolyte concentrations and high temperature Emulsions of Isopar
M-in-water are very stable under such conditions and this could be
accounted for by the high stability of the NBF The droplet size of
5050 OW emulsions prepared using 2 INUTEC regSP1 is in the region
of 1ndash10 μ m This corresponds to a capillary pressure of ~3times104 Pa for
the 1 μ m drops and ~3times103 Pa for the 10 μ m drops These capillarypressures are lower than those to which the NBF have been sub-
jected to and this clearly indicates the high stability obtained against
coalescence in these emulsions
61 WO emulsions stabilized with PHS ndashPEOndashPHS block copolymer
WO emulsions (the oil being Isopar M) were prepared using PHSndash
PEOndashPHS block copolymer at high water volume fractions (N07) The
emulsions have a narrow droplet size distribution with a z -average
radius of 183 nm [25] They also remained 1047298uid up to high water
volume fractions (N06) This could be illustrated from viscosityndash
volume fraction curves as is shown in Fig 26
Fig 20 Energyndashdistance curves for sterically stabilized systems
Fig 21 Variation of Gmin with δR
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The effective volume fraction ϕeff of the emulsions (the core
droplets plus the adsorbed layer) could be calculated from the relative
viscosity and using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth21THORN
Where η r is the relative viscosity ϕp is the maximum packing fraction(sim07) and [ η ] is the intrinsic viscosity that is equal to 25 for hard-
spheres
The calculations based on Eq (22) are shown in Fig 27 (square
symbols) From the effective volume fraction ϕeff and the core volume
fraction ϕ the adsorbed layer thickness could be calculated This was
found to be in the region of 10 nm at ϕ =04 and it decreased with
increase in ϕ
The WO emulsions prepared using the PHSndashPEOndashPHS block
copolymer remained stable both at room temperature and 50 degC
This is consistent with the structure of the block copolymer the B
chain (PEO) is soluble in water and it forms a very strong anchor at the
WO interface The PHS chains (the A chains) provide effective steric
stabilization since the chains are highly soluble in Isopar M and are
strongly solvated by its molecules
7 Suspensions stabilised using polymeric surfactants
There are generally two procedures for preparation of solidliquid
dispersions
(i) Condensation methods build-up of particles from mole-
cular units ie nucleation and growth A special procedure
is the preparation of latexes by emulsion or dispersion
polymerization
(ii) Dispersion methods in this case one starts with preformed
large particles or crystals which are dispersed in the liquid by
using a surfactant (wetting agent) with subsequent breaking up
of the large particles by milling (comminution) to achieve the
desirable particle size distribution A dispersing agent (usuallya
polymeric surfactant) is used for the dispersion process and
subsequent stabilization of the resulting suspension
There are generally two procedures for preparation of latexes
(i) Emulsion polymerization the monomers that are essentially
insoluble in the aqueous medium are emulsi1047297ed using a
surfactant and an initiator is added while heating the system
to produce the polymer particles that are stabilized electro-
statically (when using ionic surfactants) or sterically (when
using non-ionic surfactants)
(ii) Dispersion polymerization the monomers are dissolved in a
solvent in which the resulting polymer particlesare insoluble A
protective colloid (normally a block or graft copolymer) is
added to prevent 1047298occulation of the resulting polymers
particles that are produced on addition of an initiator This
method is usually applied for the production of non-aqueous
latex dispersions and is sometimes referred to as Non-Aqueous
Dispersion Polymerization (NAD)
Surfactants play a crucial role in the process of latex preparation
since they determine the stabilizing ef 1047297ciency and the effectiveness of
the surface active agent ultimately determines the number of particles
Fig 22 Schematic representation of the structure of PHSndashPEOndashPHS block copolymer
Fig 23 Conformation of PHSndash
PEOndash
PHS polymeric surfactant at the WO interface
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and their size The effectiveness of any surface active agent in
stabilizing the particles is the dominant factor and the number of
micelles formed is relatively unimportant In the NAD process themonomer normally an acrylic is dissolved in a non-aqueous solvent
normally an aliphatic hydrocarbon and an oil soluble initiator and a
stabilizer (to protect the resulting particles from1047298occulation) is added
to the reaction mixture The most successful stabilizers used in NAD
are block and graft copolymers Preformed graft stabilizers based
on poly(12-hydroxy stearic acid) (PHS) are simple to prepare and
effective in NAD polymerization
Dispersion methods are used for the preparation of suspensions of
preformed particles The role of surfactants (or polymers) in the
dispersion process can be analyzed in terms of the three processesinvolved (i) Wetting of the powder by the liquid (ii) Breaking of the
aggregates and agglomerates (iii) Comminution of the resulting
particles and their subsequent stabilization All these processes are
affected by surfactants or polymers which adsorb on the powder
surface thus aiding the wetting of the powder break-up of the
aggregates and agglomerates and subsequent reduction of particle
size by wet milling
71 Polymeric surfactants in emulsion polymerization
Recently the graft copolymer of hydrophobically modi1047297ed inulin
(INUTECreg SP1) has been used in emulsion polymerization of styrene
methyl methacrylate butyl acrylate and several other monomers [26]All lattices were prepared by emulsion polymerisation using potas-
sium persulphate as initiator The z -average particle size was
determined by photon correlation spectroscopy (PCS) and electron
micrographs were also taken
Fig 24 Optical micrographs of OW emulsions stabilized with INUTEC regSP1 stored at
50 degC for 15 weeks (a) and 14 weeks (b)
Fig 25 Variation of disjoining pressure with equivalent 1047297lm thickness at various NaCl concentrations
Fig 26 Viscosityndashvolume fraction for WO emulsion stabilized with PHSndashPEOndashPHS
block copolymer experimental data calculated using Eq (22)
293T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Emulsion polymerization of styrene or methylmethacrylate showed
an optimum weight ratio of (INUTEC)Monomer of 00033 for PS and
0001 for PMMA particles The (initiator)(Monomer) ratio was
kept constant at 000125 The monomer conversion was higher than
85 in all cases Latex dispersions of PS reaching 50 and of PMMA
reaching 40 could be obtained using such low concentration of
INUTECregSP1 Fig 27 shows the variation of particle diameter with
monomer concentration
The stability of the latexes was determined by determining the
critical coagulation concentration (CCC) using CaCl2 The CCC was low
(00175ndash005 mol dmminus3) but this was higher than that for the latex
prepared without surfactant Post addition of INUTEC regSP1 resulted in
a large increase in the CCC as is illustrated in Fig 28 which shows log
W minus log C curves (where W is the ratio between the fast 1047298occulation
rate constant to the slow 1047298occulation rate constant referred to as the
stability ratio) at various additions of INUTECregSP1
As with the emulsions the high stability of the latex when using
INUTECregSP1 is due to the strong adsorption of the polymeric
surfactant on the latex particles and formation of strongly hydrated
loops and tails of polyfructose that provide effective steric stabiliza-
tion Evidence for the strong repulsion produced when using
INUTECregSP1 was obtained from Atomic Force Microscopy investiga-
tions [27] whereby the force between hydrophobic glass spheres and
hydrophobic glass plate both containing an adsorbed layer of
INUTECregSP1 was measured as a function of distance of separation
both in water and in the presence of various Na2SO4 concentrations
The results are shown in Figs 29 and 30
Fig 27 Electron micrographs of the latexes
Fig 28 In1047298uence of post addition of INUTECreg
SP1 on the latex stability
Fig 29 Forcendashdistance curves between hydrophobised glass surfaces containing
adsorbed INUTECreg
SP1 in water
294 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
295T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
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radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 419
Clearly when χb12 B2 is positive and mixing is non-ideal leading to
positive deviation (repulsion) this occurs when the polymer chains
are in ldquogoodrdquo solvent conditions In contrast when χN12 B2 is
negative and mixing is non-ideal leading to negative deviation
(attraction) this occurs when the polymer chains are in ldquopoorrdquo
solvent conditions (precipitation of the polymer may occur under
these conditions) Since the polymer solvency depends on tempera-
ture one can also de1047297ne a theta temperature θ at which χ=12
The function [(12)minus
χ] can also be expressed in terms of twomixing parameters an enthalpy parameter κ 1 and an entropy
parameter ψ1 ie
1
2minusχ
= κ 1minusψ1 eth8THORN
The θ-temperature can also be de1047297ned in terms of κ 1 and ψ1
θ = κ 1T
ψ1
eth9THORN
Alternatively one can write
1
2
minusχ = ψ1 1minusθ
T eth10THORN
Although the FloryndashHuggins theory is sound in principle several
experimental results cannot be accounted for For example it was
found that the χ parameter depends on the polymer concentration in
solution Most serious is the fact that many polymer solutions (such as
PEO) show phase separation on heating when the theory predictsthat
it should happen only on cooling
The solution properties of copolymers are much more complicated
This is due to the fact that the two copolymer components A and B
behave differently in different solvents Only when the two compo-
nents are both soluble in the same solvent then they exhibit similar
solution properties This is the case for example for a non-polar
copolymer in a non-polar solvent It should also be emphasised that
the FloryndashHuggins theory was developed for ideal linear polymers
Indeed with branched polymers consisting of high monomer density
(eg star branched polymers) the θ-temperature depends on the
lengthof the armsand isin general lower thanthatof a linearpolymer
with the same molecular weight
Another complication arises from speci1047297c interaction with the
solvent eg hydrogen bonding between polymer and solvent
molecules (eg with PEO and PVA in water) Also aggregation insolution (lack of complete dissolution) may present another problem
One of the most useful parameters for characterising the
conformation of a polymer in solution is the root mean-square (rms)
end to end length br 2N12 which represents a con1047297guration character r
as the distance from one end group to the other of a chain molecule
Another useful parameter is the radius of gyration bs2N
12 which is a
measure of the effective size of a polymer molecule (it is the root
mean-square distance of the elements of the chain from its centre of
gravity)
For linear polymers
bs2N
1=2 = br 2N1=2
61=2 eth11THORN
The radius of gyration of a polymer in solution can be determined
from light scattering measurements
As mentioned above dilute solutions of copolymers is solvents that
are good for both components exhibit similar behaviour to hompo-
lymer chains However in a selective solvent whereby the medium is
a good solvent for one component say A and a poor solvent for the
second component B the very different solvent af 1047297nities to the two
components will have a large effect on the conformation of the
isolated chain This results in formation of aggregates involving
several macromolecules in dilute solutions (low Critical Aggregation
Concentration CAC) It is believed that the polymeric aggregates are
Fig 3 Various conformations of macromolecules on a plane surface
284 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 519
spherical [5] The CAC of these block and graft copolymers is usually
very low
Several methods may be applied to obtain the aggregate size and
shape of block and graft copolymers of which light scattering small
angle X-ray and neutron scattering are probably the most direct
Dynamic light scattering (photon correlation spectroscopy) can also
be applied to obtain the hydrodynamic radius of the aggregate This
technique is relatively easy to perform when compared with static
light scattering since it does not require special preparation of thesample
13 Adsorption and conformation of polymeric surfactants at interfaces
Understanding the adsorption and conformation of polymeric
surfactants at interfaces is key to knowing how these molecules act as
stabilizers Most basic ideas on adsorption and conformation of
polymers have been developed for the solidliquid interface [10ndash17]
The process of polymer adsorption is fairly complicated mdash it
involves PolymerSurface Interaction PolymerSolvent Interaction
SurfaceSolvent Interaction The con1047297guration (conformation) of the
polymer at the solidliquid interface The polymersurface interaction
is described in terms of adsorption energy per segment χs The
polymersolvent interaction is described in terms of the Floryndash
Huggins interaction parameter χ The polymer con1047297guration is
described by sequences of Trains segments in direct contact with
the surface Loops segments in between the trains that extend into
solution Tails ends of the molecules that also extend into solution A
schematic representation of the various polymer con1047297gurations is
given in Fig 3
For homopolymers eg poly(ethylene oxide) (PEO) or polyvinyl
pyrrolidone (PVP) a train-loop-tail con1047297guration is the case For
adsorption to occur a minimum energy of adsorption per segment χs
is required When a polymer molecule adsorbs on a surface it looses
con1047297gurational entropy and this must be compensated by an
adsorption energy χs per segment This is schematically shown in
Fig 4 where the adsorbed amount Γ is plotted versus χs The
minimum value of χs can be very small (b01kT ) since a large number
of segments per molecule are adsorbed For a polymer with say 100segments and 10 of these are in trains the adsorption energy per
molecule now reaches 1kT (with χs =01kT ) For 1000 segments the
adsorption energy per molecule is now 10kT
Homopolymers are not the most suitable for stabilisation of
dispersions For strong adsorption one needs the molecule to be
ldquoinsolublerdquo in the medium and has strong af 1047297nity (ldquoanchoringrdquo) to the
surface For stabilisation one needs the molecule to be highly soluble
in themedium andstronglysolvated by itsmolecules mdash This requires a
FloryndashHuggins interaction parameter less than 05 The above
opposing effects can be resolved by introducing ldquoshortrdquo blocks in the
moleculewhichare insolublein themedium andhavea strong af 1047297nity
to the surface as illustrated in Fig 3b Example Partially hydrolysed
polyvinyl acetate (88 hydrolysed ie with 12 acetate groups)
usually referred to as polyvinyl alcohol (PVA)
The above requirements are better satis1047297ed using AndashB AndashBndashA and
BAn graft copolymers B is chosento be highly insolublein themedium
and it should have high af 1047297nity to the surface This is essential to
ensure strong ldquoanchoringrdquo to the surface (irreversible adsorption) A is
chosen to be highly soluble in the medium and strongly solvated by
its molecules The FloryndashHuggins χ parameter can be applied in this
case For a polymer in a good solvent χ has to be lower than 05 the
smaller the χ value the better the solvent for the polymer chains
Examples of B for a hydrophobic particles in aqueous media are
polystyrene polymethylmethacrylate Examples of A in aqueous
media are polyethylene oxide polyacrylic acid polyvinyl pyrollidone
and polysaccharides For non-aqueous media such as hydrocarbons
the A chain(s) could be poly(12-hydroxystearic acid)
For full description of polymer adsorption one needs to obtain
information on the following (i) The amount of polymer adsorbed Γ (in mg or moles) per unit area of the particles It is essential to know
thesurface area of theparticles in thesuspensionNitrogenadsorption
on the powder surface may give such information (by application of
the BET equation) provided there will be no change in area on
dispersing the particles in the medium For many practical systems a
change in surface area may occur on dispersing the powder in which
case one has to use dye adsorption to measure the surface area (some
assumptions have to be made in this case) (ii)The fraction of segments
in direct contact with thesurface ie the fraction of segmentsin trains
p ( p = (Number of segments in direct contact with the surface) Total
Number) (iii)The distribution of segments in loops and tails ρ(z)
which extend in several layersfrom the surface ρ(z) is usually dif 1047297cult
to obtain experimentally although recently application of small angle
neutron scattering could obtain such information An alternative anduseful parameter for assessing ldquosteric stabilisationrdquo is the hydro-
dynamic thickness δh (thickness of the adsorbed or grafted polymer
layer plus any contribution from the hydration layer) Several methods
can be applied to measure δh as will be discussed below
2 Theories of polymer adsorption
Two main approaches have been developed to treat the problem of
polymer adsorption (i) Random Walk approach This is based on
Florys treatment of the polymer chain in solution the surface was
considered as a re1047298ecting barrier (ii) Statistical mechanical approach
Thepolymer con1047297gurationwas treated as being made of three types of
structures trains loops and tails each having a different energy state
The random walk approach is an unrealistic model to the problemof polymer adsorption since the polymer interacts in a speci1047297c
manner with the surface and the solvent The statistical mechanical
approach is a more realistic model for the problem of polymer
adsorption since it takes into account the various interactions
involved A useful model for treating polymer adsorption and
con1047297guration was suggested by Scheutjens and Fleer (SF theory)
[14ndash16] that is referred to as the step weighted random walk approach
which is summarized below
The SF theory starts without any assumption for the segment
density distribution The partition functions were derived for the
mixture of free and adsorbed polymer molecules as well as for the
solvent molecules All chain conformations were described as step
weighted random walks on a quasi-crystalline lattice which extends in
parallel layers from the surface mdash
this is schematically shown in Fig 5Fig 4 Variation of adsorption amount Γ with adsorption energy per segment χs
285T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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The partition function is written in terms of a number of con1047297gura-
tions These were treated as connectedsequencesof segments In each
layer random mixing between segments and solvent molecules was
assumed (mean 1047297eld approximation) Each step in the random walk
was assigned a weighting factor pi
pi consists of three contributions (i) An adsorption energyχs (which
exists only for the segments that are near the surface) (ii) Con1047297gura-
tional entropy of mixing (that exists in each layer) (iii) Segment-solvent
interaction parameterχ (the FloryndashHuggins interaction parameter note
that χ=0 for an athermal solvent χ=05 for a θ-solvent)
From the weighting factors the statistical weight of any chain
conformation can be calculated for a given segment concentration
pro1047297le using a matrix formulation Fig 6 shows typical adsorption
isotherms plotted as surface coverage (in equivalent monolayers)
versus polymer volume fraction ϕ in bulk solution (ϕ was taken to
vary between 0 and 10minus3 which is the normal experimental range)
The results in Fig 6 show the effect of increasing the chain length r
and effect of solvency (athermal solvent with χ=0 and theta solvent
with χ=05) The adsorption energy χs was taken to be the same and
equal to 1kT When r = 1 θ is very small and the adsorption increases linearly
with increase of ϕ (Henrys type isotherm) On the other hand when
r = 10 the isotherm deviates much from a straight line and approaches
a Langmuirian type However when r ge20 high af 1047297nity isotherms are
obtained This implies that the 1047297rst added polymer chains are
completely adsorbed resulting in extremely low polymer concentra-
tion in solution (approaching zero) This explains the irreversibility of
adsorption of polymeric surfactants with r N100 The adsorption
isotherms with r =100 and above are typical of those observed
experimentally for most polymers that are not too polydisperse ie
showing a steep rise followed by a nearly horizontal plateau (which
only increases few percent per decade increase of ϕ) In these dilute
solutions the effect of solvency is most clearly seen with poor
solvents giving the highest adsorbed amounts In good solvents θ is
much smaller and levels off for long chains to attain an adsorption
plateau which is essentially independent of molecular weight
Another point that emerges from the SF theory is the difference in
shape between the experimental and theoretical adsorption isotherms
in thelow concentration region Theexperimentalisotherms areusuallyrounded whereas those predicted fromtheory are1047298at This is accounted
for in terms of the molecular weight distribution (polydispersity) which
is encountered with most practical polymers This effect has been
explained by CohenndashStuart et al [17] With polydisperse polymers the
larger molecular weight fractions adsorb preferentially over the smaller
ones At low polymer concentrations nearly all polymer fractions are
adsorbed leaving a small fraction of the polymer with the lowest
molecular weights in solution As the polymer concentration is
increased the higher molecular weight fractions displace the lower
ones on thesurfacewhichare nowreleasedin solution thusshiftingthe
molecular weight distribution in solution to lower values This process
continues with further increase in polymer concentration leading to
fractionation whereby the higher molecular weight fractions are
adsorbed at the expense of the lower fractions which are released to
the bulk solution However in very concentrated solutions monomers
adsorb preferentially with respect to polymers and short chains with
respect to larger ones This is due to the fact that in this region the
conformational entropy term predominates the free energy disfavour-
ing the adsorption of long chains
According to the SF theory the bound fraction p is high at low
concentration and relatively independent of molecular weight when
r N20 However with increase in surface coverage andor molecular
weight p tends to decrease indicating the formation of larger loops
and tails
The structure of the adsorbed layer is described in terms of the
segment density distribution As an illustration Fig 7 shows some
calculations using the SF theory for loops and tails with r =1000
ϕ= 10minus6 and χ=05 In this example 38 of the segments are in
trains 555 in loops and 65 in tails This theory demonstrates theimportance of tails which dominate the total distribution in the outer
region
3 Experimental techniques for studying polymeric surfactant
adsorption
As mentioned above for full characterization of polymeric sur-
factant adsorption one needs to determine three parameters (i) The
adsorbed amount Γ (mg mminus2 or mol mminus2) as a function of equilibrium
concentration C eq ie the adsorption isotherm (ii) The fraction of
segments in direct contact with the surface p (number of segments in
Fig 5 Schematic representation of a polymer molecule adsorbing on a 1047298at surface mdash
quasi-crystalline lattice with segments 1047297lling layers that are parallel to the surface
(random mixing of segments and solvent molecules in each layer is assumed)
Fig 6 Adsorption isotherms for oligomers and polymers in the dilute region based on
the SF theory Fig 7 Loop tail and total segment pro1047297le according to the SF theory
286 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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The microelectrophoresis technique is based on measurement of
the electrophoretic mobility u of the particles in the presence and
absence of the polymer layer From u one can calculate the zeta
potential ζ using the Huckel equation (which is applicable for small
particles and extended double layers ie κ Rbb1 where κ is the
DebyendashHuckel parameter that is related to the salt concentration
u = 23eeon η
eth15THORN
where ε is the relative permittivity of the medium and ε o is the
permittivity of free space
By measuring ζ of the particles with and without the adsorbed
polymer layer one can obtain the hydrodynamic thickness δh For
accurate measurements one should carry the measurements at various
electrolyte concentrations and extrapolate the results to the plateau
value Several automatic instruments are available for measurement of
the electrophoretic mobility Malvern zeta sizerndashCoulter Delsa sizer and
Broekahven instrument All these instruments are easy to use and the
measurement can be carried out within few minutes
4 Examples of the adsorption isotherms of nonionic polymericsurfactants
Fig 9 shows the adsorption isotherms for PEO with different
molecular weights on PS (at room temperature It can be seen that the
amount adsorbed in mgm-2 increases with increase in the polymer
molecular weight Fig 10 shows the variation of the hydrodynamic
thickness δh with molecular weight M δh shows a linear increase with
log M δh increases with n the number of segments in the chain
according to
δhasympn08 eth16THORN
Fig 11 shows the adsorption isotherms of PVA with various
molecular weights on PS latex (at 25 degC) [19] The polymers were
obtained by fractionation of a commercial sample of PVA with an
average molecular weight of 45000 The polymer also contained 12
vinyl acetate groups As with PEO the amount of adsorption increases
with increase in M The isotherms are also of the high af 1047297nity type Γ at the plateau increases linearly with M 12
The hydrodynamic thickness was determined using PCS and theresults are given below
M 67000 43000 28000 17000 8000
δhnm 255 197 140 98 33
δh seems to increase linearly with increase in the molecular weight
The effect of solvency on adsorption was investigated by increasing
the temperature (the PVA molecules are less soluble at higher
temperature) or addition of electrolyte (KCl) [20] The results are
shown in Figs 12 and 13 for M =65100 As can be seen from Fig 12
increase of temperature results in reduction of solvency of the
mediumfor the chain (due to break down of hydrogen bonds) and this
results in an increase in the amount adsorbed Addition of KCl (which
reduces the solvency of the medium for the chain) results in anincrease in adsorption (as predicted by theory)
The adsorption of block and graft copolymers is more complex
since the intimate structure of the chain determines the extent of
adsorption [18] Randomcopolymers adsorbin an intermediate way to
that of the corresponding homopolymers Blockcopolymers retain the
adsorption preference of the individual blocks The hydrophilic block
(eg PEO) the buoy (previously referred to as the A chain) extends
away from the particle surface into the bulk solution whereas the
hydrophobic anchor block (previously referred to as the B chain) (eg
PS or PPO) provides 1047297rm attachment to the surface Fig 14 shows the
theoretical prediction of diblock copolymer adsorption according to
Fig 9 Adsorption isotherms for PEO on PS
Fig 10 Hydrodynamic thickness of PEO on PS as a function of the molecular weight
Fig 11 Adsorption isotherms of PVA with different molecular weights on polystyrene
latex at 25 degC
Fig 12 In1047298uence of temperature on adsorption
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the Scheutjens and Fleer theory The surface density σ is plotted
versus the fraction of anchor segmentsν A The adsorption depends on
the anchorbuoy composition
The amount of adsorption is higher than for homopolymers and
the adsorbed layer thickness is more extended and dense Fig 15
shows the theoretical prediction for the adsorbed layer thickness δwhich is plotted as a function of ν A For a triblock copolymer AndashBndashA
with two buoy chains (A) and one anchor chain (B) the behaviour is
similar to that of diblock copolymers This is shown in Fig16 for PEOndash
PPOndashPEO block (Pluronic)
5 Interaction between particles containing adsorbed polymeric
surfactant layers steric stabilization
When two particles each with a radius R and containing an
adsorbed polymer layer with a hydrodynamic thickness δh approach
each other to a surfacendashsurface separation distance h that is smaller
than 2 δh the polymer layers interact with each other resulting in two
main situations [21] (i) The polymer chains may overlap with each
other (ii) The polymer layer may undergo some compression In both
cases there will be an increase in the local segment density of the
polymer chains in the interaction region This is schematicallyillustrated in Fig 17 The real situation is perhaps in between the
above two cases ie the polymer chains may undergo some
interpenetration and some compression
Provided the dangling chains (the A chains in AndashB AndashBndashA block or
BAn graft copolymers) are in a good solvent this local increase in
segment density in the interaction zone will result in strong repulsion
as a result of two main effects (i) Increase in the osmotic pressure in
the overlap region as a result of the unfavourable mixing of the
polymer chains when these are in good solvent conditions This is
referred to as osmotic repulsion or mixing interaction and it is
described by a free energy of interaction Gmix (ii) Reduction of the
con1047297gurational entropy of the chains in the interaction zone this
entropy reduction results from the decrease in the volume available
for thechains when these areeither overlapped or compressed This is
referred to as volume restriction interaction entropic or elastic
interaction and it is described by a free energy of interaction Gel
Combination of Gmix and Gel is usually referred to as the steric
interaction free energy Gs ie
Gs = Gmix + Gel eth17THORN
The sign of Gmix depends on the solvency of the medium for the
chains If in a good solvent ie the FloryndashHuggins interaction
parameter χ is less than 05 then Gmix is positive and the mixing
interaction leads to repulsion (see below) In contrast if χN05 (ie the
chains are in a poor solvent condition) Gmix is negative and the
mixing interaction becomes attractive Gel is always positive and
hence in some cases one can produce stable dispersions in a relatively
poor solvent (enhanced steric stabilisation)
51 Mixing interaction Gmix
This results from the unfavourable mixing of the polymer chainswhen these are in a good solvent conditions This is schematically
shown in Fig 18
Consider two spherical particles with the same radius and each
containing an adsorbed polymer layer with thickness δ Before
overlap one can de1047297ne in each polymer layer a chemical potential
for the solvent μ iα and a volume fraction for the polymer in the
layerϕ2 In the overlap region (volume element dV ) the chemical
potential of the solvent is reduced to μ i β This results from the increase
in polymer segment concentration in this overlap region
In the overlap region the chemical potential of the polymer chains
is now higher than in the rest of the layer (with no overlap) This
Fig 13 In1047298uence of addition of KCl on adsorption
Fig 14 Prediction of Adsorption of diblock copolymer
Fig15 Theoretical predictions of the adsorbed layer thickness for a diblock copolymer
Fig 16 Adsorbed amount (mg mminus2) versus fraction of anchor segment for an AndashBndashA
triblock copolymer (PEOndash
PPOndash
PEO)
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amounts to an increase in the osmotic pressure in the overlap region
as a result solvent will diffuse from thebulk to the overlap region thus
separating the particles and hence a strong repulsive energy arises
from this effect The above repulsive energy can be calculated by
considering the free energy of mixing of two polymer solutions as for
example treated by Flory and Krigbaum [22] The free energy of
mixing is given by two terms (i) An entropy term that depends on the
volume fraction of polymer and solvent (ii)An energy term that is
determined by the FloryndashHuggins interaction parameter χ
Using the above theory one can derive an expression for the free
energy of mixing of two polymer layers (assuming a uniform segment
density distribution in each layer) surrounding two spherical particles
as a function of the separation distance h between the particles The
expression for Gmix is
Gmix
kT =
2V 22V 1
m2
2
1
2minusχ
δminus
h
2
2
3R + 2δ + h
2
eth18THORN
k is the Boltzmann constant T is the absolute temperature V 2 is the
molar volume of polymer V 1 is the molar volume of solvent and ν 2 is
the number of polymer chains per unit area
The sign of Gmix depends on the value of the FloryndashHuggins
interaction parameter χ if χb05 Gmix is positive and the interaction
is repulsive if χN05 Gmix is negative and the interaction is attractiveif χ=05 Gmix =0 and this de1047297nes the θ-condition
52 Elastic interaction Gel
Thisarises fromthe lossin con1047297gurational entropy of thechains on
the approach of a second particle As a result of this approach the
volume available for the chains becomes restricted resulting in loss of
the number of con1047297gurations This can be illustrated by considering a
simple molecule represented by a rod that rotates freely in a
hemisphere across a surface (Fig 19) When the two surfaces are
separated byan in1047297nitedistanceinfin thenumber of con1047297gurations of the
rod is Ω(infin) which is proportional to the volume of the hemisphere
When a secondparticleapproaches to a distance h suchthatit cuts the
hemisphere (loosing some volume) the volume available to the chains
is reduced and the number of con1047297gurations become Ω(h) which is
less than Ω(infin) For two 1047298at plates Gel is given by the following
expression
Gel
kT = 2m2 ln
X heth THORN
X infineth THORN
= 2m2Rel heth THORN eth19THORN
where Rel
(h) is a geometric function whose form depends on the
segment density distribution It should be stressed that Gel is always
positive and could play a major role in steric stabilisation It becomes
very strong when the separation distance between the particles
becomes comparable to the adsorbed layer thickness δ
Combination of Gmix and Gel with GA (the van der Waals attractive
energy)gives the total free energy of interaction GT (assuming there is
no contribution from any residual electrostatic interaction) ie
GT = Gmix + Gel + GA eth20THORN
A schematic representation of the variation of Gmix Gel GA and GT
with surfacendashsurface separation distance h is shown in Fig 20
Gmix increases very sharply with decrease of h when hb2δ Gel
increases very sharply with decrease of h when hbδ GT versus h
shows a minimum Gmin at separation distances comparable to 2δ
When h b2δ GT shows a rapid increase with decrease in h
The depth of the minimum depends on the Hamaker constant A
the particle radius R and adsorbed layer thickness δ Gmin increases
with increase of A and R At a given A and R Gmin increases with
decrease in δ (ie with decrease of the molecular weight M w of the
stabiliser This is illustrated in Fig 21 which shows the energyndash
distance curves as a function of δR The larger the value of δR the
smaller the value of Gmin In this case the system may approach
thermodynamic stability as is the case with nano-dispersions
6 Emulsions stabilized by polymeric surfactants
The most effective method for emulsion stabilization is to use
polymeric surfactants that stronglyadsorb at the OWor WO interface
Fig 17 Schematic representation of the interaction between particles containing adsorbed polymer layers
Fig 18 Schematic representation of polymer layer overlap
Fig 19 Schematic representation of con1047297gurational entropy loss on approach of a
second particle
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and produce effective steric stabilization against strong 1047298occulation
coalescence and Ostwald ripening [23]
As mentioned above a graft copolymer of the ABn type was
synthesized by grafting several alkyl groups on an inulin (polyfruc-
tose) chain The polymeric surfactant (INUTECregSP1) consists of a
linear polyfructose chain (the stabilizing A chain) and several alkyl
groups (the B chains) that provide multi-anchor attachment to the oil
droplets This polymeric surfactant produces enhanced steric stabili-
zation both in water and high electrolyte concentrations as will be
discussed later
For water-in-oil (WO) emulsions an AndashBndashA block copolymer of
poly (12-hydroxystearic acid) (PHS) (the A chains) and poly (ethylene
oxide) (PEO) (the B chain) PHSndashPEOndashPHS is commercially available
(Arlacel P135 UNIQEMA) The PEO chain (that is soluble in the water
droplets) forms the anchor chain whereas the PHS chains form the
stabilizing chains PHS is highly soluble in most hydrocarbon solvents
and is strongly solvated by its molecules The structure of the PHSndash
PEOndashPHS block copolymer is schematically shown in Fig 22The conformation of the polymeric surfactant at the WO interface
is schematically shown in Fig 23
Emulsions of Isopar Mwater and cyclomethiconewater were
prepared using INUTECregSP1 5050 (vv) OW emulsions were
prepared and the emulsi1047297er concentration was varied from 025 to 2
(wv) based on the oil phase 05 (wv) emulsi1047297erwas suf 1047297cient for
stabilization of these 5050 (vv) emulsions [23]
The emulsions were stored at room temperature and 50 degC and
optical micrographs were taken at intervals of time (for a year) in
order to check the stability Emulsions prepared in water were very
stableshowingno change in droplet size distributionover more than a
year period and this indicated absence of coalescence Any weak
1047298occulation that occurred was reversible and the emulsion could be
redispersed by gentle shaking Fig 24 shows an optical micrograph fora dilute 5050 (vv) emulsion that was stored for 15 and 14 weeks at
50 degC
No change in droplet size was observed after storage for more than
1 year at 50 degC indicating absence of coalescence The same result was
obtained when using different oils Emulsions were also stable against
coalescence in the presence of high electrolyte concentrations (up to
4 mol dmminus3 or ~25 NaCl
The above stability in high electrolyte concentrations is not
observed with polymeric surfactants based on polethylene oxide
The high stability observed using INUTECregSP1 is related to its
strong hydration both in water and in electrolyte solutions The
hydration of inulin (the backbone of HMI) could be assessed using
cloud point measurements A comparison was also made with PEO
with two molecular weights namely 4000 and 20000
Solutionsof PEO 4000 and 20000 showeda systematic decrease of
cloud point with increase in NaCl or MgSO4 concentration In contrast
inulin showed no cloud point up to 4 mol dmminus3 NaCl and up to 1 mol
dmminus3 MgSO4
The above results explain the difference between PEO and inulin
With PEO the chains show dehydration when the NaCl concentration
is increased above 2 mol dmminus3 or 05 mol dmminus3 MgSO4 The inulin
chains remain hydrated at much higher electrolyte concentrations It
seems that the linear polyfructose chains remain strongly hydrated athigh temperature and high electrolyte concentrations
The high emulsion stability obtained whenusingINUTECregSP1 can be
accounted for by the following factors (i) The multi-point attachment
of the polymer by several alkyl chains that are grafted on the backbone
(ii) The strong hydration of the polyfructose ldquoloopsrdquo both in water and
high electrolyte concentrations (χ remains below 05 under these
conditions) (iii) Thehigh volumefraction (concentration) of the loops at
the interface (iv) Enhanced steric stabilization this is the case with
multi-point attachment which produces strong elastic interaction
Evidence for the high stability of the liquid 1047297lm between emulsion
droplets when using INUTECregSP1 was obtained by Exerowa et al [24]
using disjoining pressure measurements This is illustrated in Fig 25
which shows a plot of disjoining pressure versus separation distance
between two emulsion droplets at various electrolyte concentrations
The results show that by increasing the capillary pressure a stable
Newton Black Film (NBF) is obtained at a 1047297lm thickness of sim7 nm
The lack of rupture of the 1047297lm at the highest pressure applied of
45times104 Pa indicate the high stability of the 1047297lm in water and in high
electrolyte concentrations (up to 20 mol dmminus3 NaCl)
The lack of rupture of the NBF up to the highest pressure applied
namely 45times104 Pa clearly indicatesthe high stability of the liquid1047297lm
in the presence of high NaCl concentrations (up to 2 mol dm minus3) This
result is consistent with the high emulsion stability obtained at high
electrolyte concentrations and high temperature Emulsions of Isopar
M-in-water are very stable under such conditions and this could be
accounted for by the high stability of the NBF The droplet size of
5050 OW emulsions prepared using 2 INUTEC regSP1 is in the region
of 1ndash10 μ m This corresponds to a capillary pressure of ~3times104 Pa for
the 1 μ m drops and ~3times103 Pa for the 10 μ m drops These capillarypressures are lower than those to which the NBF have been sub-
jected to and this clearly indicates the high stability obtained against
coalescence in these emulsions
61 WO emulsions stabilized with PHS ndashPEOndashPHS block copolymer
WO emulsions (the oil being Isopar M) were prepared using PHSndash
PEOndashPHS block copolymer at high water volume fractions (N07) The
emulsions have a narrow droplet size distribution with a z -average
radius of 183 nm [25] They also remained 1047298uid up to high water
volume fractions (N06) This could be illustrated from viscosityndash
volume fraction curves as is shown in Fig 26
Fig 20 Energyndashdistance curves for sterically stabilized systems
Fig 21 Variation of Gmin with δR
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The effective volume fraction ϕeff of the emulsions (the core
droplets plus the adsorbed layer) could be calculated from the relative
viscosity and using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth21THORN
Where η r is the relative viscosity ϕp is the maximum packing fraction(sim07) and [ η ] is the intrinsic viscosity that is equal to 25 for hard-
spheres
The calculations based on Eq (22) are shown in Fig 27 (square
symbols) From the effective volume fraction ϕeff and the core volume
fraction ϕ the adsorbed layer thickness could be calculated This was
found to be in the region of 10 nm at ϕ =04 and it decreased with
increase in ϕ
The WO emulsions prepared using the PHSndashPEOndashPHS block
copolymer remained stable both at room temperature and 50 degC
This is consistent with the structure of the block copolymer the B
chain (PEO) is soluble in water and it forms a very strong anchor at the
WO interface The PHS chains (the A chains) provide effective steric
stabilization since the chains are highly soluble in Isopar M and are
strongly solvated by its molecules
7 Suspensions stabilised using polymeric surfactants
There are generally two procedures for preparation of solidliquid
dispersions
(i) Condensation methods build-up of particles from mole-
cular units ie nucleation and growth A special procedure
is the preparation of latexes by emulsion or dispersion
polymerization
(ii) Dispersion methods in this case one starts with preformed
large particles or crystals which are dispersed in the liquid by
using a surfactant (wetting agent) with subsequent breaking up
of the large particles by milling (comminution) to achieve the
desirable particle size distribution A dispersing agent (usuallya
polymeric surfactant) is used for the dispersion process and
subsequent stabilization of the resulting suspension
There are generally two procedures for preparation of latexes
(i) Emulsion polymerization the monomers that are essentially
insoluble in the aqueous medium are emulsi1047297ed using a
surfactant and an initiator is added while heating the system
to produce the polymer particles that are stabilized electro-
statically (when using ionic surfactants) or sterically (when
using non-ionic surfactants)
(ii) Dispersion polymerization the monomers are dissolved in a
solvent in which the resulting polymer particlesare insoluble A
protective colloid (normally a block or graft copolymer) is
added to prevent 1047298occulation of the resulting polymers
particles that are produced on addition of an initiator This
method is usually applied for the production of non-aqueous
latex dispersions and is sometimes referred to as Non-Aqueous
Dispersion Polymerization (NAD)
Surfactants play a crucial role in the process of latex preparation
since they determine the stabilizing ef 1047297ciency and the effectiveness of
the surface active agent ultimately determines the number of particles
Fig 22 Schematic representation of the structure of PHSndashPEOndashPHS block copolymer
Fig 23 Conformation of PHSndash
PEOndash
PHS polymeric surfactant at the WO interface
292 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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and their size The effectiveness of any surface active agent in
stabilizing the particles is the dominant factor and the number of
micelles formed is relatively unimportant In the NAD process themonomer normally an acrylic is dissolved in a non-aqueous solvent
normally an aliphatic hydrocarbon and an oil soluble initiator and a
stabilizer (to protect the resulting particles from1047298occulation) is added
to the reaction mixture The most successful stabilizers used in NAD
are block and graft copolymers Preformed graft stabilizers based
on poly(12-hydroxy stearic acid) (PHS) are simple to prepare and
effective in NAD polymerization
Dispersion methods are used for the preparation of suspensions of
preformed particles The role of surfactants (or polymers) in the
dispersion process can be analyzed in terms of the three processesinvolved (i) Wetting of the powder by the liquid (ii) Breaking of the
aggregates and agglomerates (iii) Comminution of the resulting
particles and their subsequent stabilization All these processes are
affected by surfactants or polymers which adsorb on the powder
surface thus aiding the wetting of the powder break-up of the
aggregates and agglomerates and subsequent reduction of particle
size by wet milling
71 Polymeric surfactants in emulsion polymerization
Recently the graft copolymer of hydrophobically modi1047297ed inulin
(INUTECreg SP1) has been used in emulsion polymerization of styrene
methyl methacrylate butyl acrylate and several other monomers [26]All lattices were prepared by emulsion polymerisation using potas-
sium persulphate as initiator The z -average particle size was
determined by photon correlation spectroscopy (PCS) and electron
micrographs were also taken
Fig 24 Optical micrographs of OW emulsions stabilized with INUTEC regSP1 stored at
50 degC for 15 weeks (a) and 14 weeks (b)
Fig 25 Variation of disjoining pressure with equivalent 1047297lm thickness at various NaCl concentrations
Fig 26 Viscosityndashvolume fraction for WO emulsion stabilized with PHSndashPEOndashPHS
block copolymer experimental data calculated using Eq (22)
293T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Emulsion polymerization of styrene or methylmethacrylate showed
an optimum weight ratio of (INUTEC)Monomer of 00033 for PS and
0001 for PMMA particles The (initiator)(Monomer) ratio was
kept constant at 000125 The monomer conversion was higher than
85 in all cases Latex dispersions of PS reaching 50 and of PMMA
reaching 40 could be obtained using such low concentration of
INUTECregSP1 Fig 27 shows the variation of particle diameter with
monomer concentration
The stability of the latexes was determined by determining the
critical coagulation concentration (CCC) using CaCl2 The CCC was low
(00175ndash005 mol dmminus3) but this was higher than that for the latex
prepared without surfactant Post addition of INUTEC regSP1 resulted in
a large increase in the CCC as is illustrated in Fig 28 which shows log
W minus log C curves (where W is the ratio between the fast 1047298occulation
rate constant to the slow 1047298occulation rate constant referred to as the
stability ratio) at various additions of INUTECregSP1
As with the emulsions the high stability of the latex when using
INUTECregSP1 is due to the strong adsorption of the polymeric
surfactant on the latex particles and formation of strongly hydrated
loops and tails of polyfructose that provide effective steric stabiliza-
tion Evidence for the strong repulsion produced when using
INUTECregSP1 was obtained from Atomic Force Microscopy investiga-
tions [27] whereby the force between hydrophobic glass spheres and
hydrophobic glass plate both containing an adsorbed layer of
INUTECregSP1 was measured as a function of distance of separation
both in water and in the presence of various Na2SO4 concentrations
The results are shown in Figs 29 and 30
Fig 27 Electron micrographs of the latexes
Fig 28 In1047298uence of post addition of INUTECreg
SP1 on the latex stability
Fig 29 Forcendashdistance curves between hydrophobised glass surfaces containing
adsorbed INUTECreg
SP1 in water
294 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
295T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
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radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 519
spherical [5] The CAC of these block and graft copolymers is usually
very low
Several methods may be applied to obtain the aggregate size and
shape of block and graft copolymers of which light scattering small
angle X-ray and neutron scattering are probably the most direct
Dynamic light scattering (photon correlation spectroscopy) can also
be applied to obtain the hydrodynamic radius of the aggregate This
technique is relatively easy to perform when compared with static
light scattering since it does not require special preparation of thesample
13 Adsorption and conformation of polymeric surfactants at interfaces
Understanding the adsorption and conformation of polymeric
surfactants at interfaces is key to knowing how these molecules act as
stabilizers Most basic ideas on adsorption and conformation of
polymers have been developed for the solidliquid interface [10ndash17]
The process of polymer adsorption is fairly complicated mdash it
involves PolymerSurface Interaction PolymerSolvent Interaction
SurfaceSolvent Interaction The con1047297guration (conformation) of the
polymer at the solidliquid interface The polymersurface interaction
is described in terms of adsorption energy per segment χs The
polymersolvent interaction is described in terms of the Floryndash
Huggins interaction parameter χ The polymer con1047297guration is
described by sequences of Trains segments in direct contact with
the surface Loops segments in between the trains that extend into
solution Tails ends of the molecules that also extend into solution A
schematic representation of the various polymer con1047297gurations is
given in Fig 3
For homopolymers eg poly(ethylene oxide) (PEO) or polyvinyl
pyrrolidone (PVP) a train-loop-tail con1047297guration is the case For
adsorption to occur a minimum energy of adsorption per segment χs
is required When a polymer molecule adsorbs on a surface it looses
con1047297gurational entropy and this must be compensated by an
adsorption energy χs per segment This is schematically shown in
Fig 4 where the adsorbed amount Γ is plotted versus χs The
minimum value of χs can be very small (b01kT ) since a large number
of segments per molecule are adsorbed For a polymer with say 100segments and 10 of these are in trains the adsorption energy per
molecule now reaches 1kT (with χs =01kT ) For 1000 segments the
adsorption energy per molecule is now 10kT
Homopolymers are not the most suitable for stabilisation of
dispersions For strong adsorption one needs the molecule to be
ldquoinsolublerdquo in the medium and has strong af 1047297nity (ldquoanchoringrdquo) to the
surface For stabilisation one needs the molecule to be highly soluble
in themedium andstronglysolvated by itsmolecules mdash This requires a
FloryndashHuggins interaction parameter less than 05 The above
opposing effects can be resolved by introducing ldquoshortrdquo blocks in the
moleculewhichare insolublein themedium andhavea strong af 1047297nity
to the surface as illustrated in Fig 3b Example Partially hydrolysed
polyvinyl acetate (88 hydrolysed ie with 12 acetate groups)
usually referred to as polyvinyl alcohol (PVA)
The above requirements are better satis1047297ed using AndashB AndashBndashA and
BAn graft copolymers B is chosento be highly insolublein themedium
and it should have high af 1047297nity to the surface This is essential to
ensure strong ldquoanchoringrdquo to the surface (irreversible adsorption) A is
chosen to be highly soluble in the medium and strongly solvated by
its molecules The FloryndashHuggins χ parameter can be applied in this
case For a polymer in a good solvent χ has to be lower than 05 the
smaller the χ value the better the solvent for the polymer chains
Examples of B for a hydrophobic particles in aqueous media are
polystyrene polymethylmethacrylate Examples of A in aqueous
media are polyethylene oxide polyacrylic acid polyvinyl pyrollidone
and polysaccharides For non-aqueous media such as hydrocarbons
the A chain(s) could be poly(12-hydroxystearic acid)
For full description of polymer adsorption one needs to obtain
information on the following (i) The amount of polymer adsorbed Γ (in mg or moles) per unit area of the particles It is essential to know
thesurface area of theparticles in thesuspensionNitrogenadsorption
on the powder surface may give such information (by application of
the BET equation) provided there will be no change in area on
dispersing the particles in the medium For many practical systems a
change in surface area may occur on dispersing the powder in which
case one has to use dye adsorption to measure the surface area (some
assumptions have to be made in this case) (ii)The fraction of segments
in direct contact with thesurface ie the fraction of segmentsin trains
p ( p = (Number of segments in direct contact with the surface) Total
Number) (iii)The distribution of segments in loops and tails ρ(z)
which extend in several layersfrom the surface ρ(z) is usually dif 1047297cult
to obtain experimentally although recently application of small angle
neutron scattering could obtain such information An alternative anduseful parameter for assessing ldquosteric stabilisationrdquo is the hydro-
dynamic thickness δh (thickness of the adsorbed or grafted polymer
layer plus any contribution from the hydration layer) Several methods
can be applied to measure δh as will be discussed below
2 Theories of polymer adsorption
Two main approaches have been developed to treat the problem of
polymer adsorption (i) Random Walk approach This is based on
Florys treatment of the polymer chain in solution the surface was
considered as a re1047298ecting barrier (ii) Statistical mechanical approach
Thepolymer con1047297gurationwas treated as being made of three types of
structures trains loops and tails each having a different energy state
The random walk approach is an unrealistic model to the problemof polymer adsorption since the polymer interacts in a speci1047297c
manner with the surface and the solvent The statistical mechanical
approach is a more realistic model for the problem of polymer
adsorption since it takes into account the various interactions
involved A useful model for treating polymer adsorption and
con1047297guration was suggested by Scheutjens and Fleer (SF theory)
[14ndash16] that is referred to as the step weighted random walk approach
which is summarized below
The SF theory starts without any assumption for the segment
density distribution The partition functions were derived for the
mixture of free and adsorbed polymer molecules as well as for the
solvent molecules All chain conformations were described as step
weighted random walks on a quasi-crystalline lattice which extends in
parallel layers from the surface mdash
this is schematically shown in Fig 5Fig 4 Variation of adsorption amount Γ with adsorption energy per segment χs
285T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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The partition function is written in terms of a number of con1047297gura-
tions These were treated as connectedsequencesof segments In each
layer random mixing between segments and solvent molecules was
assumed (mean 1047297eld approximation) Each step in the random walk
was assigned a weighting factor pi
pi consists of three contributions (i) An adsorption energyχs (which
exists only for the segments that are near the surface) (ii) Con1047297gura-
tional entropy of mixing (that exists in each layer) (iii) Segment-solvent
interaction parameterχ (the FloryndashHuggins interaction parameter note
that χ=0 for an athermal solvent χ=05 for a θ-solvent)
From the weighting factors the statistical weight of any chain
conformation can be calculated for a given segment concentration
pro1047297le using a matrix formulation Fig 6 shows typical adsorption
isotherms plotted as surface coverage (in equivalent monolayers)
versus polymer volume fraction ϕ in bulk solution (ϕ was taken to
vary between 0 and 10minus3 which is the normal experimental range)
The results in Fig 6 show the effect of increasing the chain length r
and effect of solvency (athermal solvent with χ=0 and theta solvent
with χ=05) The adsorption energy χs was taken to be the same and
equal to 1kT When r = 1 θ is very small and the adsorption increases linearly
with increase of ϕ (Henrys type isotherm) On the other hand when
r = 10 the isotherm deviates much from a straight line and approaches
a Langmuirian type However when r ge20 high af 1047297nity isotherms are
obtained This implies that the 1047297rst added polymer chains are
completely adsorbed resulting in extremely low polymer concentra-
tion in solution (approaching zero) This explains the irreversibility of
adsorption of polymeric surfactants with r N100 The adsorption
isotherms with r =100 and above are typical of those observed
experimentally for most polymers that are not too polydisperse ie
showing a steep rise followed by a nearly horizontal plateau (which
only increases few percent per decade increase of ϕ) In these dilute
solutions the effect of solvency is most clearly seen with poor
solvents giving the highest adsorbed amounts In good solvents θ is
much smaller and levels off for long chains to attain an adsorption
plateau which is essentially independent of molecular weight
Another point that emerges from the SF theory is the difference in
shape between the experimental and theoretical adsorption isotherms
in thelow concentration region Theexperimentalisotherms areusuallyrounded whereas those predicted fromtheory are1047298at This is accounted
for in terms of the molecular weight distribution (polydispersity) which
is encountered with most practical polymers This effect has been
explained by CohenndashStuart et al [17] With polydisperse polymers the
larger molecular weight fractions adsorb preferentially over the smaller
ones At low polymer concentrations nearly all polymer fractions are
adsorbed leaving a small fraction of the polymer with the lowest
molecular weights in solution As the polymer concentration is
increased the higher molecular weight fractions displace the lower
ones on thesurfacewhichare nowreleasedin solution thusshiftingthe
molecular weight distribution in solution to lower values This process
continues with further increase in polymer concentration leading to
fractionation whereby the higher molecular weight fractions are
adsorbed at the expense of the lower fractions which are released to
the bulk solution However in very concentrated solutions monomers
adsorb preferentially with respect to polymers and short chains with
respect to larger ones This is due to the fact that in this region the
conformational entropy term predominates the free energy disfavour-
ing the adsorption of long chains
According to the SF theory the bound fraction p is high at low
concentration and relatively independent of molecular weight when
r N20 However with increase in surface coverage andor molecular
weight p tends to decrease indicating the formation of larger loops
and tails
The structure of the adsorbed layer is described in terms of the
segment density distribution As an illustration Fig 7 shows some
calculations using the SF theory for loops and tails with r =1000
ϕ= 10minus6 and χ=05 In this example 38 of the segments are in
trains 555 in loops and 65 in tails This theory demonstrates theimportance of tails which dominate the total distribution in the outer
region
3 Experimental techniques for studying polymeric surfactant
adsorption
As mentioned above for full characterization of polymeric sur-
factant adsorption one needs to determine three parameters (i) The
adsorbed amount Γ (mg mminus2 or mol mminus2) as a function of equilibrium
concentration C eq ie the adsorption isotherm (ii) The fraction of
segments in direct contact with the surface p (number of segments in
Fig 5 Schematic representation of a polymer molecule adsorbing on a 1047298at surface mdash
quasi-crystalline lattice with segments 1047297lling layers that are parallel to the surface
(random mixing of segments and solvent molecules in each layer is assumed)
Fig 6 Adsorption isotherms for oligomers and polymers in the dilute region based on
the SF theory Fig 7 Loop tail and total segment pro1047297le according to the SF theory
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The microelectrophoresis technique is based on measurement of
the electrophoretic mobility u of the particles in the presence and
absence of the polymer layer From u one can calculate the zeta
potential ζ using the Huckel equation (which is applicable for small
particles and extended double layers ie κ Rbb1 where κ is the
DebyendashHuckel parameter that is related to the salt concentration
u = 23eeon η
eth15THORN
where ε is the relative permittivity of the medium and ε o is the
permittivity of free space
By measuring ζ of the particles with and without the adsorbed
polymer layer one can obtain the hydrodynamic thickness δh For
accurate measurements one should carry the measurements at various
electrolyte concentrations and extrapolate the results to the plateau
value Several automatic instruments are available for measurement of
the electrophoretic mobility Malvern zeta sizerndashCoulter Delsa sizer and
Broekahven instrument All these instruments are easy to use and the
measurement can be carried out within few minutes
4 Examples of the adsorption isotherms of nonionic polymericsurfactants
Fig 9 shows the adsorption isotherms for PEO with different
molecular weights on PS (at room temperature It can be seen that the
amount adsorbed in mgm-2 increases with increase in the polymer
molecular weight Fig 10 shows the variation of the hydrodynamic
thickness δh with molecular weight M δh shows a linear increase with
log M δh increases with n the number of segments in the chain
according to
δhasympn08 eth16THORN
Fig 11 shows the adsorption isotherms of PVA with various
molecular weights on PS latex (at 25 degC) [19] The polymers were
obtained by fractionation of a commercial sample of PVA with an
average molecular weight of 45000 The polymer also contained 12
vinyl acetate groups As with PEO the amount of adsorption increases
with increase in M The isotherms are also of the high af 1047297nity type Γ at the plateau increases linearly with M 12
The hydrodynamic thickness was determined using PCS and theresults are given below
M 67000 43000 28000 17000 8000
δhnm 255 197 140 98 33
δh seems to increase linearly with increase in the molecular weight
The effect of solvency on adsorption was investigated by increasing
the temperature (the PVA molecules are less soluble at higher
temperature) or addition of electrolyte (KCl) [20] The results are
shown in Figs 12 and 13 for M =65100 As can be seen from Fig 12
increase of temperature results in reduction of solvency of the
mediumfor the chain (due to break down of hydrogen bonds) and this
results in an increase in the amount adsorbed Addition of KCl (which
reduces the solvency of the medium for the chain) results in anincrease in adsorption (as predicted by theory)
The adsorption of block and graft copolymers is more complex
since the intimate structure of the chain determines the extent of
adsorption [18] Randomcopolymers adsorbin an intermediate way to
that of the corresponding homopolymers Blockcopolymers retain the
adsorption preference of the individual blocks The hydrophilic block
(eg PEO) the buoy (previously referred to as the A chain) extends
away from the particle surface into the bulk solution whereas the
hydrophobic anchor block (previously referred to as the B chain) (eg
PS or PPO) provides 1047297rm attachment to the surface Fig 14 shows the
theoretical prediction of diblock copolymer adsorption according to
Fig 9 Adsorption isotherms for PEO on PS
Fig 10 Hydrodynamic thickness of PEO on PS as a function of the molecular weight
Fig 11 Adsorption isotherms of PVA with different molecular weights on polystyrene
latex at 25 degC
Fig 12 In1047298uence of temperature on adsorption
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the Scheutjens and Fleer theory The surface density σ is plotted
versus the fraction of anchor segmentsν A The adsorption depends on
the anchorbuoy composition
The amount of adsorption is higher than for homopolymers and
the adsorbed layer thickness is more extended and dense Fig 15
shows the theoretical prediction for the adsorbed layer thickness δwhich is plotted as a function of ν A For a triblock copolymer AndashBndashA
with two buoy chains (A) and one anchor chain (B) the behaviour is
similar to that of diblock copolymers This is shown in Fig16 for PEOndash
PPOndashPEO block (Pluronic)
5 Interaction between particles containing adsorbed polymeric
surfactant layers steric stabilization
When two particles each with a radius R and containing an
adsorbed polymer layer with a hydrodynamic thickness δh approach
each other to a surfacendashsurface separation distance h that is smaller
than 2 δh the polymer layers interact with each other resulting in two
main situations [21] (i) The polymer chains may overlap with each
other (ii) The polymer layer may undergo some compression In both
cases there will be an increase in the local segment density of the
polymer chains in the interaction region This is schematicallyillustrated in Fig 17 The real situation is perhaps in between the
above two cases ie the polymer chains may undergo some
interpenetration and some compression
Provided the dangling chains (the A chains in AndashB AndashBndashA block or
BAn graft copolymers) are in a good solvent this local increase in
segment density in the interaction zone will result in strong repulsion
as a result of two main effects (i) Increase in the osmotic pressure in
the overlap region as a result of the unfavourable mixing of the
polymer chains when these are in good solvent conditions This is
referred to as osmotic repulsion or mixing interaction and it is
described by a free energy of interaction Gmix (ii) Reduction of the
con1047297gurational entropy of the chains in the interaction zone this
entropy reduction results from the decrease in the volume available
for thechains when these areeither overlapped or compressed This is
referred to as volume restriction interaction entropic or elastic
interaction and it is described by a free energy of interaction Gel
Combination of Gmix and Gel is usually referred to as the steric
interaction free energy Gs ie
Gs = Gmix + Gel eth17THORN
The sign of Gmix depends on the solvency of the medium for the
chains If in a good solvent ie the FloryndashHuggins interaction
parameter χ is less than 05 then Gmix is positive and the mixing
interaction leads to repulsion (see below) In contrast if χN05 (ie the
chains are in a poor solvent condition) Gmix is negative and the
mixing interaction becomes attractive Gel is always positive and
hence in some cases one can produce stable dispersions in a relatively
poor solvent (enhanced steric stabilisation)
51 Mixing interaction Gmix
This results from the unfavourable mixing of the polymer chainswhen these are in a good solvent conditions This is schematically
shown in Fig 18
Consider two spherical particles with the same radius and each
containing an adsorbed polymer layer with thickness δ Before
overlap one can de1047297ne in each polymer layer a chemical potential
for the solvent μ iα and a volume fraction for the polymer in the
layerϕ2 In the overlap region (volume element dV ) the chemical
potential of the solvent is reduced to μ i β This results from the increase
in polymer segment concentration in this overlap region
In the overlap region the chemical potential of the polymer chains
is now higher than in the rest of the layer (with no overlap) This
Fig 13 In1047298uence of addition of KCl on adsorption
Fig 14 Prediction of Adsorption of diblock copolymer
Fig15 Theoretical predictions of the adsorbed layer thickness for a diblock copolymer
Fig 16 Adsorbed amount (mg mminus2) versus fraction of anchor segment for an AndashBndashA
triblock copolymer (PEOndash
PPOndash
PEO)
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amounts to an increase in the osmotic pressure in the overlap region
as a result solvent will diffuse from thebulk to the overlap region thus
separating the particles and hence a strong repulsive energy arises
from this effect The above repulsive energy can be calculated by
considering the free energy of mixing of two polymer solutions as for
example treated by Flory and Krigbaum [22] The free energy of
mixing is given by two terms (i) An entropy term that depends on the
volume fraction of polymer and solvent (ii)An energy term that is
determined by the FloryndashHuggins interaction parameter χ
Using the above theory one can derive an expression for the free
energy of mixing of two polymer layers (assuming a uniform segment
density distribution in each layer) surrounding two spherical particles
as a function of the separation distance h between the particles The
expression for Gmix is
Gmix
kT =
2V 22V 1
m2
2
1
2minusχ
δminus
h
2
2
3R + 2δ + h
2
eth18THORN
k is the Boltzmann constant T is the absolute temperature V 2 is the
molar volume of polymer V 1 is the molar volume of solvent and ν 2 is
the number of polymer chains per unit area
The sign of Gmix depends on the value of the FloryndashHuggins
interaction parameter χ if χb05 Gmix is positive and the interaction
is repulsive if χN05 Gmix is negative and the interaction is attractiveif χ=05 Gmix =0 and this de1047297nes the θ-condition
52 Elastic interaction Gel
Thisarises fromthe lossin con1047297gurational entropy of thechains on
the approach of a second particle As a result of this approach the
volume available for the chains becomes restricted resulting in loss of
the number of con1047297gurations This can be illustrated by considering a
simple molecule represented by a rod that rotates freely in a
hemisphere across a surface (Fig 19) When the two surfaces are
separated byan in1047297nitedistanceinfin thenumber of con1047297gurations of the
rod is Ω(infin) which is proportional to the volume of the hemisphere
When a secondparticleapproaches to a distance h suchthatit cuts the
hemisphere (loosing some volume) the volume available to the chains
is reduced and the number of con1047297gurations become Ω(h) which is
less than Ω(infin) For two 1047298at plates Gel is given by the following
expression
Gel
kT = 2m2 ln
X heth THORN
X infineth THORN
= 2m2Rel heth THORN eth19THORN
where Rel
(h) is a geometric function whose form depends on the
segment density distribution It should be stressed that Gel is always
positive and could play a major role in steric stabilisation It becomes
very strong when the separation distance between the particles
becomes comparable to the adsorbed layer thickness δ
Combination of Gmix and Gel with GA (the van der Waals attractive
energy)gives the total free energy of interaction GT (assuming there is
no contribution from any residual electrostatic interaction) ie
GT = Gmix + Gel + GA eth20THORN
A schematic representation of the variation of Gmix Gel GA and GT
with surfacendashsurface separation distance h is shown in Fig 20
Gmix increases very sharply with decrease of h when hb2δ Gel
increases very sharply with decrease of h when hbδ GT versus h
shows a minimum Gmin at separation distances comparable to 2δ
When h b2δ GT shows a rapid increase with decrease in h
The depth of the minimum depends on the Hamaker constant A
the particle radius R and adsorbed layer thickness δ Gmin increases
with increase of A and R At a given A and R Gmin increases with
decrease in δ (ie with decrease of the molecular weight M w of the
stabiliser This is illustrated in Fig 21 which shows the energyndash
distance curves as a function of δR The larger the value of δR the
smaller the value of Gmin In this case the system may approach
thermodynamic stability as is the case with nano-dispersions
6 Emulsions stabilized by polymeric surfactants
The most effective method for emulsion stabilization is to use
polymeric surfactants that stronglyadsorb at the OWor WO interface
Fig 17 Schematic representation of the interaction between particles containing adsorbed polymer layers
Fig 18 Schematic representation of polymer layer overlap
Fig 19 Schematic representation of con1047297gurational entropy loss on approach of a
second particle
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and produce effective steric stabilization against strong 1047298occulation
coalescence and Ostwald ripening [23]
As mentioned above a graft copolymer of the ABn type was
synthesized by grafting several alkyl groups on an inulin (polyfruc-
tose) chain The polymeric surfactant (INUTECregSP1) consists of a
linear polyfructose chain (the stabilizing A chain) and several alkyl
groups (the B chains) that provide multi-anchor attachment to the oil
droplets This polymeric surfactant produces enhanced steric stabili-
zation both in water and high electrolyte concentrations as will be
discussed later
For water-in-oil (WO) emulsions an AndashBndashA block copolymer of
poly (12-hydroxystearic acid) (PHS) (the A chains) and poly (ethylene
oxide) (PEO) (the B chain) PHSndashPEOndashPHS is commercially available
(Arlacel P135 UNIQEMA) The PEO chain (that is soluble in the water
droplets) forms the anchor chain whereas the PHS chains form the
stabilizing chains PHS is highly soluble in most hydrocarbon solvents
and is strongly solvated by its molecules The structure of the PHSndash
PEOndashPHS block copolymer is schematically shown in Fig 22The conformation of the polymeric surfactant at the WO interface
is schematically shown in Fig 23
Emulsions of Isopar Mwater and cyclomethiconewater were
prepared using INUTECregSP1 5050 (vv) OW emulsions were
prepared and the emulsi1047297er concentration was varied from 025 to 2
(wv) based on the oil phase 05 (wv) emulsi1047297erwas suf 1047297cient for
stabilization of these 5050 (vv) emulsions [23]
The emulsions were stored at room temperature and 50 degC and
optical micrographs were taken at intervals of time (for a year) in
order to check the stability Emulsions prepared in water were very
stableshowingno change in droplet size distributionover more than a
year period and this indicated absence of coalescence Any weak
1047298occulation that occurred was reversible and the emulsion could be
redispersed by gentle shaking Fig 24 shows an optical micrograph fora dilute 5050 (vv) emulsion that was stored for 15 and 14 weeks at
50 degC
No change in droplet size was observed after storage for more than
1 year at 50 degC indicating absence of coalescence The same result was
obtained when using different oils Emulsions were also stable against
coalescence in the presence of high electrolyte concentrations (up to
4 mol dmminus3 or ~25 NaCl
The above stability in high electrolyte concentrations is not
observed with polymeric surfactants based on polethylene oxide
The high stability observed using INUTECregSP1 is related to its
strong hydration both in water and in electrolyte solutions The
hydration of inulin (the backbone of HMI) could be assessed using
cloud point measurements A comparison was also made with PEO
with two molecular weights namely 4000 and 20000
Solutionsof PEO 4000 and 20000 showeda systematic decrease of
cloud point with increase in NaCl or MgSO4 concentration In contrast
inulin showed no cloud point up to 4 mol dmminus3 NaCl and up to 1 mol
dmminus3 MgSO4
The above results explain the difference between PEO and inulin
With PEO the chains show dehydration when the NaCl concentration
is increased above 2 mol dmminus3 or 05 mol dmminus3 MgSO4 The inulin
chains remain hydrated at much higher electrolyte concentrations It
seems that the linear polyfructose chains remain strongly hydrated athigh temperature and high electrolyte concentrations
The high emulsion stability obtained whenusingINUTECregSP1 can be
accounted for by the following factors (i) The multi-point attachment
of the polymer by several alkyl chains that are grafted on the backbone
(ii) The strong hydration of the polyfructose ldquoloopsrdquo both in water and
high electrolyte concentrations (χ remains below 05 under these
conditions) (iii) Thehigh volumefraction (concentration) of the loops at
the interface (iv) Enhanced steric stabilization this is the case with
multi-point attachment which produces strong elastic interaction
Evidence for the high stability of the liquid 1047297lm between emulsion
droplets when using INUTECregSP1 was obtained by Exerowa et al [24]
using disjoining pressure measurements This is illustrated in Fig 25
which shows a plot of disjoining pressure versus separation distance
between two emulsion droplets at various electrolyte concentrations
The results show that by increasing the capillary pressure a stable
Newton Black Film (NBF) is obtained at a 1047297lm thickness of sim7 nm
The lack of rupture of the 1047297lm at the highest pressure applied of
45times104 Pa indicate the high stability of the 1047297lm in water and in high
electrolyte concentrations (up to 20 mol dmminus3 NaCl)
The lack of rupture of the NBF up to the highest pressure applied
namely 45times104 Pa clearly indicatesthe high stability of the liquid1047297lm
in the presence of high NaCl concentrations (up to 2 mol dm minus3) This
result is consistent with the high emulsion stability obtained at high
electrolyte concentrations and high temperature Emulsions of Isopar
M-in-water are very stable under such conditions and this could be
accounted for by the high stability of the NBF The droplet size of
5050 OW emulsions prepared using 2 INUTEC regSP1 is in the region
of 1ndash10 μ m This corresponds to a capillary pressure of ~3times104 Pa for
the 1 μ m drops and ~3times103 Pa for the 10 μ m drops These capillarypressures are lower than those to which the NBF have been sub-
jected to and this clearly indicates the high stability obtained against
coalescence in these emulsions
61 WO emulsions stabilized with PHS ndashPEOndashPHS block copolymer
WO emulsions (the oil being Isopar M) were prepared using PHSndash
PEOndashPHS block copolymer at high water volume fractions (N07) The
emulsions have a narrow droplet size distribution with a z -average
radius of 183 nm [25] They also remained 1047298uid up to high water
volume fractions (N06) This could be illustrated from viscosityndash
volume fraction curves as is shown in Fig 26
Fig 20 Energyndashdistance curves for sterically stabilized systems
Fig 21 Variation of Gmin with δR
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The effective volume fraction ϕeff of the emulsions (the core
droplets plus the adsorbed layer) could be calculated from the relative
viscosity and using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth21THORN
Where η r is the relative viscosity ϕp is the maximum packing fraction(sim07) and [ η ] is the intrinsic viscosity that is equal to 25 for hard-
spheres
The calculations based on Eq (22) are shown in Fig 27 (square
symbols) From the effective volume fraction ϕeff and the core volume
fraction ϕ the adsorbed layer thickness could be calculated This was
found to be in the region of 10 nm at ϕ =04 and it decreased with
increase in ϕ
The WO emulsions prepared using the PHSndashPEOndashPHS block
copolymer remained stable both at room temperature and 50 degC
This is consistent with the structure of the block copolymer the B
chain (PEO) is soluble in water and it forms a very strong anchor at the
WO interface The PHS chains (the A chains) provide effective steric
stabilization since the chains are highly soluble in Isopar M and are
strongly solvated by its molecules
7 Suspensions stabilised using polymeric surfactants
There are generally two procedures for preparation of solidliquid
dispersions
(i) Condensation methods build-up of particles from mole-
cular units ie nucleation and growth A special procedure
is the preparation of latexes by emulsion or dispersion
polymerization
(ii) Dispersion methods in this case one starts with preformed
large particles or crystals which are dispersed in the liquid by
using a surfactant (wetting agent) with subsequent breaking up
of the large particles by milling (comminution) to achieve the
desirable particle size distribution A dispersing agent (usuallya
polymeric surfactant) is used for the dispersion process and
subsequent stabilization of the resulting suspension
There are generally two procedures for preparation of latexes
(i) Emulsion polymerization the monomers that are essentially
insoluble in the aqueous medium are emulsi1047297ed using a
surfactant and an initiator is added while heating the system
to produce the polymer particles that are stabilized electro-
statically (when using ionic surfactants) or sterically (when
using non-ionic surfactants)
(ii) Dispersion polymerization the monomers are dissolved in a
solvent in which the resulting polymer particlesare insoluble A
protective colloid (normally a block or graft copolymer) is
added to prevent 1047298occulation of the resulting polymers
particles that are produced on addition of an initiator This
method is usually applied for the production of non-aqueous
latex dispersions and is sometimes referred to as Non-Aqueous
Dispersion Polymerization (NAD)
Surfactants play a crucial role in the process of latex preparation
since they determine the stabilizing ef 1047297ciency and the effectiveness of
the surface active agent ultimately determines the number of particles
Fig 22 Schematic representation of the structure of PHSndashPEOndashPHS block copolymer
Fig 23 Conformation of PHSndash
PEOndash
PHS polymeric surfactant at the WO interface
292 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
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and their size The effectiveness of any surface active agent in
stabilizing the particles is the dominant factor and the number of
micelles formed is relatively unimportant In the NAD process themonomer normally an acrylic is dissolved in a non-aqueous solvent
normally an aliphatic hydrocarbon and an oil soluble initiator and a
stabilizer (to protect the resulting particles from1047298occulation) is added
to the reaction mixture The most successful stabilizers used in NAD
are block and graft copolymers Preformed graft stabilizers based
on poly(12-hydroxy stearic acid) (PHS) are simple to prepare and
effective in NAD polymerization
Dispersion methods are used for the preparation of suspensions of
preformed particles The role of surfactants (or polymers) in the
dispersion process can be analyzed in terms of the three processesinvolved (i) Wetting of the powder by the liquid (ii) Breaking of the
aggregates and agglomerates (iii) Comminution of the resulting
particles and their subsequent stabilization All these processes are
affected by surfactants or polymers which adsorb on the powder
surface thus aiding the wetting of the powder break-up of the
aggregates and agglomerates and subsequent reduction of particle
size by wet milling
71 Polymeric surfactants in emulsion polymerization
Recently the graft copolymer of hydrophobically modi1047297ed inulin
(INUTECreg SP1) has been used in emulsion polymerization of styrene
methyl methacrylate butyl acrylate and several other monomers [26]All lattices were prepared by emulsion polymerisation using potas-
sium persulphate as initiator The z -average particle size was
determined by photon correlation spectroscopy (PCS) and electron
micrographs were also taken
Fig 24 Optical micrographs of OW emulsions stabilized with INUTEC regSP1 stored at
50 degC for 15 weeks (a) and 14 weeks (b)
Fig 25 Variation of disjoining pressure with equivalent 1047297lm thickness at various NaCl concentrations
Fig 26 Viscosityndashvolume fraction for WO emulsion stabilized with PHSndashPEOndashPHS
block copolymer experimental data calculated using Eq (22)
293T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
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Emulsion polymerization of styrene or methylmethacrylate showed
an optimum weight ratio of (INUTEC)Monomer of 00033 for PS and
0001 for PMMA particles The (initiator)(Monomer) ratio was
kept constant at 000125 The monomer conversion was higher than
85 in all cases Latex dispersions of PS reaching 50 and of PMMA
reaching 40 could be obtained using such low concentration of
INUTECregSP1 Fig 27 shows the variation of particle diameter with
monomer concentration
The stability of the latexes was determined by determining the
critical coagulation concentration (CCC) using CaCl2 The CCC was low
(00175ndash005 mol dmminus3) but this was higher than that for the latex
prepared without surfactant Post addition of INUTEC regSP1 resulted in
a large increase in the CCC as is illustrated in Fig 28 which shows log
W minus log C curves (where W is the ratio between the fast 1047298occulation
rate constant to the slow 1047298occulation rate constant referred to as the
stability ratio) at various additions of INUTECregSP1
As with the emulsions the high stability of the latex when using
INUTECregSP1 is due to the strong adsorption of the polymeric
surfactant on the latex particles and formation of strongly hydrated
loops and tails of polyfructose that provide effective steric stabiliza-
tion Evidence for the strong repulsion produced when using
INUTECregSP1 was obtained from Atomic Force Microscopy investiga-
tions [27] whereby the force between hydrophobic glass spheres and
hydrophobic glass plate both containing an adsorbed layer of
INUTECregSP1 was measured as a function of distance of separation
both in water and in the presence of various Na2SO4 concentrations
The results are shown in Figs 29 and 30
Fig 27 Electron micrographs of the latexes
Fig 28 In1047298uence of post addition of INUTECreg
SP1 on the latex stability
Fig 29 Forcendashdistance curves between hydrophobised glass surfaces containing
adsorbed INUTECreg
SP1 in water
294 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
295T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
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Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
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radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 619
The partition function is written in terms of a number of con1047297gura-
tions These were treated as connectedsequencesof segments In each
layer random mixing between segments and solvent molecules was
assumed (mean 1047297eld approximation) Each step in the random walk
was assigned a weighting factor pi
pi consists of three contributions (i) An adsorption energyχs (which
exists only for the segments that are near the surface) (ii) Con1047297gura-
tional entropy of mixing (that exists in each layer) (iii) Segment-solvent
interaction parameterχ (the FloryndashHuggins interaction parameter note
that χ=0 for an athermal solvent χ=05 for a θ-solvent)
From the weighting factors the statistical weight of any chain
conformation can be calculated for a given segment concentration
pro1047297le using a matrix formulation Fig 6 shows typical adsorption
isotherms plotted as surface coverage (in equivalent monolayers)
versus polymer volume fraction ϕ in bulk solution (ϕ was taken to
vary between 0 and 10minus3 which is the normal experimental range)
The results in Fig 6 show the effect of increasing the chain length r
and effect of solvency (athermal solvent with χ=0 and theta solvent
with χ=05) The adsorption energy χs was taken to be the same and
equal to 1kT When r = 1 θ is very small and the adsorption increases linearly
with increase of ϕ (Henrys type isotherm) On the other hand when
r = 10 the isotherm deviates much from a straight line and approaches
a Langmuirian type However when r ge20 high af 1047297nity isotherms are
obtained This implies that the 1047297rst added polymer chains are
completely adsorbed resulting in extremely low polymer concentra-
tion in solution (approaching zero) This explains the irreversibility of
adsorption of polymeric surfactants with r N100 The adsorption
isotherms with r =100 and above are typical of those observed
experimentally for most polymers that are not too polydisperse ie
showing a steep rise followed by a nearly horizontal plateau (which
only increases few percent per decade increase of ϕ) In these dilute
solutions the effect of solvency is most clearly seen with poor
solvents giving the highest adsorbed amounts In good solvents θ is
much smaller and levels off for long chains to attain an adsorption
plateau which is essentially independent of molecular weight
Another point that emerges from the SF theory is the difference in
shape between the experimental and theoretical adsorption isotherms
in thelow concentration region Theexperimentalisotherms areusuallyrounded whereas those predicted fromtheory are1047298at This is accounted
for in terms of the molecular weight distribution (polydispersity) which
is encountered with most practical polymers This effect has been
explained by CohenndashStuart et al [17] With polydisperse polymers the
larger molecular weight fractions adsorb preferentially over the smaller
ones At low polymer concentrations nearly all polymer fractions are
adsorbed leaving a small fraction of the polymer with the lowest
molecular weights in solution As the polymer concentration is
increased the higher molecular weight fractions displace the lower
ones on thesurfacewhichare nowreleasedin solution thusshiftingthe
molecular weight distribution in solution to lower values This process
continues with further increase in polymer concentration leading to
fractionation whereby the higher molecular weight fractions are
adsorbed at the expense of the lower fractions which are released to
the bulk solution However in very concentrated solutions monomers
adsorb preferentially with respect to polymers and short chains with
respect to larger ones This is due to the fact that in this region the
conformational entropy term predominates the free energy disfavour-
ing the adsorption of long chains
According to the SF theory the bound fraction p is high at low
concentration and relatively independent of molecular weight when
r N20 However with increase in surface coverage andor molecular
weight p tends to decrease indicating the formation of larger loops
and tails
The structure of the adsorbed layer is described in terms of the
segment density distribution As an illustration Fig 7 shows some
calculations using the SF theory for loops and tails with r =1000
ϕ= 10minus6 and χ=05 In this example 38 of the segments are in
trains 555 in loops and 65 in tails This theory demonstrates theimportance of tails which dominate the total distribution in the outer
region
3 Experimental techniques for studying polymeric surfactant
adsorption
As mentioned above for full characterization of polymeric sur-
factant adsorption one needs to determine three parameters (i) The
adsorbed amount Γ (mg mminus2 or mol mminus2) as a function of equilibrium
concentration C eq ie the adsorption isotherm (ii) The fraction of
segments in direct contact with the surface p (number of segments in
Fig 5 Schematic representation of a polymer molecule adsorbing on a 1047298at surface mdash
quasi-crystalline lattice with segments 1047297lling layers that are parallel to the surface
(random mixing of segments and solvent molecules in each layer is assumed)
Fig 6 Adsorption isotherms for oligomers and polymers in the dilute region based on
the SF theory Fig 7 Loop tail and total segment pro1047297le according to the SF theory
286 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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The microelectrophoresis technique is based on measurement of
the electrophoretic mobility u of the particles in the presence and
absence of the polymer layer From u one can calculate the zeta
potential ζ using the Huckel equation (which is applicable for small
particles and extended double layers ie κ Rbb1 where κ is the
DebyendashHuckel parameter that is related to the salt concentration
u = 23eeon η
eth15THORN
where ε is the relative permittivity of the medium and ε o is the
permittivity of free space
By measuring ζ of the particles with and without the adsorbed
polymer layer one can obtain the hydrodynamic thickness δh For
accurate measurements one should carry the measurements at various
electrolyte concentrations and extrapolate the results to the plateau
value Several automatic instruments are available for measurement of
the electrophoretic mobility Malvern zeta sizerndashCoulter Delsa sizer and
Broekahven instrument All these instruments are easy to use and the
measurement can be carried out within few minutes
4 Examples of the adsorption isotherms of nonionic polymericsurfactants
Fig 9 shows the adsorption isotherms for PEO with different
molecular weights on PS (at room temperature It can be seen that the
amount adsorbed in mgm-2 increases with increase in the polymer
molecular weight Fig 10 shows the variation of the hydrodynamic
thickness δh with molecular weight M δh shows a linear increase with
log M δh increases with n the number of segments in the chain
according to
δhasympn08 eth16THORN
Fig 11 shows the adsorption isotherms of PVA with various
molecular weights on PS latex (at 25 degC) [19] The polymers were
obtained by fractionation of a commercial sample of PVA with an
average molecular weight of 45000 The polymer also contained 12
vinyl acetate groups As with PEO the amount of adsorption increases
with increase in M The isotherms are also of the high af 1047297nity type Γ at the plateau increases linearly with M 12
The hydrodynamic thickness was determined using PCS and theresults are given below
M 67000 43000 28000 17000 8000
δhnm 255 197 140 98 33
δh seems to increase linearly with increase in the molecular weight
The effect of solvency on adsorption was investigated by increasing
the temperature (the PVA molecules are less soluble at higher
temperature) or addition of electrolyte (KCl) [20] The results are
shown in Figs 12 and 13 for M =65100 As can be seen from Fig 12
increase of temperature results in reduction of solvency of the
mediumfor the chain (due to break down of hydrogen bonds) and this
results in an increase in the amount adsorbed Addition of KCl (which
reduces the solvency of the medium for the chain) results in anincrease in adsorption (as predicted by theory)
The adsorption of block and graft copolymers is more complex
since the intimate structure of the chain determines the extent of
adsorption [18] Randomcopolymers adsorbin an intermediate way to
that of the corresponding homopolymers Blockcopolymers retain the
adsorption preference of the individual blocks The hydrophilic block
(eg PEO) the buoy (previously referred to as the A chain) extends
away from the particle surface into the bulk solution whereas the
hydrophobic anchor block (previously referred to as the B chain) (eg
PS or PPO) provides 1047297rm attachment to the surface Fig 14 shows the
theoretical prediction of diblock copolymer adsorption according to
Fig 9 Adsorption isotherms for PEO on PS
Fig 10 Hydrodynamic thickness of PEO on PS as a function of the molecular weight
Fig 11 Adsorption isotherms of PVA with different molecular weights on polystyrene
latex at 25 degC
Fig 12 In1047298uence of temperature on adsorption
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the Scheutjens and Fleer theory The surface density σ is plotted
versus the fraction of anchor segmentsν A The adsorption depends on
the anchorbuoy composition
The amount of adsorption is higher than for homopolymers and
the adsorbed layer thickness is more extended and dense Fig 15
shows the theoretical prediction for the adsorbed layer thickness δwhich is plotted as a function of ν A For a triblock copolymer AndashBndashA
with two buoy chains (A) and one anchor chain (B) the behaviour is
similar to that of diblock copolymers This is shown in Fig16 for PEOndash
PPOndashPEO block (Pluronic)
5 Interaction between particles containing adsorbed polymeric
surfactant layers steric stabilization
When two particles each with a radius R and containing an
adsorbed polymer layer with a hydrodynamic thickness δh approach
each other to a surfacendashsurface separation distance h that is smaller
than 2 δh the polymer layers interact with each other resulting in two
main situations [21] (i) The polymer chains may overlap with each
other (ii) The polymer layer may undergo some compression In both
cases there will be an increase in the local segment density of the
polymer chains in the interaction region This is schematicallyillustrated in Fig 17 The real situation is perhaps in between the
above two cases ie the polymer chains may undergo some
interpenetration and some compression
Provided the dangling chains (the A chains in AndashB AndashBndashA block or
BAn graft copolymers) are in a good solvent this local increase in
segment density in the interaction zone will result in strong repulsion
as a result of two main effects (i) Increase in the osmotic pressure in
the overlap region as a result of the unfavourable mixing of the
polymer chains when these are in good solvent conditions This is
referred to as osmotic repulsion or mixing interaction and it is
described by a free energy of interaction Gmix (ii) Reduction of the
con1047297gurational entropy of the chains in the interaction zone this
entropy reduction results from the decrease in the volume available
for thechains when these areeither overlapped or compressed This is
referred to as volume restriction interaction entropic or elastic
interaction and it is described by a free energy of interaction Gel
Combination of Gmix and Gel is usually referred to as the steric
interaction free energy Gs ie
Gs = Gmix + Gel eth17THORN
The sign of Gmix depends on the solvency of the medium for the
chains If in a good solvent ie the FloryndashHuggins interaction
parameter χ is less than 05 then Gmix is positive and the mixing
interaction leads to repulsion (see below) In contrast if χN05 (ie the
chains are in a poor solvent condition) Gmix is negative and the
mixing interaction becomes attractive Gel is always positive and
hence in some cases one can produce stable dispersions in a relatively
poor solvent (enhanced steric stabilisation)
51 Mixing interaction Gmix
This results from the unfavourable mixing of the polymer chainswhen these are in a good solvent conditions This is schematically
shown in Fig 18
Consider two spherical particles with the same radius and each
containing an adsorbed polymer layer with thickness δ Before
overlap one can de1047297ne in each polymer layer a chemical potential
for the solvent μ iα and a volume fraction for the polymer in the
layerϕ2 In the overlap region (volume element dV ) the chemical
potential of the solvent is reduced to μ i β This results from the increase
in polymer segment concentration in this overlap region
In the overlap region the chemical potential of the polymer chains
is now higher than in the rest of the layer (with no overlap) This
Fig 13 In1047298uence of addition of KCl on adsorption
Fig 14 Prediction of Adsorption of diblock copolymer
Fig15 Theoretical predictions of the adsorbed layer thickness for a diblock copolymer
Fig 16 Adsorbed amount (mg mminus2) versus fraction of anchor segment for an AndashBndashA
triblock copolymer (PEOndash
PPOndash
PEO)
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amounts to an increase in the osmotic pressure in the overlap region
as a result solvent will diffuse from thebulk to the overlap region thus
separating the particles and hence a strong repulsive energy arises
from this effect The above repulsive energy can be calculated by
considering the free energy of mixing of two polymer solutions as for
example treated by Flory and Krigbaum [22] The free energy of
mixing is given by two terms (i) An entropy term that depends on the
volume fraction of polymer and solvent (ii)An energy term that is
determined by the FloryndashHuggins interaction parameter χ
Using the above theory one can derive an expression for the free
energy of mixing of two polymer layers (assuming a uniform segment
density distribution in each layer) surrounding two spherical particles
as a function of the separation distance h between the particles The
expression for Gmix is
Gmix
kT =
2V 22V 1
m2
2
1
2minusχ
δminus
h
2
2
3R + 2δ + h
2
eth18THORN
k is the Boltzmann constant T is the absolute temperature V 2 is the
molar volume of polymer V 1 is the molar volume of solvent and ν 2 is
the number of polymer chains per unit area
The sign of Gmix depends on the value of the FloryndashHuggins
interaction parameter χ if χb05 Gmix is positive and the interaction
is repulsive if χN05 Gmix is negative and the interaction is attractiveif χ=05 Gmix =0 and this de1047297nes the θ-condition
52 Elastic interaction Gel
Thisarises fromthe lossin con1047297gurational entropy of thechains on
the approach of a second particle As a result of this approach the
volume available for the chains becomes restricted resulting in loss of
the number of con1047297gurations This can be illustrated by considering a
simple molecule represented by a rod that rotates freely in a
hemisphere across a surface (Fig 19) When the two surfaces are
separated byan in1047297nitedistanceinfin thenumber of con1047297gurations of the
rod is Ω(infin) which is proportional to the volume of the hemisphere
When a secondparticleapproaches to a distance h suchthatit cuts the
hemisphere (loosing some volume) the volume available to the chains
is reduced and the number of con1047297gurations become Ω(h) which is
less than Ω(infin) For two 1047298at plates Gel is given by the following
expression
Gel
kT = 2m2 ln
X heth THORN
X infineth THORN
= 2m2Rel heth THORN eth19THORN
where Rel
(h) is a geometric function whose form depends on the
segment density distribution It should be stressed that Gel is always
positive and could play a major role in steric stabilisation It becomes
very strong when the separation distance between the particles
becomes comparable to the adsorbed layer thickness δ
Combination of Gmix and Gel with GA (the van der Waals attractive
energy)gives the total free energy of interaction GT (assuming there is
no contribution from any residual electrostatic interaction) ie
GT = Gmix + Gel + GA eth20THORN
A schematic representation of the variation of Gmix Gel GA and GT
with surfacendashsurface separation distance h is shown in Fig 20
Gmix increases very sharply with decrease of h when hb2δ Gel
increases very sharply with decrease of h when hbδ GT versus h
shows a minimum Gmin at separation distances comparable to 2δ
When h b2δ GT shows a rapid increase with decrease in h
The depth of the minimum depends on the Hamaker constant A
the particle radius R and adsorbed layer thickness δ Gmin increases
with increase of A and R At a given A and R Gmin increases with
decrease in δ (ie with decrease of the molecular weight M w of the
stabiliser This is illustrated in Fig 21 which shows the energyndash
distance curves as a function of δR The larger the value of δR the
smaller the value of Gmin In this case the system may approach
thermodynamic stability as is the case with nano-dispersions
6 Emulsions stabilized by polymeric surfactants
The most effective method for emulsion stabilization is to use
polymeric surfactants that stronglyadsorb at the OWor WO interface
Fig 17 Schematic representation of the interaction between particles containing adsorbed polymer layers
Fig 18 Schematic representation of polymer layer overlap
Fig 19 Schematic representation of con1047297gurational entropy loss on approach of a
second particle
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and produce effective steric stabilization against strong 1047298occulation
coalescence and Ostwald ripening [23]
As mentioned above a graft copolymer of the ABn type was
synthesized by grafting several alkyl groups on an inulin (polyfruc-
tose) chain The polymeric surfactant (INUTECregSP1) consists of a
linear polyfructose chain (the stabilizing A chain) and several alkyl
groups (the B chains) that provide multi-anchor attachment to the oil
droplets This polymeric surfactant produces enhanced steric stabili-
zation both in water and high electrolyte concentrations as will be
discussed later
For water-in-oil (WO) emulsions an AndashBndashA block copolymer of
poly (12-hydroxystearic acid) (PHS) (the A chains) and poly (ethylene
oxide) (PEO) (the B chain) PHSndashPEOndashPHS is commercially available
(Arlacel P135 UNIQEMA) The PEO chain (that is soluble in the water
droplets) forms the anchor chain whereas the PHS chains form the
stabilizing chains PHS is highly soluble in most hydrocarbon solvents
and is strongly solvated by its molecules The structure of the PHSndash
PEOndashPHS block copolymer is schematically shown in Fig 22The conformation of the polymeric surfactant at the WO interface
is schematically shown in Fig 23
Emulsions of Isopar Mwater and cyclomethiconewater were
prepared using INUTECregSP1 5050 (vv) OW emulsions were
prepared and the emulsi1047297er concentration was varied from 025 to 2
(wv) based on the oil phase 05 (wv) emulsi1047297erwas suf 1047297cient for
stabilization of these 5050 (vv) emulsions [23]
The emulsions were stored at room temperature and 50 degC and
optical micrographs were taken at intervals of time (for a year) in
order to check the stability Emulsions prepared in water were very
stableshowingno change in droplet size distributionover more than a
year period and this indicated absence of coalescence Any weak
1047298occulation that occurred was reversible and the emulsion could be
redispersed by gentle shaking Fig 24 shows an optical micrograph fora dilute 5050 (vv) emulsion that was stored for 15 and 14 weeks at
50 degC
No change in droplet size was observed after storage for more than
1 year at 50 degC indicating absence of coalescence The same result was
obtained when using different oils Emulsions were also stable against
coalescence in the presence of high electrolyte concentrations (up to
4 mol dmminus3 or ~25 NaCl
The above stability in high electrolyte concentrations is not
observed with polymeric surfactants based on polethylene oxide
The high stability observed using INUTECregSP1 is related to its
strong hydration both in water and in electrolyte solutions The
hydration of inulin (the backbone of HMI) could be assessed using
cloud point measurements A comparison was also made with PEO
with two molecular weights namely 4000 and 20000
Solutionsof PEO 4000 and 20000 showeda systematic decrease of
cloud point with increase in NaCl or MgSO4 concentration In contrast
inulin showed no cloud point up to 4 mol dmminus3 NaCl and up to 1 mol
dmminus3 MgSO4
The above results explain the difference between PEO and inulin
With PEO the chains show dehydration when the NaCl concentration
is increased above 2 mol dmminus3 or 05 mol dmminus3 MgSO4 The inulin
chains remain hydrated at much higher electrolyte concentrations It
seems that the linear polyfructose chains remain strongly hydrated athigh temperature and high electrolyte concentrations
The high emulsion stability obtained whenusingINUTECregSP1 can be
accounted for by the following factors (i) The multi-point attachment
of the polymer by several alkyl chains that are grafted on the backbone
(ii) The strong hydration of the polyfructose ldquoloopsrdquo both in water and
high electrolyte concentrations (χ remains below 05 under these
conditions) (iii) Thehigh volumefraction (concentration) of the loops at
the interface (iv) Enhanced steric stabilization this is the case with
multi-point attachment which produces strong elastic interaction
Evidence for the high stability of the liquid 1047297lm between emulsion
droplets when using INUTECregSP1 was obtained by Exerowa et al [24]
using disjoining pressure measurements This is illustrated in Fig 25
which shows a plot of disjoining pressure versus separation distance
between two emulsion droplets at various electrolyte concentrations
The results show that by increasing the capillary pressure a stable
Newton Black Film (NBF) is obtained at a 1047297lm thickness of sim7 nm
The lack of rupture of the 1047297lm at the highest pressure applied of
45times104 Pa indicate the high stability of the 1047297lm in water and in high
electrolyte concentrations (up to 20 mol dmminus3 NaCl)
The lack of rupture of the NBF up to the highest pressure applied
namely 45times104 Pa clearly indicatesthe high stability of the liquid1047297lm
in the presence of high NaCl concentrations (up to 2 mol dm minus3) This
result is consistent with the high emulsion stability obtained at high
electrolyte concentrations and high temperature Emulsions of Isopar
M-in-water are very stable under such conditions and this could be
accounted for by the high stability of the NBF The droplet size of
5050 OW emulsions prepared using 2 INUTEC regSP1 is in the region
of 1ndash10 μ m This corresponds to a capillary pressure of ~3times104 Pa for
the 1 μ m drops and ~3times103 Pa for the 10 μ m drops These capillarypressures are lower than those to which the NBF have been sub-
jected to and this clearly indicates the high stability obtained against
coalescence in these emulsions
61 WO emulsions stabilized with PHS ndashPEOndashPHS block copolymer
WO emulsions (the oil being Isopar M) were prepared using PHSndash
PEOndashPHS block copolymer at high water volume fractions (N07) The
emulsions have a narrow droplet size distribution with a z -average
radius of 183 nm [25] They also remained 1047298uid up to high water
volume fractions (N06) This could be illustrated from viscosityndash
volume fraction curves as is shown in Fig 26
Fig 20 Energyndashdistance curves for sterically stabilized systems
Fig 21 Variation of Gmin with δR
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The effective volume fraction ϕeff of the emulsions (the core
droplets plus the adsorbed layer) could be calculated from the relative
viscosity and using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth21THORN
Where η r is the relative viscosity ϕp is the maximum packing fraction(sim07) and [ η ] is the intrinsic viscosity that is equal to 25 for hard-
spheres
The calculations based on Eq (22) are shown in Fig 27 (square
symbols) From the effective volume fraction ϕeff and the core volume
fraction ϕ the adsorbed layer thickness could be calculated This was
found to be in the region of 10 nm at ϕ =04 and it decreased with
increase in ϕ
The WO emulsions prepared using the PHSndashPEOndashPHS block
copolymer remained stable both at room temperature and 50 degC
This is consistent with the structure of the block copolymer the B
chain (PEO) is soluble in water and it forms a very strong anchor at the
WO interface The PHS chains (the A chains) provide effective steric
stabilization since the chains are highly soluble in Isopar M and are
strongly solvated by its molecules
7 Suspensions stabilised using polymeric surfactants
There are generally two procedures for preparation of solidliquid
dispersions
(i) Condensation methods build-up of particles from mole-
cular units ie nucleation and growth A special procedure
is the preparation of latexes by emulsion or dispersion
polymerization
(ii) Dispersion methods in this case one starts with preformed
large particles or crystals which are dispersed in the liquid by
using a surfactant (wetting agent) with subsequent breaking up
of the large particles by milling (comminution) to achieve the
desirable particle size distribution A dispersing agent (usuallya
polymeric surfactant) is used for the dispersion process and
subsequent stabilization of the resulting suspension
There are generally two procedures for preparation of latexes
(i) Emulsion polymerization the monomers that are essentially
insoluble in the aqueous medium are emulsi1047297ed using a
surfactant and an initiator is added while heating the system
to produce the polymer particles that are stabilized electro-
statically (when using ionic surfactants) or sterically (when
using non-ionic surfactants)
(ii) Dispersion polymerization the monomers are dissolved in a
solvent in which the resulting polymer particlesare insoluble A
protective colloid (normally a block or graft copolymer) is
added to prevent 1047298occulation of the resulting polymers
particles that are produced on addition of an initiator This
method is usually applied for the production of non-aqueous
latex dispersions and is sometimes referred to as Non-Aqueous
Dispersion Polymerization (NAD)
Surfactants play a crucial role in the process of latex preparation
since they determine the stabilizing ef 1047297ciency and the effectiveness of
the surface active agent ultimately determines the number of particles
Fig 22 Schematic representation of the structure of PHSndashPEOndashPHS block copolymer
Fig 23 Conformation of PHSndash
PEOndash
PHS polymeric surfactant at the WO interface
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and their size The effectiveness of any surface active agent in
stabilizing the particles is the dominant factor and the number of
micelles formed is relatively unimportant In the NAD process themonomer normally an acrylic is dissolved in a non-aqueous solvent
normally an aliphatic hydrocarbon and an oil soluble initiator and a
stabilizer (to protect the resulting particles from1047298occulation) is added
to the reaction mixture The most successful stabilizers used in NAD
are block and graft copolymers Preformed graft stabilizers based
on poly(12-hydroxy stearic acid) (PHS) are simple to prepare and
effective in NAD polymerization
Dispersion methods are used for the preparation of suspensions of
preformed particles The role of surfactants (or polymers) in the
dispersion process can be analyzed in terms of the three processesinvolved (i) Wetting of the powder by the liquid (ii) Breaking of the
aggregates and agglomerates (iii) Comminution of the resulting
particles and their subsequent stabilization All these processes are
affected by surfactants or polymers which adsorb on the powder
surface thus aiding the wetting of the powder break-up of the
aggregates and agglomerates and subsequent reduction of particle
size by wet milling
71 Polymeric surfactants in emulsion polymerization
Recently the graft copolymer of hydrophobically modi1047297ed inulin
(INUTECreg SP1) has been used in emulsion polymerization of styrene
methyl methacrylate butyl acrylate and several other monomers [26]All lattices were prepared by emulsion polymerisation using potas-
sium persulphate as initiator The z -average particle size was
determined by photon correlation spectroscopy (PCS) and electron
micrographs were also taken
Fig 24 Optical micrographs of OW emulsions stabilized with INUTEC regSP1 stored at
50 degC for 15 weeks (a) and 14 weeks (b)
Fig 25 Variation of disjoining pressure with equivalent 1047297lm thickness at various NaCl concentrations
Fig 26 Viscosityndashvolume fraction for WO emulsion stabilized with PHSndashPEOndashPHS
block copolymer experimental data calculated using Eq (22)
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Emulsion polymerization of styrene or methylmethacrylate showed
an optimum weight ratio of (INUTEC)Monomer of 00033 for PS and
0001 for PMMA particles The (initiator)(Monomer) ratio was
kept constant at 000125 The monomer conversion was higher than
85 in all cases Latex dispersions of PS reaching 50 and of PMMA
reaching 40 could be obtained using such low concentration of
INUTECregSP1 Fig 27 shows the variation of particle diameter with
monomer concentration
The stability of the latexes was determined by determining the
critical coagulation concentration (CCC) using CaCl2 The CCC was low
(00175ndash005 mol dmminus3) but this was higher than that for the latex
prepared without surfactant Post addition of INUTEC regSP1 resulted in
a large increase in the CCC as is illustrated in Fig 28 which shows log
W minus log C curves (where W is the ratio between the fast 1047298occulation
rate constant to the slow 1047298occulation rate constant referred to as the
stability ratio) at various additions of INUTECregSP1
As with the emulsions the high stability of the latex when using
INUTECregSP1 is due to the strong adsorption of the polymeric
surfactant on the latex particles and formation of strongly hydrated
loops and tails of polyfructose that provide effective steric stabiliza-
tion Evidence for the strong repulsion produced when using
INUTECregSP1 was obtained from Atomic Force Microscopy investiga-
tions [27] whereby the force between hydrophobic glass spheres and
hydrophobic glass plate both containing an adsorbed layer of
INUTECregSP1 was measured as a function of distance of separation
both in water and in the presence of various Na2SO4 concentrations
The results are shown in Figs 29 and 30
Fig 27 Electron micrographs of the latexes
Fig 28 In1047298uence of post addition of INUTECreg
SP1 on the latex stability
Fig 29 Forcendashdistance curves between hydrophobised glass surfaces containing
adsorbed INUTECreg
SP1 in water
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711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
295T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
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radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 719
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 819
The microelectrophoresis technique is based on measurement of
the electrophoretic mobility u of the particles in the presence and
absence of the polymer layer From u one can calculate the zeta
potential ζ using the Huckel equation (which is applicable for small
particles and extended double layers ie κ Rbb1 where κ is the
DebyendashHuckel parameter that is related to the salt concentration
u = 23eeon η
eth15THORN
where ε is the relative permittivity of the medium and ε o is the
permittivity of free space
By measuring ζ of the particles with and without the adsorbed
polymer layer one can obtain the hydrodynamic thickness δh For
accurate measurements one should carry the measurements at various
electrolyte concentrations and extrapolate the results to the plateau
value Several automatic instruments are available for measurement of
the electrophoretic mobility Malvern zeta sizerndashCoulter Delsa sizer and
Broekahven instrument All these instruments are easy to use and the
measurement can be carried out within few minutes
4 Examples of the adsorption isotherms of nonionic polymericsurfactants
Fig 9 shows the adsorption isotherms for PEO with different
molecular weights on PS (at room temperature It can be seen that the
amount adsorbed in mgm-2 increases with increase in the polymer
molecular weight Fig 10 shows the variation of the hydrodynamic
thickness δh with molecular weight M δh shows a linear increase with
log M δh increases with n the number of segments in the chain
according to
δhasympn08 eth16THORN
Fig 11 shows the adsorption isotherms of PVA with various
molecular weights on PS latex (at 25 degC) [19] The polymers were
obtained by fractionation of a commercial sample of PVA with an
average molecular weight of 45000 The polymer also contained 12
vinyl acetate groups As with PEO the amount of adsorption increases
with increase in M The isotherms are also of the high af 1047297nity type Γ at the plateau increases linearly with M 12
The hydrodynamic thickness was determined using PCS and theresults are given below
M 67000 43000 28000 17000 8000
δhnm 255 197 140 98 33
δh seems to increase linearly with increase in the molecular weight
The effect of solvency on adsorption was investigated by increasing
the temperature (the PVA molecules are less soluble at higher
temperature) or addition of electrolyte (KCl) [20] The results are
shown in Figs 12 and 13 for M =65100 As can be seen from Fig 12
increase of temperature results in reduction of solvency of the
mediumfor the chain (due to break down of hydrogen bonds) and this
results in an increase in the amount adsorbed Addition of KCl (which
reduces the solvency of the medium for the chain) results in anincrease in adsorption (as predicted by theory)
The adsorption of block and graft copolymers is more complex
since the intimate structure of the chain determines the extent of
adsorption [18] Randomcopolymers adsorbin an intermediate way to
that of the corresponding homopolymers Blockcopolymers retain the
adsorption preference of the individual blocks The hydrophilic block
(eg PEO) the buoy (previously referred to as the A chain) extends
away from the particle surface into the bulk solution whereas the
hydrophobic anchor block (previously referred to as the B chain) (eg
PS or PPO) provides 1047297rm attachment to the surface Fig 14 shows the
theoretical prediction of diblock copolymer adsorption according to
Fig 9 Adsorption isotherms for PEO on PS
Fig 10 Hydrodynamic thickness of PEO on PS as a function of the molecular weight
Fig 11 Adsorption isotherms of PVA with different molecular weights on polystyrene
latex at 25 degC
Fig 12 In1047298uence of temperature on adsorption
288 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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the Scheutjens and Fleer theory The surface density σ is plotted
versus the fraction of anchor segmentsν A The adsorption depends on
the anchorbuoy composition
The amount of adsorption is higher than for homopolymers and
the adsorbed layer thickness is more extended and dense Fig 15
shows the theoretical prediction for the adsorbed layer thickness δwhich is plotted as a function of ν A For a triblock copolymer AndashBndashA
with two buoy chains (A) and one anchor chain (B) the behaviour is
similar to that of diblock copolymers This is shown in Fig16 for PEOndash
PPOndashPEO block (Pluronic)
5 Interaction between particles containing adsorbed polymeric
surfactant layers steric stabilization
When two particles each with a radius R and containing an
adsorbed polymer layer with a hydrodynamic thickness δh approach
each other to a surfacendashsurface separation distance h that is smaller
than 2 δh the polymer layers interact with each other resulting in two
main situations [21] (i) The polymer chains may overlap with each
other (ii) The polymer layer may undergo some compression In both
cases there will be an increase in the local segment density of the
polymer chains in the interaction region This is schematicallyillustrated in Fig 17 The real situation is perhaps in between the
above two cases ie the polymer chains may undergo some
interpenetration and some compression
Provided the dangling chains (the A chains in AndashB AndashBndashA block or
BAn graft copolymers) are in a good solvent this local increase in
segment density in the interaction zone will result in strong repulsion
as a result of two main effects (i) Increase in the osmotic pressure in
the overlap region as a result of the unfavourable mixing of the
polymer chains when these are in good solvent conditions This is
referred to as osmotic repulsion or mixing interaction and it is
described by a free energy of interaction Gmix (ii) Reduction of the
con1047297gurational entropy of the chains in the interaction zone this
entropy reduction results from the decrease in the volume available
for thechains when these areeither overlapped or compressed This is
referred to as volume restriction interaction entropic or elastic
interaction and it is described by a free energy of interaction Gel
Combination of Gmix and Gel is usually referred to as the steric
interaction free energy Gs ie
Gs = Gmix + Gel eth17THORN
The sign of Gmix depends on the solvency of the medium for the
chains If in a good solvent ie the FloryndashHuggins interaction
parameter χ is less than 05 then Gmix is positive and the mixing
interaction leads to repulsion (see below) In contrast if χN05 (ie the
chains are in a poor solvent condition) Gmix is negative and the
mixing interaction becomes attractive Gel is always positive and
hence in some cases one can produce stable dispersions in a relatively
poor solvent (enhanced steric stabilisation)
51 Mixing interaction Gmix
This results from the unfavourable mixing of the polymer chainswhen these are in a good solvent conditions This is schematically
shown in Fig 18
Consider two spherical particles with the same radius and each
containing an adsorbed polymer layer with thickness δ Before
overlap one can de1047297ne in each polymer layer a chemical potential
for the solvent μ iα and a volume fraction for the polymer in the
layerϕ2 In the overlap region (volume element dV ) the chemical
potential of the solvent is reduced to μ i β This results from the increase
in polymer segment concentration in this overlap region
In the overlap region the chemical potential of the polymer chains
is now higher than in the rest of the layer (with no overlap) This
Fig 13 In1047298uence of addition of KCl on adsorption
Fig 14 Prediction of Adsorption of diblock copolymer
Fig15 Theoretical predictions of the adsorbed layer thickness for a diblock copolymer
Fig 16 Adsorbed amount (mg mminus2) versus fraction of anchor segment for an AndashBndashA
triblock copolymer (PEOndash
PPOndash
PEO)
289T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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amounts to an increase in the osmotic pressure in the overlap region
as a result solvent will diffuse from thebulk to the overlap region thus
separating the particles and hence a strong repulsive energy arises
from this effect The above repulsive energy can be calculated by
considering the free energy of mixing of two polymer solutions as for
example treated by Flory and Krigbaum [22] The free energy of
mixing is given by two terms (i) An entropy term that depends on the
volume fraction of polymer and solvent (ii)An energy term that is
determined by the FloryndashHuggins interaction parameter χ
Using the above theory one can derive an expression for the free
energy of mixing of two polymer layers (assuming a uniform segment
density distribution in each layer) surrounding two spherical particles
as a function of the separation distance h between the particles The
expression for Gmix is
Gmix
kT =
2V 22V 1
m2
2
1
2minusχ
δminus
h
2
2
3R + 2δ + h
2
eth18THORN
k is the Boltzmann constant T is the absolute temperature V 2 is the
molar volume of polymer V 1 is the molar volume of solvent and ν 2 is
the number of polymer chains per unit area
The sign of Gmix depends on the value of the FloryndashHuggins
interaction parameter χ if χb05 Gmix is positive and the interaction
is repulsive if χN05 Gmix is negative and the interaction is attractiveif χ=05 Gmix =0 and this de1047297nes the θ-condition
52 Elastic interaction Gel
Thisarises fromthe lossin con1047297gurational entropy of thechains on
the approach of a second particle As a result of this approach the
volume available for the chains becomes restricted resulting in loss of
the number of con1047297gurations This can be illustrated by considering a
simple molecule represented by a rod that rotates freely in a
hemisphere across a surface (Fig 19) When the two surfaces are
separated byan in1047297nitedistanceinfin thenumber of con1047297gurations of the
rod is Ω(infin) which is proportional to the volume of the hemisphere
When a secondparticleapproaches to a distance h suchthatit cuts the
hemisphere (loosing some volume) the volume available to the chains
is reduced and the number of con1047297gurations become Ω(h) which is
less than Ω(infin) For two 1047298at plates Gel is given by the following
expression
Gel
kT = 2m2 ln
X heth THORN
X infineth THORN
= 2m2Rel heth THORN eth19THORN
where Rel
(h) is a geometric function whose form depends on the
segment density distribution It should be stressed that Gel is always
positive and could play a major role in steric stabilisation It becomes
very strong when the separation distance between the particles
becomes comparable to the adsorbed layer thickness δ
Combination of Gmix and Gel with GA (the van der Waals attractive
energy)gives the total free energy of interaction GT (assuming there is
no contribution from any residual electrostatic interaction) ie
GT = Gmix + Gel + GA eth20THORN
A schematic representation of the variation of Gmix Gel GA and GT
with surfacendashsurface separation distance h is shown in Fig 20
Gmix increases very sharply with decrease of h when hb2δ Gel
increases very sharply with decrease of h when hbδ GT versus h
shows a minimum Gmin at separation distances comparable to 2δ
When h b2δ GT shows a rapid increase with decrease in h
The depth of the minimum depends on the Hamaker constant A
the particle radius R and adsorbed layer thickness δ Gmin increases
with increase of A and R At a given A and R Gmin increases with
decrease in δ (ie with decrease of the molecular weight M w of the
stabiliser This is illustrated in Fig 21 which shows the energyndash
distance curves as a function of δR The larger the value of δR the
smaller the value of Gmin In this case the system may approach
thermodynamic stability as is the case with nano-dispersions
6 Emulsions stabilized by polymeric surfactants
The most effective method for emulsion stabilization is to use
polymeric surfactants that stronglyadsorb at the OWor WO interface
Fig 17 Schematic representation of the interaction between particles containing adsorbed polymer layers
Fig 18 Schematic representation of polymer layer overlap
Fig 19 Schematic representation of con1047297gurational entropy loss on approach of a
second particle
290 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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and produce effective steric stabilization against strong 1047298occulation
coalescence and Ostwald ripening [23]
As mentioned above a graft copolymer of the ABn type was
synthesized by grafting several alkyl groups on an inulin (polyfruc-
tose) chain The polymeric surfactant (INUTECregSP1) consists of a
linear polyfructose chain (the stabilizing A chain) and several alkyl
groups (the B chains) that provide multi-anchor attachment to the oil
droplets This polymeric surfactant produces enhanced steric stabili-
zation both in water and high electrolyte concentrations as will be
discussed later
For water-in-oil (WO) emulsions an AndashBndashA block copolymer of
poly (12-hydroxystearic acid) (PHS) (the A chains) and poly (ethylene
oxide) (PEO) (the B chain) PHSndashPEOndashPHS is commercially available
(Arlacel P135 UNIQEMA) The PEO chain (that is soluble in the water
droplets) forms the anchor chain whereas the PHS chains form the
stabilizing chains PHS is highly soluble in most hydrocarbon solvents
and is strongly solvated by its molecules The structure of the PHSndash
PEOndashPHS block copolymer is schematically shown in Fig 22The conformation of the polymeric surfactant at the WO interface
is schematically shown in Fig 23
Emulsions of Isopar Mwater and cyclomethiconewater were
prepared using INUTECregSP1 5050 (vv) OW emulsions were
prepared and the emulsi1047297er concentration was varied from 025 to 2
(wv) based on the oil phase 05 (wv) emulsi1047297erwas suf 1047297cient for
stabilization of these 5050 (vv) emulsions [23]
The emulsions were stored at room temperature and 50 degC and
optical micrographs were taken at intervals of time (for a year) in
order to check the stability Emulsions prepared in water were very
stableshowingno change in droplet size distributionover more than a
year period and this indicated absence of coalescence Any weak
1047298occulation that occurred was reversible and the emulsion could be
redispersed by gentle shaking Fig 24 shows an optical micrograph fora dilute 5050 (vv) emulsion that was stored for 15 and 14 weeks at
50 degC
No change in droplet size was observed after storage for more than
1 year at 50 degC indicating absence of coalescence The same result was
obtained when using different oils Emulsions were also stable against
coalescence in the presence of high electrolyte concentrations (up to
4 mol dmminus3 or ~25 NaCl
The above stability in high electrolyte concentrations is not
observed with polymeric surfactants based on polethylene oxide
The high stability observed using INUTECregSP1 is related to its
strong hydration both in water and in electrolyte solutions The
hydration of inulin (the backbone of HMI) could be assessed using
cloud point measurements A comparison was also made with PEO
with two molecular weights namely 4000 and 20000
Solutionsof PEO 4000 and 20000 showeda systematic decrease of
cloud point with increase in NaCl or MgSO4 concentration In contrast
inulin showed no cloud point up to 4 mol dmminus3 NaCl and up to 1 mol
dmminus3 MgSO4
The above results explain the difference between PEO and inulin
With PEO the chains show dehydration when the NaCl concentration
is increased above 2 mol dmminus3 or 05 mol dmminus3 MgSO4 The inulin
chains remain hydrated at much higher electrolyte concentrations It
seems that the linear polyfructose chains remain strongly hydrated athigh temperature and high electrolyte concentrations
The high emulsion stability obtained whenusingINUTECregSP1 can be
accounted for by the following factors (i) The multi-point attachment
of the polymer by several alkyl chains that are grafted on the backbone
(ii) The strong hydration of the polyfructose ldquoloopsrdquo both in water and
high electrolyte concentrations (χ remains below 05 under these
conditions) (iii) Thehigh volumefraction (concentration) of the loops at
the interface (iv) Enhanced steric stabilization this is the case with
multi-point attachment which produces strong elastic interaction
Evidence for the high stability of the liquid 1047297lm between emulsion
droplets when using INUTECregSP1 was obtained by Exerowa et al [24]
using disjoining pressure measurements This is illustrated in Fig 25
which shows a plot of disjoining pressure versus separation distance
between two emulsion droplets at various electrolyte concentrations
The results show that by increasing the capillary pressure a stable
Newton Black Film (NBF) is obtained at a 1047297lm thickness of sim7 nm
The lack of rupture of the 1047297lm at the highest pressure applied of
45times104 Pa indicate the high stability of the 1047297lm in water and in high
electrolyte concentrations (up to 20 mol dmminus3 NaCl)
The lack of rupture of the NBF up to the highest pressure applied
namely 45times104 Pa clearly indicatesthe high stability of the liquid1047297lm
in the presence of high NaCl concentrations (up to 2 mol dm minus3) This
result is consistent with the high emulsion stability obtained at high
electrolyte concentrations and high temperature Emulsions of Isopar
M-in-water are very stable under such conditions and this could be
accounted for by the high stability of the NBF The droplet size of
5050 OW emulsions prepared using 2 INUTEC regSP1 is in the region
of 1ndash10 μ m This corresponds to a capillary pressure of ~3times104 Pa for
the 1 μ m drops and ~3times103 Pa for the 10 μ m drops These capillarypressures are lower than those to which the NBF have been sub-
jected to and this clearly indicates the high stability obtained against
coalescence in these emulsions
61 WO emulsions stabilized with PHS ndashPEOndashPHS block copolymer
WO emulsions (the oil being Isopar M) were prepared using PHSndash
PEOndashPHS block copolymer at high water volume fractions (N07) The
emulsions have a narrow droplet size distribution with a z -average
radius of 183 nm [25] They also remained 1047298uid up to high water
volume fractions (N06) This could be illustrated from viscosityndash
volume fraction curves as is shown in Fig 26
Fig 20 Energyndashdistance curves for sterically stabilized systems
Fig 21 Variation of Gmin with δR
291T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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The effective volume fraction ϕeff of the emulsions (the core
droplets plus the adsorbed layer) could be calculated from the relative
viscosity and using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth21THORN
Where η r is the relative viscosity ϕp is the maximum packing fraction(sim07) and [ η ] is the intrinsic viscosity that is equal to 25 for hard-
spheres
The calculations based on Eq (22) are shown in Fig 27 (square
symbols) From the effective volume fraction ϕeff and the core volume
fraction ϕ the adsorbed layer thickness could be calculated This was
found to be in the region of 10 nm at ϕ =04 and it decreased with
increase in ϕ
The WO emulsions prepared using the PHSndashPEOndashPHS block
copolymer remained stable both at room temperature and 50 degC
This is consistent with the structure of the block copolymer the B
chain (PEO) is soluble in water and it forms a very strong anchor at the
WO interface The PHS chains (the A chains) provide effective steric
stabilization since the chains are highly soluble in Isopar M and are
strongly solvated by its molecules
7 Suspensions stabilised using polymeric surfactants
There are generally two procedures for preparation of solidliquid
dispersions
(i) Condensation methods build-up of particles from mole-
cular units ie nucleation and growth A special procedure
is the preparation of latexes by emulsion or dispersion
polymerization
(ii) Dispersion methods in this case one starts with preformed
large particles or crystals which are dispersed in the liquid by
using a surfactant (wetting agent) with subsequent breaking up
of the large particles by milling (comminution) to achieve the
desirable particle size distribution A dispersing agent (usuallya
polymeric surfactant) is used for the dispersion process and
subsequent stabilization of the resulting suspension
There are generally two procedures for preparation of latexes
(i) Emulsion polymerization the monomers that are essentially
insoluble in the aqueous medium are emulsi1047297ed using a
surfactant and an initiator is added while heating the system
to produce the polymer particles that are stabilized electro-
statically (when using ionic surfactants) or sterically (when
using non-ionic surfactants)
(ii) Dispersion polymerization the monomers are dissolved in a
solvent in which the resulting polymer particlesare insoluble A
protective colloid (normally a block or graft copolymer) is
added to prevent 1047298occulation of the resulting polymers
particles that are produced on addition of an initiator This
method is usually applied for the production of non-aqueous
latex dispersions and is sometimes referred to as Non-Aqueous
Dispersion Polymerization (NAD)
Surfactants play a crucial role in the process of latex preparation
since they determine the stabilizing ef 1047297ciency and the effectiveness of
the surface active agent ultimately determines the number of particles
Fig 22 Schematic representation of the structure of PHSndashPEOndashPHS block copolymer
Fig 23 Conformation of PHSndash
PEOndash
PHS polymeric surfactant at the WO interface
292 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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and their size The effectiveness of any surface active agent in
stabilizing the particles is the dominant factor and the number of
micelles formed is relatively unimportant In the NAD process themonomer normally an acrylic is dissolved in a non-aqueous solvent
normally an aliphatic hydrocarbon and an oil soluble initiator and a
stabilizer (to protect the resulting particles from1047298occulation) is added
to the reaction mixture The most successful stabilizers used in NAD
are block and graft copolymers Preformed graft stabilizers based
on poly(12-hydroxy stearic acid) (PHS) are simple to prepare and
effective in NAD polymerization
Dispersion methods are used for the preparation of suspensions of
preformed particles The role of surfactants (or polymers) in the
dispersion process can be analyzed in terms of the three processesinvolved (i) Wetting of the powder by the liquid (ii) Breaking of the
aggregates and agglomerates (iii) Comminution of the resulting
particles and their subsequent stabilization All these processes are
affected by surfactants or polymers which adsorb on the powder
surface thus aiding the wetting of the powder break-up of the
aggregates and agglomerates and subsequent reduction of particle
size by wet milling
71 Polymeric surfactants in emulsion polymerization
Recently the graft copolymer of hydrophobically modi1047297ed inulin
(INUTECreg SP1) has been used in emulsion polymerization of styrene
methyl methacrylate butyl acrylate and several other monomers [26]All lattices were prepared by emulsion polymerisation using potas-
sium persulphate as initiator The z -average particle size was
determined by photon correlation spectroscopy (PCS) and electron
micrographs were also taken
Fig 24 Optical micrographs of OW emulsions stabilized with INUTEC regSP1 stored at
50 degC for 15 weeks (a) and 14 weeks (b)
Fig 25 Variation of disjoining pressure with equivalent 1047297lm thickness at various NaCl concentrations
Fig 26 Viscosityndashvolume fraction for WO emulsion stabilized with PHSndashPEOndashPHS
block copolymer experimental data calculated using Eq (22)
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Emulsion polymerization of styrene or methylmethacrylate showed
an optimum weight ratio of (INUTEC)Monomer of 00033 for PS and
0001 for PMMA particles The (initiator)(Monomer) ratio was
kept constant at 000125 The monomer conversion was higher than
85 in all cases Latex dispersions of PS reaching 50 and of PMMA
reaching 40 could be obtained using such low concentration of
INUTECregSP1 Fig 27 shows the variation of particle diameter with
monomer concentration
The stability of the latexes was determined by determining the
critical coagulation concentration (CCC) using CaCl2 The CCC was low
(00175ndash005 mol dmminus3) but this was higher than that for the latex
prepared without surfactant Post addition of INUTEC regSP1 resulted in
a large increase in the CCC as is illustrated in Fig 28 which shows log
W minus log C curves (where W is the ratio between the fast 1047298occulation
rate constant to the slow 1047298occulation rate constant referred to as the
stability ratio) at various additions of INUTECregSP1
As with the emulsions the high stability of the latex when using
INUTECregSP1 is due to the strong adsorption of the polymeric
surfactant on the latex particles and formation of strongly hydrated
loops and tails of polyfructose that provide effective steric stabiliza-
tion Evidence for the strong repulsion produced when using
INUTECregSP1 was obtained from Atomic Force Microscopy investiga-
tions [27] whereby the force between hydrophobic glass spheres and
hydrophobic glass plate both containing an adsorbed layer of
INUTECregSP1 was measured as a function of distance of separation
both in water and in the presence of various Na2SO4 concentrations
The results are shown in Figs 29 and 30
Fig 27 Electron micrographs of the latexes
Fig 28 In1047298uence of post addition of INUTECreg
SP1 on the latex stability
Fig 29 Forcendashdistance curves between hydrophobised glass surfaces containing
adsorbed INUTECreg
SP1 in water
294 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
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layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
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radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1919
With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 819
The microelectrophoresis technique is based on measurement of
the electrophoretic mobility u of the particles in the presence and
absence of the polymer layer From u one can calculate the zeta
potential ζ using the Huckel equation (which is applicable for small
particles and extended double layers ie κ Rbb1 where κ is the
DebyendashHuckel parameter that is related to the salt concentration
u = 23eeon η
eth15THORN
where ε is the relative permittivity of the medium and ε o is the
permittivity of free space
By measuring ζ of the particles with and without the adsorbed
polymer layer one can obtain the hydrodynamic thickness δh For
accurate measurements one should carry the measurements at various
electrolyte concentrations and extrapolate the results to the plateau
value Several automatic instruments are available for measurement of
the electrophoretic mobility Malvern zeta sizerndashCoulter Delsa sizer and
Broekahven instrument All these instruments are easy to use and the
measurement can be carried out within few minutes
4 Examples of the adsorption isotherms of nonionic polymericsurfactants
Fig 9 shows the adsorption isotherms for PEO with different
molecular weights on PS (at room temperature It can be seen that the
amount adsorbed in mgm-2 increases with increase in the polymer
molecular weight Fig 10 shows the variation of the hydrodynamic
thickness δh with molecular weight M δh shows a linear increase with
log M δh increases with n the number of segments in the chain
according to
δhasympn08 eth16THORN
Fig 11 shows the adsorption isotherms of PVA with various
molecular weights on PS latex (at 25 degC) [19] The polymers were
obtained by fractionation of a commercial sample of PVA with an
average molecular weight of 45000 The polymer also contained 12
vinyl acetate groups As with PEO the amount of adsorption increases
with increase in M The isotherms are also of the high af 1047297nity type Γ at the plateau increases linearly with M 12
The hydrodynamic thickness was determined using PCS and theresults are given below
M 67000 43000 28000 17000 8000
δhnm 255 197 140 98 33
δh seems to increase linearly with increase in the molecular weight
The effect of solvency on adsorption was investigated by increasing
the temperature (the PVA molecules are less soluble at higher
temperature) or addition of electrolyte (KCl) [20] The results are
shown in Figs 12 and 13 for M =65100 As can be seen from Fig 12
increase of temperature results in reduction of solvency of the
mediumfor the chain (due to break down of hydrogen bonds) and this
results in an increase in the amount adsorbed Addition of KCl (which
reduces the solvency of the medium for the chain) results in anincrease in adsorption (as predicted by theory)
The adsorption of block and graft copolymers is more complex
since the intimate structure of the chain determines the extent of
adsorption [18] Randomcopolymers adsorbin an intermediate way to
that of the corresponding homopolymers Blockcopolymers retain the
adsorption preference of the individual blocks The hydrophilic block
(eg PEO) the buoy (previously referred to as the A chain) extends
away from the particle surface into the bulk solution whereas the
hydrophobic anchor block (previously referred to as the B chain) (eg
PS or PPO) provides 1047297rm attachment to the surface Fig 14 shows the
theoretical prediction of diblock copolymer adsorption according to
Fig 9 Adsorption isotherms for PEO on PS
Fig 10 Hydrodynamic thickness of PEO on PS as a function of the molecular weight
Fig 11 Adsorption isotherms of PVA with different molecular weights on polystyrene
latex at 25 degC
Fig 12 In1047298uence of temperature on adsorption
288 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
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the Scheutjens and Fleer theory The surface density σ is plotted
versus the fraction of anchor segmentsν A The adsorption depends on
the anchorbuoy composition
The amount of adsorption is higher than for homopolymers and
the adsorbed layer thickness is more extended and dense Fig 15
shows the theoretical prediction for the adsorbed layer thickness δwhich is plotted as a function of ν A For a triblock copolymer AndashBndashA
with two buoy chains (A) and one anchor chain (B) the behaviour is
similar to that of diblock copolymers This is shown in Fig16 for PEOndash
PPOndashPEO block (Pluronic)
5 Interaction between particles containing adsorbed polymeric
surfactant layers steric stabilization
When two particles each with a radius R and containing an
adsorbed polymer layer with a hydrodynamic thickness δh approach
each other to a surfacendashsurface separation distance h that is smaller
than 2 δh the polymer layers interact with each other resulting in two
main situations [21] (i) The polymer chains may overlap with each
other (ii) The polymer layer may undergo some compression In both
cases there will be an increase in the local segment density of the
polymer chains in the interaction region This is schematicallyillustrated in Fig 17 The real situation is perhaps in between the
above two cases ie the polymer chains may undergo some
interpenetration and some compression
Provided the dangling chains (the A chains in AndashB AndashBndashA block or
BAn graft copolymers) are in a good solvent this local increase in
segment density in the interaction zone will result in strong repulsion
as a result of two main effects (i) Increase in the osmotic pressure in
the overlap region as a result of the unfavourable mixing of the
polymer chains when these are in good solvent conditions This is
referred to as osmotic repulsion or mixing interaction and it is
described by a free energy of interaction Gmix (ii) Reduction of the
con1047297gurational entropy of the chains in the interaction zone this
entropy reduction results from the decrease in the volume available
for thechains when these areeither overlapped or compressed This is
referred to as volume restriction interaction entropic or elastic
interaction and it is described by a free energy of interaction Gel
Combination of Gmix and Gel is usually referred to as the steric
interaction free energy Gs ie
Gs = Gmix + Gel eth17THORN
The sign of Gmix depends on the solvency of the medium for the
chains If in a good solvent ie the FloryndashHuggins interaction
parameter χ is less than 05 then Gmix is positive and the mixing
interaction leads to repulsion (see below) In contrast if χN05 (ie the
chains are in a poor solvent condition) Gmix is negative and the
mixing interaction becomes attractive Gel is always positive and
hence in some cases one can produce stable dispersions in a relatively
poor solvent (enhanced steric stabilisation)
51 Mixing interaction Gmix
This results from the unfavourable mixing of the polymer chainswhen these are in a good solvent conditions This is schematically
shown in Fig 18
Consider two spherical particles with the same radius and each
containing an adsorbed polymer layer with thickness δ Before
overlap one can de1047297ne in each polymer layer a chemical potential
for the solvent μ iα and a volume fraction for the polymer in the
layerϕ2 In the overlap region (volume element dV ) the chemical
potential of the solvent is reduced to μ i β This results from the increase
in polymer segment concentration in this overlap region
In the overlap region the chemical potential of the polymer chains
is now higher than in the rest of the layer (with no overlap) This
Fig 13 In1047298uence of addition of KCl on adsorption
Fig 14 Prediction of Adsorption of diblock copolymer
Fig15 Theoretical predictions of the adsorbed layer thickness for a diblock copolymer
Fig 16 Adsorbed amount (mg mminus2) versus fraction of anchor segment for an AndashBndashA
triblock copolymer (PEOndash
PPOndash
PEO)
289T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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amounts to an increase in the osmotic pressure in the overlap region
as a result solvent will diffuse from thebulk to the overlap region thus
separating the particles and hence a strong repulsive energy arises
from this effect The above repulsive energy can be calculated by
considering the free energy of mixing of two polymer solutions as for
example treated by Flory and Krigbaum [22] The free energy of
mixing is given by two terms (i) An entropy term that depends on the
volume fraction of polymer and solvent (ii)An energy term that is
determined by the FloryndashHuggins interaction parameter χ
Using the above theory one can derive an expression for the free
energy of mixing of two polymer layers (assuming a uniform segment
density distribution in each layer) surrounding two spherical particles
as a function of the separation distance h between the particles The
expression for Gmix is
Gmix
kT =
2V 22V 1
m2
2
1
2minusχ
δminus
h
2
2
3R + 2δ + h
2
eth18THORN
k is the Boltzmann constant T is the absolute temperature V 2 is the
molar volume of polymer V 1 is the molar volume of solvent and ν 2 is
the number of polymer chains per unit area
The sign of Gmix depends on the value of the FloryndashHuggins
interaction parameter χ if χb05 Gmix is positive and the interaction
is repulsive if χN05 Gmix is negative and the interaction is attractiveif χ=05 Gmix =0 and this de1047297nes the θ-condition
52 Elastic interaction Gel
Thisarises fromthe lossin con1047297gurational entropy of thechains on
the approach of a second particle As a result of this approach the
volume available for the chains becomes restricted resulting in loss of
the number of con1047297gurations This can be illustrated by considering a
simple molecule represented by a rod that rotates freely in a
hemisphere across a surface (Fig 19) When the two surfaces are
separated byan in1047297nitedistanceinfin thenumber of con1047297gurations of the
rod is Ω(infin) which is proportional to the volume of the hemisphere
When a secondparticleapproaches to a distance h suchthatit cuts the
hemisphere (loosing some volume) the volume available to the chains
is reduced and the number of con1047297gurations become Ω(h) which is
less than Ω(infin) For two 1047298at plates Gel is given by the following
expression
Gel
kT = 2m2 ln
X heth THORN
X infineth THORN
= 2m2Rel heth THORN eth19THORN
where Rel
(h) is a geometric function whose form depends on the
segment density distribution It should be stressed that Gel is always
positive and could play a major role in steric stabilisation It becomes
very strong when the separation distance between the particles
becomes comparable to the adsorbed layer thickness δ
Combination of Gmix and Gel with GA (the van der Waals attractive
energy)gives the total free energy of interaction GT (assuming there is
no contribution from any residual electrostatic interaction) ie
GT = Gmix + Gel + GA eth20THORN
A schematic representation of the variation of Gmix Gel GA and GT
with surfacendashsurface separation distance h is shown in Fig 20
Gmix increases very sharply with decrease of h when hb2δ Gel
increases very sharply with decrease of h when hbδ GT versus h
shows a minimum Gmin at separation distances comparable to 2δ
When h b2δ GT shows a rapid increase with decrease in h
The depth of the minimum depends on the Hamaker constant A
the particle radius R and adsorbed layer thickness δ Gmin increases
with increase of A and R At a given A and R Gmin increases with
decrease in δ (ie with decrease of the molecular weight M w of the
stabiliser This is illustrated in Fig 21 which shows the energyndash
distance curves as a function of δR The larger the value of δR the
smaller the value of Gmin In this case the system may approach
thermodynamic stability as is the case with nano-dispersions
6 Emulsions stabilized by polymeric surfactants
The most effective method for emulsion stabilization is to use
polymeric surfactants that stronglyadsorb at the OWor WO interface
Fig 17 Schematic representation of the interaction between particles containing adsorbed polymer layers
Fig 18 Schematic representation of polymer layer overlap
Fig 19 Schematic representation of con1047297gurational entropy loss on approach of a
second particle
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and produce effective steric stabilization against strong 1047298occulation
coalescence and Ostwald ripening [23]
As mentioned above a graft copolymer of the ABn type was
synthesized by grafting several alkyl groups on an inulin (polyfruc-
tose) chain The polymeric surfactant (INUTECregSP1) consists of a
linear polyfructose chain (the stabilizing A chain) and several alkyl
groups (the B chains) that provide multi-anchor attachment to the oil
droplets This polymeric surfactant produces enhanced steric stabili-
zation both in water and high electrolyte concentrations as will be
discussed later
For water-in-oil (WO) emulsions an AndashBndashA block copolymer of
poly (12-hydroxystearic acid) (PHS) (the A chains) and poly (ethylene
oxide) (PEO) (the B chain) PHSndashPEOndashPHS is commercially available
(Arlacel P135 UNIQEMA) The PEO chain (that is soluble in the water
droplets) forms the anchor chain whereas the PHS chains form the
stabilizing chains PHS is highly soluble in most hydrocarbon solvents
and is strongly solvated by its molecules The structure of the PHSndash
PEOndashPHS block copolymer is schematically shown in Fig 22The conformation of the polymeric surfactant at the WO interface
is schematically shown in Fig 23
Emulsions of Isopar Mwater and cyclomethiconewater were
prepared using INUTECregSP1 5050 (vv) OW emulsions were
prepared and the emulsi1047297er concentration was varied from 025 to 2
(wv) based on the oil phase 05 (wv) emulsi1047297erwas suf 1047297cient for
stabilization of these 5050 (vv) emulsions [23]
The emulsions were stored at room temperature and 50 degC and
optical micrographs were taken at intervals of time (for a year) in
order to check the stability Emulsions prepared in water were very
stableshowingno change in droplet size distributionover more than a
year period and this indicated absence of coalescence Any weak
1047298occulation that occurred was reversible and the emulsion could be
redispersed by gentle shaking Fig 24 shows an optical micrograph fora dilute 5050 (vv) emulsion that was stored for 15 and 14 weeks at
50 degC
No change in droplet size was observed after storage for more than
1 year at 50 degC indicating absence of coalescence The same result was
obtained when using different oils Emulsions were also stable against
coalescence in the presence of high electrolyte concentrations (up to
4 mol dmminus3 or ~25 NaCl
The above stability in high electrolyte concentrations is not
observed with polymeric surfactants based on polethylene oxide
The high stability observed using INUTECregSP1 is related to its
strong hydration both in water and in electrolyte solutions The
hydration of inulin (the backbone of HMI) could be assessed using
cloud point measurements A comparison was also made with PEO
with two molecular weights namely 4000 and 20000
Solutionsof PEO 4000 and 20000 showeda systematic decrease of
cloud point with increase in NaCl or MgSO4 concentration In contrast
inulin showed no cloud point up to 4 mol dmminus3 NaCl and up to 1 mol
dmminus3 MgSO4
The above results explain the difference between PEO and inulin
With PEO the chains show dehydration when the NaCl concentration
is increased above 2 mol dmminus3 or 05 mol dmminus3 MgSO4 The inulin
chains remain hydrated at much higher electrolyte concentrations It
seems that the linear polyfructose chains remain strongly hydrated athigh temperature and high electrolyte concentrations
The high emulsion stability obtained whenusingINUTECregSP1 can be
accounted for by the following factors (i) The multi-point attachment
of the polymer by several alkyl chains that are grafted on the backbone
(ii) The strong hydration of the polyfructose ldquoloopsrdquo both in water and
high electrolyte concentrations (χ remains below 05 under these
conditions) (iii) Thehigh volumefraction (concentration) of the loops at
the interface (iv) Enhanced steric stabilization this is the case with
multi-point attachment which produces strong elastic interaction
Evidence for the high stability of the liquid 1047297lm between emulsion
droplets when using INUTECregSP1 was obtained by Exerowa et al [24]
using disjoining pressure measurements This is illustrated in Fig 25
which shows a plot of disjoining pressure versus separation distance
between two emulsion droplets at various electrolyte concentrations
The results show that by increasing the capillary pressure a stable
Newton Black Film (NBF) is obtained at a 1047297lm thickness of sim7 nm
The lack of rupture of the 1047297lm at the highest pressure applied of
45times104 Pa indicate the high stability of the 1047297lm in water and in high
electrolyte concentrations (up to 20 mol dmminus3 NaCl)
The lack of rupture of the NBF up to the highest pressure applied
namely 45times104 Pa clearly indicatesthe high stability of the liquid1047297lm
in the presence of high NaCl concentrations (up to 2 mol dm minus3) This
result is consistent with the high emulsion stability obtained at high
electrolyte concentrations and high temperature Emulsions of Isopar
M-in-water are very stable under such conditions and this could be
accounted for by the high stability of the NBF The droplet size of
5050 OW emulsions prepared using 2 INUTEC regSP1 is in the region
of 1ndash10 μ m This corresponds to a capillary pressure of ~3times104 Pa for
the 1 μ m drops and ~3times103 Pa for the 10 μ m drops These capillarypressures are lower than those to which the NBF have been sub-
jected to and this clearly indicates the high stability obtained against
coalescence in these emulsions
61 WO emulsions stabilized with PHS ndashPEOndashPHS block copolymer
WO emulsions (the oil being Isopar M) were prepared using PHSndash
PEOndashPHS block copolymer at high water volume fractions (N07) The
emulsions have a narrow droplet size distribution with a z -average
radius of 183 nm [25] They also remained 1047298uid up to high water
volume fractions (N06) This could be illustrated from viscosityndash
volume fraction curves as is shown in Fig 26
Fig 20 Energyndashdistance curves for sterically stabilized systems
Fig 21 Variation of Gmin with δR
291T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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The effective volume fraction ϕeff of the emulsions (the core
droplets plus the adsorbed layer) could be calculated from the relative
viscosity and using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth21THORN
Where η r is the relative viscosity ϕp is the maximum packing fraction(sim07) and [ η ] is the intrinsic viscosity that is equal to 25 for hard-
spheres
The calculations based on Eq (22) are shown in Fig 27 (square
symbols) From the effective volume fraction ϕeff and the core volume
fraction ϕ the adsorbed layer thickness could be calculated This was
found to be in the region of 10 nm at ϕ =04 and it decreased with
increase in ϕ
The WO emulsions prepared using the PHSndashPEOndashPHS block
copolymer remained stable both at room temperature and 50 degC
This is consistent with the structure of the block copolymer the B
chain (PEO) is soluble in water and it forms a very strong anchor at the
WO interface The PHS chains (the A chains) provide effective steric
stabilization since the chains are highly soluble in Isopar M and are
strongly solvated by its molecules
7 Suspensions stabilised using polymeric surfactants
There are generally two procedures for preparation of solidliquid
dispersions
(i) Condensation methods build-up of particles from mole-
cular units ie nucleation and growth A special procedure
is the preparation of latexes by emulsion or dispersion
polymerization
(ii) Dispersion methods in this case one starts with preformed
large particles or crystals which are dispersed in the liquid by
using a surfactant (wetting agent) with subsequent breaking up
of the large particles by milling (comminution) to achieve the
desirable particle size distribution A dispersing agent (usuallya
polymeric surfactant) is used for the dispersion process and
subsequent stabilization of the resulting suspension
There are generally two procedures for preparation of latexes
(i) Emulsion polymerization the monomers that are essentially
insoluble in the aqueous medium are emulsi1047297ed using a
surfactant and an initiator is added while heating the system
to produce the polymer particles that are stabilized electro-
statically (when using ionic surfactants) or sterically (when
using non-ionic surfactants)
(ii) Dispersion polymerization the monomers are dissolved in a
solvent in which the resulting polymer particlesare insoluble A
protective colloid (normally a block or graft copolymer) is
added to prevent 1047298occulation of the resulting polymers
particles that are produced on addition of an initiator This
method is usually applied for the production of non-aqueous
latex dispersions and is sometimes referred to as Non-Aqueous
Dispersion Polymerization (NAD)
Surfactants play a crucial role in the process of latex preparation
since they determine the stabilizing ef 1047297ciency and the effectiveness of
the surface active agent ultimately determines the number of particles
Fig 22 Schematic representation of the structure of PHSndashPEOndashPHS block copolymer
Fig 23 Conformation of PHSndash
PEOndash
PHS polymeric surfactant at the WO interface
292 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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and their size The effectiveness of any surface active agent in
stabilizing the particles is the dominant factor and the number of
micelles formed is relatively unimportant In the NAD process themonomer normally an acrylic is dissolved in a non-aqueous solvent
normally an aliphatic hydrocarbon and an oil soluble initiator and a
stabilizer (to protect the resulting particles from1047298occulation) is added
to the reaction mixture The most successful stabilizers used in NAD
are block and graft copolymers Preformed graft stabilizers based
on poly(12-hydroxy stearic acid) (PHS) are simple to prepare and
effective in NAD polymerization
Dispersion methods are used for the preparation of suspensions of
preformed particles The role of surfactants (or polymers) in the
dispersion process can be analyzed in terms of the three processesinvolved (i) Wetting of the powder by the liquid (ii) Breaking of the
aggregates and agglomerates (iii) Comminution of the resulting
particles and their subsequent stabilization All these processes are
affected by surfactants or polymers which adsorb on the powder
surface thus aiding the wetting of the powder break-up of the
aggregates and agglomerates and subsequent reduction of particle
size by wet milling
71 Polymeric surfactants in emulsion polymerization
Recently the graft copolymer of hydrophobically modi1047297ed inulin
(INUTECreg SP1) has been used in emulsion polymerization of styrene
methyl methacrylate butyl acrylate and several other monomers [26]All lattices were prepared by emulsion polymerisation using potas-
sium persulphate as initiator The z -average particle size was
determined by photon correlation spectroscopy (PCS) and electron
micrographs were also taken
Fig 24 Optical micrographs of OW emulsions stabilized with INUTEC regSP1 stored at
50 degC for 15 weeks (a) and 14 weeks (b)
Fig 25 Variation of disjoining pressure with equivalent 1047297lm thickness at various NaCl concentrations
Fig 26 Viscosityndashvolume fraction for WO emulsion stabilized with PHSndashPEOndashPHS
block copolymer experimental data calculated using Eq (22)
293T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Emulsion polymerization of styrene or methylmethacrylate showed
an optimum weight ratio of (INUTEC)Monomer of 00033 for PS and
0001 for PMMA particles The (initiator)(Monomer) ratio was
kept constant at 000125 The monomer conversion was higher than
85 in all cases Latex dispersions of PS reaching 50 and of PMMA
reaching 40 could be obtained using such low concentration of
INUTECregSP1 Fig 27 shows the variation of particle diameter with
monomer concentration
The stability of the latexes was determined by determining the
critical coagulation concentration (CCC) using CaCl2 The CCC was low
(00175ndash005 mol dmminus3) but this was higher than that for the latex
prepared without surfactant Post addition of INUTEC regSP1 resulted in
a large increase in the CCC as is illustrated in Fig 28 which shows log
W minus log C curves (where W is the ratio between the fast 1047298occulation
rate constant to the slow 1047298occulation rate constant referred to as the
stability ratio) at various additions of INUTECregSP1
As with the emulsions the high stability of the latex when using
INUTECregSP1 is due to the strong adsorption of the polymeric
surfactant on the latex particles and formation of strongly hydrated
loops and tails of polyfructose that provide effective steric stabiliza-
tion Evidence for the strong repulsion produced when using
INUTECregSP1 was obtained from Atomic Force Microscopy investiga-
tions [27] whereby the force between hydrophobic glass spheres and
hydrophobic glass plate both containing an adsorbed layer of
INUTECregSP1 was measured as a function of distance of separation
both in water and in the presence of various Na2SO4 concentrations
The results are shown in Figs 29 and 30
Fig 27 Electron micrographs of the latexes
Fig 28 In1047298uence of post addition of INUTECreg
SP1 on the latex stability
Fig 29 Forcendashdistance curves between hydrophobised glass surfaces containing
adsorbed INUTECreg
SP1 in water
294 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
295T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
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radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 919
the Scheutjens and Fleer theory The surface density σ is plotted
versus the fraction of anchor segmentsν A The adsorption depends on
the anchorbuoy composition
The amount of adsorption is higher than for homopolymers and
the adsorbed layer thickness is more extended and dense Fig 15
shows the theoretical prediction for the adsorbed layer thickness δwhich is plotted as a function of ν A For a triblock copolymer AndashBndashA
with two buoy chains (A) and one anchor chain (B) the behaviour is
similar to that of diblock copolymers This is shown in Fig16 for PEOndash
PPOndashPEO block (Pluronic)
5 Interaction between particles containing adsorbed polymeric
surfactant layers steric stabilization
When two particles each with a radius R and containing an
adsorbed polymer layer with a hydrodynamic thickness δh approach
each other to a surfacendashsurface separation distance h that is smaller
than 2 δh the polymer layers interact with each other resulting in two
main situations [21] (i) The polymer chains may overlap with each
other (ii) The polymer layer may undergo some compression In both
cases there will be an increase in the local segment density of the
polymer chains in the interaction region This is schematicallyillustrated in Fig 17 The real situation is perhaps in between the
above two cases ie the polymer chains may undergo some
interpenetration and some compression
Provided the dangling chains (the A chains in AndashB AndashBndashA block or
BAn graft copolymers) are in a good solvent this local increase in
segment density in the interaction zone will result in strong repulsion
as a result of two main effects (i) Increase in the osmotic pressure in
the overlap region as a result of the unfavourable mixing of the
polymer chains when these are in good solvent conditions This is
referred to as osmotic repulsion or mixing interaction and it is
described by a free energy of interaction Gmix (ii) Reduction of the
con1047297gurational entropy of the chains in the interaction zone this
entropy reduction results from the decrease in the volume available
for thechains when these areeither overlapped or compressed This is
referred to as volume restriction interaction entropic or elastic
interaction and it is described by a free energy of interaction Gel
Combination of Gmix and Gel is usually referred to as the steric
interaction free energy Gs ie
Gs = Gmix + Gel eth17THORN
The sign of Gmix depends on the solvency of the medium for the
chains If in a good solvent ie the FloryndashHuggins interaction
parameter χ is less than 05 then Gmix is positive and the mixing
interaction leads to repulsion (see below) In contrast if χN05 (ie the
chains are in a poor solvent condition) Gmix is negative and the
mixing interaction becomes attractive Gel is always positive and
hence in some cases one can produce stable dispersions in a relatively
poor solvent (enhanced steric stabilisation)
51 Mixing interaction Gmix
This results from the unfavourable mixing of the polymer chainswhen these are in a good solvent conditions This is schematically
shown in Fig 18
Consider two spherical particles with the same radius and each
containing an adsorbed polymer layer with thickness δ Before
overlap one can de1047297ne in each polymer layer a chemical potential
for the solvent μ iα and a volume fraction for the polymer in the
layerϕ2 In the overlap region (volume element dV ) the chemical
potential of the solvent is reduced to μ i β This results from the increase
in polymer segment concentration in this overlap region
In the overlap region the chemical potential of the polymer chains
is now higher than in the rest of the layer (with no overlap) This
Fig 13 In1047298uence of addition of KCl on adsorption
Fig 14 Prediction of Adsorption of diblock copolymer
Fig15 Theoretical predictions of the adsorbed layer thickness for a diblock copolymer
Fig 16 Adsorbed amount (mg mminus2) versus fraction of anchor segment for an AndashBndashA
triblock copolymer (PEOndash
PPOndash
PEO)
289T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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amounts to an increase in the osmotic pressure in the overlap region
as a result solvent will diffuse from thebulk to the overlap region thus
separating the particles and hence a strong repulsive energy arises
from this effect The above repulsive energy can be calculated by
considering the free energy of mixing of two polymer solutions as for
example treated by Flory and Krigbaum [22] The free energy of
mixing is given by two terms (i) An entropy term that depends on the
volume fraction of polymer and solvent (ii)An energy term that is
determined by the FloryndashHuggins interaction parameter χ
Using the above theory one can derive an expression for the free
energy of mixing of two polymer layers (assuming a uniform segment
density distribution in each layer) surrounding two spherical particles
as a function of the separation distance h between the particles The
expression for Gmix is
Gmix
kT =
2V 22V 1
m2
2
1
2minusχ
δminus
h
2
2
3R + 2δ + h
2
eth18THORN
k is the Boltzmann constant T is the absolute temperature V 2 is the
molar volume of polymer V 1 is the molar volume of solvent and ν 2 is
the number of polymer chains per unit area
The sign of Gmix depends on the value of the FloryndashHuggins
interaction parameter χ if χb05 Gmix is positive and the interaction
is repulsive if χN05 Gmix is negative and the interaction is attractiveif χ=05 Gmix =0 and this de1047297nes the θ-condition
52 Elastic interaction Gel
Thisarises fromthe lossin con1047297gurational entropy of thechains on
the approach of a second particle As a result of this approach the
volume available for the chains becomes restricted resulting in loss of
the number of con1047297gurations This can be illustrated by considering a
simple molecule represented by a rod that rotates freely in a
hemisphere across a surface (Fig 19) When the two surfaces are
separated byan in1047297nitedistanceinfin thenumber of con1047297gurations of the
rod is Ω(infin) which is proportional to the volume of the hemisphere
When a secondparticleapproaches to a distance h suchthatit cuts the
hemisphere (loosing some volume) the volume available to the chains
is reduced and the number of con1047297gurations become Ω(h) which is
less than Ω(infin) For two 1047298at plates Gel is given by the following
expression
Gel
kT = 2m2 ln
X heth THORN
X infineth THORN
= 2m2Rel heth THORN eth19THORN
where Rel
(h) is a geometric function whose form depends on the
segment density distribution It should be stressed that Gel is always
positive and could play a major role in steric stabilisation It becomes
very strong when the separation distance between the particles
becomes comparable to the adsorbed layer thickness δ
Combination of Gmix and Gel with GA (the van der Waals attractive
energy)gives the total free energy of interaction GT (assuming there is
no contribution from any residual electrostatic interaction) ie
GT = Gmix + Gel + GA eth20THORN
A schematic representation of the variation of Gmix Gel GA and GT
with surfacendashsurface separation distance h is shown in Fig 20
Gmix increases very sharply with decrease of h when hb2δ Gel
increases very sharply with decrease of h when hbδ GT versus h
shows a minimum Gmin at separation distances comparable to 2δ
When h b2δ GT shows a rapid increase with decrease in h
The depth of the minimum depends on the Hamaker constant A
the particle radius R and adsorbed layer thickness δ Gmin increases
with increase of A and R At a given A and R Gmin increases with
decrease in δ (ie with decrease of the molecular weight M w of the
stabiliser This is illustrated in Fig 21 which shows the energyndash
distance curves as a function of δR The larger the value of δR the
smaller the value of Gmin In this case the system may approach
thermodynamic stability as is the case with nano-dispersions
6 Emulsions stabilized by polymeric surfactants
The most effective method for emulsion stabilization is to use
polymeric surfactants that stronglyadsorb at the OWor WO interface
Fig 17 Schematic representation of the interaction between particles containing adsorbed polymer layers
Fig 18 Schematic representation of polymer layer overlap
Fig 19 Schematic representation of con1047297gurational entropy loss on approach of a
second particle
290 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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and produce effective steric stabilization against strong 1047298occulation
coalescence and Ostwald ripening [23]
As mentioned above a graft copolymer of the ABn type was
synthesized by grafting several alkyl groups on an inulin (polyfruc-
tose) chain The polymeric surfactant (INUTECregSP1) consists of a
linear polyfructose chain (the stabilizing A chain) and several alkyl
groups (the B chains) that provide multi-anchor attachment to the oil
droplets This polymeric surfactant produces enhanced steric stabili-
zation both in water and high electrolyte concentrations as will be
discussed later
For water-in-oil (WO) emulsions an AndashBndashA block copolymer of
poly (12-hydroxystearic acid) (PHS) (the A chains) and poly (ethylene
oxide) (PEO) (the B chain) PHSndashPEOndashPHS is commercially available
(Arlacel P135 UNIQEMA) The PEO chain (that is soluble in the water
droplets) forms the anchor chain whereas the PHS chains form the
stabilizing chains PHS is highly soluble in most hydrocarbon solvents
and is strongly solvated by its molecules The structure of the PHSndash
PEOndashPHS block copolymer is schematically shown in Fig 22The conformation of the polymeric surfactant at the WO interface
is schematically shown in Fig 23
Emulsions of Isopar Mwater and cyclomethiconewater were
prepared using INUTECregSP1 5050 (vv) OW emulsions were
prepared and the emulsi1047297er concentration was varied from 025 to 2
(wv) based on the oil phase 05 (wv) emulsi1047297erwas suf 1047297cient for
stabilization of these 5050 (vv) emulsions [23]
The emulsions were stored at room temperature and 50 degC and
optical micrographs were taken at intervals of time (for a year) in
order to check the stability Emulsions prepared in water were very
stableshowingno change in droplet size distributionover more than a
year period and this indicated absence of coalescence Any weak
1047298occulation that occurred was reversible and the emulsion could be
redispersed by gentle shaking Fig 24 shows an optical micrograph fora dilute 5050 (vv) emulsion that was stored for 15 and 14 weeks at
50 degC
No change in droplet size was observed after storage for more than
1 year at 50 degC indicating absence of coalescence The same result was
obtained when using different oils Emulsions were also stable against
coalescence in the presence of high electrolyte concentrations (up to
4 mol dmminus3 or ~25 NaCl
The above stability in high electrolyte concentrations is not
observed with polymeric surfactants based on polethylene oxide
The high stability observed using INUTECregSP1 is related to its
strong hydration both in water and in electrolyte solutions The
hydration of inulin (the backbone of HMI) could be assessed using
cloud point measurements A comparison was also made with PEO
with two molecular weights namely 4000 and 20000
Solutionsof PEO 4000 and 20000 showeda systematic decrease of
cloud point with increase in NaCl or MgSO4 concentration In contrast
inulin showed no cloud point up to 4 mol dmminus3 NaCl and up to 1 mol
dmminus3 MgSO4
The above results explain the difference between PEO and inulin
With PEO the chains show dehydration when the NaCl concentration
is increased above 2 mol dmminus3 or 05 mol dmminus3 MgSO4 The inulin
chains remain hydrated at much higher electrolyte concentrations It
seems that the linear polyfructose chains remain strongly hydrated athigh temperature and high electrolyte concentrations
The high emulsion stability obtained whenusingINUTECregSP1 can be
accounted for by the following factors (i) The multi-point attachment
of the polymer by several alkyl chains that are grafted on the backbone
(ii) The strong hydration of the polyfructose ldquoloopsrdquo both in water and
high electrolyte concentrations (χ remains below 05 under these
conditions) (iii) Thehigh volumefraction (concentration) of the loops at
the interface (iv) Enhanced steric stabilization this is the case with
multi-point attachment which produces strong elastic interaction
Evidence for the high stability of the liquid 1047297lm between emulsion
droplets when using INUTECregSP1 was obtained by Exerowa et al [24]
using disjoining pressure measurements This is illustrated in Fig 25
which shows a plot of disjoining pressure versus separation distance
between two emulsion droplets at various electrolyte concentrations
The results show that by increasing the capillary pressure a stable
Newton Black Film (NBF) is obtained at a 1047297lm thickness of sim7 nm
The lack of rupture of the 1047297lm at the highest pressure applied of
45times104 Pa indicate the high stability of the 1047297lm in water and in high
electrolyte concentrations (up to 20 mol dmminus3 NaCl)
The lack of rupture of the NBF up to the highest pressure applied
namely 45times104 Pa clearly indicatesthe high stability of the liquid1047297lm
in the presence of high NaCl concentrations (up to 2 mol dm minus3) This
result is consistent with the high emulsion stability obtained at high
electrolyte concentrations and high temperature Emulsions of Isopar
M-in-water are very stable under such conditions and this could be
accounted for by the high stability of the NBF The droplet size of
5050 OW emulsions prepared using 2 INUTEC regSP1 is in the region
of 1ndash10 μ m This corresponds to a capillary pressure of ~3times104 Pa for
the 1 μ m drops and ~3times103 Pa for the 10 μ m drops These capillarypressures are lower than those to which the NBF have been sub-
jected to and this clearly indicates the high stability obtained against
coalescence in these emulsions
61 WO emulsions stabilized with PHS ndashPEOndashPHS block copolymer
WO emulsions (the oil being Isopar M) were prepared using PHSndash
PEOndashPHS block copolymer at high water volume fractions (N07) The
emulsions have a narrow droplet size distribution with a z -average
radius of 183 nm [25] They also remained 1047298uid up to high water
volume fractions (N06) This could be illustrated from viscosityndash
volume fraction curves as is shown in Fig 26
Fig 20 Energyndashdistance curves for sterically stabilized systems
Fig 21 Variation of Gmin with δR
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The effective volume fraction ϕeff of the emulsions (the core
droplets plus the adsorbed layer) could be calculated from the relative
viscosity and using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth21THORN
Where η r is the relative viscosity ϕp is the maximum packing fraction(sim07) and [ η ] is the intrinsic viscosity that is equal to 25 for hard-
spheres
The calculations based on Eq (22) are shown in Fig 27 (square
symbols) From the effective volume fraction ϕeff and the core volume
fraction ϕ the adsorbed layer thickness could be calculated This was
found to be in the region of 10 nm at ϕ =04 and it decreased with
increase in ϕ
The WO emulsions prepared using the PHSndashPEOndashPHS block
copolymer remained stable both at room temperature and 50 degC
This is consistent with the structure of the block copolymer the B
chain (PEO) is soluble in water and it forms a very strong anchor at the
WO interface The PHS chains (the A chains) provide effective steric
stabilization since the chains are highly soluble in Isopar M and are
strongly solvated by its molecules
7 Suspensions stabilised using polymeric surfactants
There are generally two procedures for preparation of solidliquid
dispersions
(i) Condensation methods build-up of particles from mole-
cular units ie nucleation and growth A special procedure
is the preparation of latexes by emulsion or dispersion
polymerization
(ii) Dispersion methods in this case one starts with preformed
large particles or crystals which are dispersed in the liquid by
using a surfactant (wetting agent) with subsequent breaking up
of the large particles by milling (comminution) to achieve the
desirable particle size distribution A dispersing agent (usuallya
polymeric surfactant) is used for the dispersion process and
subsequent stabilization of the resulting suspension
There are generally two procedures for preparation of latexes
(i) Emulsion polymerization the monomers that are essentially
insoluble in the aqueous medium are emulsi1047297ed using a
surfactant and an initiator is added while heating the system
to produce the polymer particles that are stabilized electro-
statically (when using ionic surfactants) or sterically (when
using non-ionic surfactants)
(ii) Dispersion polymerization the monomers are dissolved in a
solvent in which the resulting polymer particlesare insoluble A
protective colloid (normally a block or graft copolymer) is
added to prevent 1047298occulation of the resulting polymers
particles that are produced on addition of an initiator This
method is usually applied for the production of non-aqueous
latex dispersions and is sometimes referred to as Non-Aqueous
Dispersion Polymerization (NAD)
Surfactants play a crucial role in the process of latex preparation
since they determine the stabilizing ef 1047297ciency and the effectiveness of
the surface active agent ultimately determines the number of particles
Fig 22 Schematic representation of the structure of PHSndashPEOndashPHS block copolymer
Fig 23 Conformation of PHSndash
PEOndash
PHS polymeric surfactant at the WO interface
292 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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and their size The effectiveness of any surface active agent in
stabilizing the particles is the dominant factor and the number of
micelles formed is relatively unimportant In the NAD process themonomer normally an acrylic is dissolved in a non-aqueous solvent
normally an aliphatic hydrocarbon and an oil soluble initiator and a
stabilizer (to protect the resulting particles from1047298occulation) is added
to the reaction mixture The most successful stabilizers used in NAD
are block and graft copolymers Preformed graft stabilizers based
on poly(12-hydroxy stearic acid) (PHS) are simple to prepare and
effective in NAD polymerization
Dispersion methods are used for the preparation of suspensions of
preformed particles The role of surfactants (or polymers) in the
dispersion process can be analyzed in terms of the three processesinvolved (i) Wetting of the powder by the liquid (ii) Breaking of the
aggregates and agglomerates (iii) Comminution of the resulting
particles and their subsequent stabilization All these processes are
affected by surfactants or polymers which adsorb on the powder
surface thus aiding the wetting of the powder break-up of the
aggregates and agglomerates and subsequent reduction of particle
size by wet milling
71 Polymeric surfactants in emulsion polymerization
Recently the graft copolymer of hydrophobically modi1047297ed inulin
(INUTECreg SP1) has been used in emulsion polymerization of styrene
methyl methacrylate butyl acrylate and several other monomers [26]All lattices were prepared by emulsion polymerisation using potas-
sium persulphate as initiator The z -average particle size was
determined by photon correlation spectroscopy (PCS) and electron
micrographs were also taken
Fig 24 Optical micrographs of OW emulsions stabilized with INUTEC regSP1 stored at
50 degC for 15 weeks (a) and 14 weeks (b)
Fig 25 Variation of disjoining pressure with equivalent 1047297lm thickness at various NaCl concentrations
Fig 26 Viscosityndashvolume fraction for WO emulsion stabilized with PHSndashPEOndashPHS
block copolymer experimental data calculated using Eq (22)
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Emulsion polymerization of styrene or methylmethacrylate showed
an optimum weight ratio of (INUTEC)Monomer of 00033 for PS and
0001 for PMMA particles The (initiator)(Monomer) ratio was
kept constant at 000125 The monomer conversion was higher than
85 in all cases Latex dispersions of PS reaching 50 and of PMMA
reaching 40 could be obtained using such low concentration of
INUTECregSP1 Fig 27 shows the variation of particle diameter with
monomer concentration
The stability of the latexes was determined by determining the
critical coagulation concentration (CCC) using CaCl2 The CCC was low
(00175ndash005 mol dmminus3) but this was higher than that for the latex
prepared without surfactant Post addition of INUTEC regSP1 resulted in
a large increase in the CCC as is illustrated in Fig 28 which shows log
W minus log C curves (where W is the ratio between the fast 1047298occulation
rate constant to the slow 1047298occulation rate constant referred to as the
stability ratio) at various additions of INUTECregSP1
As with the emulsions the high stability of the latex when using
INUTECregSP1 is due to the strong adsorption of the polymeric
surfactant on the latex particles and formation of strongly hydrated
loops and tails of polyfructose that provide effective steric stabiliza-
tion Evidence for the strong repulsion produced when using
INUTECregSP1 was obtained from Atomic Force Microscopy investiga-
tions [27] whereby the force between hydrophobic glass spheres and
hydrophobic glass plate both containing an adsorbed layer of
INUTECregSP1 was measured as a function of distance of separation
both in water and in the presence of various Na2SO4 concentrations
The results are shown in Figs 29 and 30
Fig 27 Electron micrographs of the latexes
Fig 28 In1047298uence of post addition of INUTECreg
SP1 on the latex stability
Fig 29 Forcendashdistance curves between hydrophobised glass surfaces containing
adsorbed INUTECreg
SP1 in water
294 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
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layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
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radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1919
With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1019
amounts to an increase in the osmotic pressure in the overlap region
as a result solvent will diffuse from thebulk to the overlap region thus
separating the particles and hence a strong repulsive energy arises
from this effect The above repulsive energy can be calculated by
considering the free energy of mixing of two polymer solutions as for
example treated by Flory and Krigbaum [22] The free energy of
mixing is given by two terms (i) An entropy term that depends on the
volume fraction of polymer and solvent (ii)An energy term that is
determined by the FloryndashHuggins interaction parameter χ
Using the above theory one can derive an expression for the free
energy of mixing of two polymer layers (assuming a uniform segment
density distribution in each layer) surrounding two spherical particles
as a function of the separation distance h between the particles The
expression for Gmix is
Gmix
kT =
2V 22V 1
m2
2
1
2minusχ
δminus
h
2
2
3R + 2δ + h
2
eth18THORN
k is the Boltzmann constant T is the absolute temperature V 2 is the
molar volume of polymer V 1 is the molar volume of solvent and ν 2 is
the number of polymer chains per unit area
The sign of Gmix depends on the value of the FloryndashHuggins
interaction parameter χ if χb05 Gmix is positive and the interaction
is repulsive if χN05 Gmix is negative and the interaction is attractiveif χ=05 Gmix =0 and this de1047297nes the θ-condition
52 Elastic interaction Gel
Thisarises fromthe lossin con1047297gurational entropy of thechains on
the approach of a second particle As a result of this approach the
volume available for the chains becomes restricted resulting in loss of
the number of con1047297gurations This can be illustrated by considering a
simple molecule represented by a rod that rotates freely in a
hemisphere across a surface (Fig 19) When the two surfaces are
separated byan in1047297nitedistanceinfin thenumber of con1047297gurations of the
rod is Ω(infin) which is proportional to the volume of the hemisphere
When a secondparticleapproaches to a distance h suchthatit cuts the
hemisphere (loosing some volume) the volume available to the chains
is reduced and the number of con1047297gurations become Ω(h) which is
less than Ω(infin) For two 1047298at plates Gel is given by the following
expression
Gel
kT = 2m2 ln
X heth THORN
X infineth THORN
= 2m2Rel heth THORN eth19THORN
where Rel
(h) is a geometric function whose form depends on the
segment density distribution It should be stressed that Gel is always
positive and could play a major role in steric stabilisation It becomes
very strong when the separation distance between the particles
becomes comparable to the adsorbed layer thickness δ
Combination of Gmix and Gel with GA (the van der Waals attractive
energy)gives the total free energy of interaction GT (assuming there is
no contribution from any residual electrostatic interaction) ie
GT = Gmix + Gel + GA eth20THORN
A schematic representation of the variation of Gmix Gel GA and GT
with surfacendashsurface separation distance h is shown in Fig 20
Gmix increases very sharply with decrease of h when hb2δ Gel
increases very sharply with decrease of h when hbδ GT versus h
shows a minimum Gmin at separation distances comparable to 2δ
When h b2δ GT shows a rapid increase with decrease in h
The depth of the minimum depends on the Hamaker constant A
the particle radius R and adsorbed layer thickness δ Gmin increases
with increase of A and R At a given A and R Gmin increases with
decrease in δ (ie with decrease of the molecular weight M w of the
stabiliser This is illustrated in Fig 21 which shows the energyndash
distance curves as a function of δR The larger the value of δR the
smaller the value of Gmin In this case the system may approach
thermodynamic stability as is the case with nano-dispersions
6 Emulsions stabilized by polymeric surfactants
The most effective method for emulsion stabilization is to use
polymeric surfactants that stronglyadsorb at the OWor WO interface
Fig 17 Schematic representation of the interaction between particles containing adsorbed polymer layers
Fig 18 Schematic representation of polymer layer overlap
Fig 19 Schematic representation of con1047297gurational entropy loss on approach of a
second particle
290 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
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and produce effective steric stabilization against strong 1047298occulation
coalescence and Ostwald ripening [23]
As mentioned above a graft copolymer of the ABn type was
synthesized by grafting several alkyl groups on an inulin (polyfruc-
tose) chain The polymeric surfactant (INUTECregSP1) consists of a
linear polyfructose chain (the stabilizing A chain) and several alkyl
groups (the B chains) that provide multi-anchor attachment to the oil
droplets This polymeric surfactant produces enhanced steric stabili-
zation both in water and high electrolyte concentrations as will be
discussed later
For water-in-oil (WO) emulsions an AndashBndashA block copolymer of
poly (12-hydroxystearic acid) (PHS) (the A chains) and poly (ethylene
oxide) (PEO) (the B chain) PHSndashPEOndashPHS is commercially available
(Arlacel P135 UNIQEMA) The PEO chain (that is soluble in the water
droplets) forms the anchor chain whereas the PHS chains form the
stabilizing chains PHS is highly soluble in most hydrocarbon solvents
and is strongly solvated by its molecules The structure of the PHSndash
PEOndashPHS block copolymer is schematically shown in Fig 22The conformation of the polymeric surfactant at the WO interface
is schematically shown in Fig 23
Emulsions of Isopar Mwater and cyclomethiconewater were
prepared using INUTECregSP1 5050 (vv) OW emulsions were
prepared and the emulsi1047297er concentration was varied from 025 to 2
(wv) based on the oil phase 05 (wv) emulsi1047297erwas suf 1047297cient for
stabilization of these 5050 (vv) emulsions [23]
The emulsions were stored at room temperature and 50 degC and
optical micrographs were taken at intervals of time (for a year) in
order to check the stability Emulsions prepared in water were very
stableshowingno change in droplet size distributionover more than a
year period and this indicated absence of coalescence Any weak
1047298occulation that occurred was reversible and the emulsion could be
redispersed by gentle shaking Fig 24 shows an optical micrograph fora dilute 5050 (vv) emulsion that was stored for 15 and 14 weeks at
50 degC
No change in droplet size was observed after storage for more than
1 year at 50 degC indicating absence of coalescence The same result was
obtained when using different oils Emulsions were also stable against
coalescence in the presence of high electrolyte concentrations (up to
4 mol dmminus3 or ~25 NaCl
The above stability in high electrolyte concentrations is not
observed with polymeric surfactants based on polethylene oxide
The high stability observed using INUTECregSP1 is related to its
strong hydration both in water and in electrolyte solutions The
hydration of inulin (the backbone of HMI) could be assessed using
cloud point measurements A comparison was also made with PEO
with two molecular weights namely 4000 and 20000
Solutionsof PEO 4000 and 20000 showeda systematic decrease of
cloud point with increase in NaCl or MgSO4 concentration In contrast
inulin showed no cloud point up to 4 mol dmminus3 NaCl and up to 1 mol
dmminus3 MgSO4
The above results explain the difference between PEO and inulin
With PEO the chains show dehydration when the NaCl concentration
is increased above 2 mol dmminus3 or 05 mol dmminus3 MgSO4 The inulin
chains remain hydrated at much higher electrolyte concentrations It
seems that the linear polyfructose chains remain strongly hydrated athigh temperature and high electrolyte concentrations
The high emulsion stability obtained whenusingINUTECregSP1 can be
accounted for by the following factors (i) The multi-point attachment
of the polymer by several alkyl chains that are grafted on the backbone
(ii) The strong hydration of the polyfructose ldquoloopsrdquo both in water and
high electrolyte concentrations (χ remains below 05 under these
conditions) (iii) Thehigh volumefraction (concentration) of the loops at
the interface (iv) Enhanced steric stabilization this is the case with
multi-point attachment which produces strong elastic interaction
Evidence for the high stability of the liquid 1047297lm between emulsion
droplets when using INUTECregSP1 was obtained by Exerowa et al [24]
using disjoining pressure measurements This is illustrated in Fig 25
which shows a plot of disjoining pressure versus separation distance
between two emulsion droplets at various electrolyte concentrations
The results show that by increasing the capillary pressure a stable
Newton Black Film (NBF) is obtained at a 1047297lm thickness of sim7 nm
The lack of rupture of the 1047297lm at the highest pressure applied of
45times104 Pa indicate the high stability of the 1047297lm in water and in high
electrolyte concentrations (up to 20 mol dmminus3 NaCl)
The lack of rupture of the NBF up to the highest pressure applied
namely 45times104 Pa clearly indicatesthe high stability of the liquid1047297lm
in the presence of high NaCl concentrations (up to 2 mol dm minus3) This
result is consistent with the high emulsion stability obtained at high
electrolyte concentrations and high temperature Emulsions of Isopar
M-in-water are very stable under such conditions and this could be
accounted for by the high stability of the NBF The droplet size of
5050 OW emulsions prepared using 2 INUTEC regSP1 is in the region
of 1ndash10 μ m This corresponds to a capillary pressure of ~3times104 Pa for
the 1 μ m drops and ~3times103 Pa for the 10 μ m drops These capillarypressures are lower than those to which the NBF have been sub-
jected to and this clearly indicates the high stability obtained against
coalescence in these emulsions
61 WO emulsions stabilized with PHS ndashPEOndashPHS block copolymer
WO emulsions (the oil being Isopar M) were prepared using PHSndash
PEOndashPHS block copolymer at high water volume fractions (N07) The
emulsions have a narrow droplet size distribution with a z -average
radius of 183 nm [25] They also remained 1047298uid up to high water
volume fractions (N06) This could be illustrated from viscosityndash
volume fraction curves as is shown in Fig 26
Fig 20 Energyndashdistance curves for sterically stabilized systems
Fig 21 Variation of Gmin with δR
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The effective volume fraction ϕeff of the emulsions (the core
droplets plus the adsorbed layer) could be calculated from the relative
viscosity and using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth21THORN
Where η r is the relative viscosity ϕp is the maximum packing fraction(sim07) and [ η ] is the intrinsic viscosity that is equal to 25 for hard-
spheres
The calculations based on Eq (22) are shown in Fig 27 (square
symbols) From the effective volume fraction ϕeff and the core volume
fraction ϕ the adsorbed layer thickness could be calculated This was
found to be in the region of 10 nm at ϕ =04 and it decreased with
increase in ϕ
The WO emulsions prepared using the PHSndashPEOndashPHS block
copolymer remained stable both at room temperature and 50 degC
This is consistent with the structure of the block copolymer the B
chain (PEO) is soluble in water and it forms a very strong anchor at the
WO interface The PHS chains (the A chains) provide effective steric
stabilization since the chains are highly soluble in Isopar M and are
strongly solvated by its molecules
7 Suspensions stabilised using polymeric surfactants
There are generally two procedures for preparation of solidliquid
dispersions
(i) Condensation methods build-up of particles from mole-
cular units ie nucleation and growth A special procedure
is the preparation of latexes by emulsion or dispersion
polymerization
(ii) Dispersion methods in this case one starts with preformed
large particles or crystals which are dispersed in the liquid by
using a surfactant (wetting agent) with subsequent breaking up
of the large particles by milling (comminution) to achieve the
desirable particle size distribution A dispersing agent (usuallya
polymeric surfactant) is used for the dispersion process and
subsequent stabilization of the resulting suspension
There are generally two procedures for preparation of latexes
(i) Emulsion polymerization the monomers that are essentially
insoluble in the aqueous medium are emulsi1047297ed using a
surfactant and an initiator is added while heating the system
to produce the polymer particles that are stabilized electro-
statically (when using ionic surfactants) or sterically (when
using non-ionic surfactants)
(ii) Dispersion polymerization the monomers are dissolved in a
solvent in which the resulting polymer particlesare insoluble A
protective colloid (normally a block or graft copolymer) is
added to prevent 1047298occulation of the resulting polymers
particles that are produced on addition of an initiator This
method is usually applied for the production of non-aqueous
latex dispersions and is sometimes referred to as Non-Aqueous
Dispersion Polymerization (NAD)
Surfactants play a crucial role in the process of latex preparation
since they determine the stabilizing ef 1047297ciency and the effectiveness of
the surface active agent ultimately determines the number of particles
Fig 22 Schematic representation of the structure of PHSndashPEOndashPHS block copolymer
Fig 23 Conformation of PHSndash
PEOndash
PHS polymeric surfactant at the WO interface
292 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
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and their size The effectiveness of any surface active agent in
stabilizing the particles is the dominant factor and the number of
micelles formed is relatively unimportant In the NAD process themonomer normally an acrylic is dissolved in a non-aqueous solvent
normally an aliphatic hydrocarbon and an oil soluble initiator and a
stabilizer (to protect the resulting particles from1047298occulation) is added
to the reaction mixture The most successful stabilizers used in NAD
are block and graft copolymers Preformed graft stabilizers based
on poly(12-hydroxy stearic acid) (PHS) are simple to prepare and
effective in NAD polymerization
Dispersion methods are used for the preparation of suspensions of
preformed particles The role of surfactants (or polymers) in the
dispersion process can be analyzed in terms of the three processesinvolved (i) Wetting of the powder by the liquid (ii) Breaking of the
aggregates and agglomerates (iii) Comminution of the resulting
particles and their subsequent stabilization All these processes are
affected by surfactants or polymers which adsorb on the powder
surface thus aiding the wetting of the powder break-up of the
aggregates and agglomerates and subsequent reduction of particle
size by wet milling
71 Polymeric surfactants in emulsion polymerization
Recently the graft copolymer of hydrophobically modi1047297ed inulin
(INUTECreg SP1) has been used in emulsion polymerization of styrene
methyl methacrylate butyl acrylate and several other monomers [26]All lattices were prepared by emulsion polymerisation using potas-
sium persulphate as initiator The z -average particle size was
determined by photon correlation spectroscopy (PCS) and electron
micrographs were also taken
Fig 24 Optical micrographs of OW emulsions stabilized with INUTEC regSP1 stored at
50 degC for 15 weeks (a) and 14 weeks (b)
Fig 25 Variation of disjoining pressure with equivalent 1047297lm thickness at various NaCl concentrations
Fig 26 Viscosityndashvolume fraction for WO emulsion stabilized with PHSndashPEOndashPHS
block copolymer experimental data calculated using Eq (22)
293T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Emulsion polymerization of styrene or methylmethacrylate showed
an optimum weight ratio of (INUTEC)Monomer of 00033 for PS and
0001 for PMMA particles The (initiator)(Monomer) ratio was
kept constant at 000125 The monomer conversion was higher than
85 in all cases Latex dispersions of PS reaching 50 and of PMMA
reaching 40 could be obtained using such low concentration of
INUTECregSP1 Fig 27 shows the variation of particle diameter with
monomer concentration
The stability of the latexes was determined by determining the
critical coagulation concentration (CCC) using CaCl2 The CCC was low
(00175ndash005 mol dmminus3) but this was higher than that for the latex
prepared without surfactant Post addition of INUTEC regSP1 resulted in
a large increase in the CCC as is illustrated in Fig 28 which shows log
W minus log C curves (where W is the ratio between the fast 1047298occulation
rate constant to the slow 1047298occulation rate constant referred to as the
stability ratio) at various additions of INUTECregSP1
As with the emulsions the high stability of the latex when using
INUTECregSP1 is due to the strong adsorption of the polymeric
surfactant on the latex particles and formation of strongly hydrated
loops and tails of polyfructose that provide effective steric stabiliza-
tion Evidence for the strong repulsion produced when using
INUTECregSP1 was obtained from Atomic Force Microscopy investiga-
tions [27] whereby the force between hydrophobic glass spheres and
hydrophobic glass plate both containing an adsorbed layer of
INUTECregSP1 was measured as a function of distance of separation
both in water and in the presence of various Na2SO4 concentrations
The results are shown in Figs 29 and 30
Fig 27 Electron micrographs of the latexes
Fig 28 In1047298uence of post addition of INUTECreg
SP1 on the latex stability
Fig 29 Forcendashdistance curves between hydrophobised glass surfaces containing
adsorbed INUTECreg
SP1 in water
294 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
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711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
295T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
297T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1119
and produce effective steric stabilization against strong 1047298occulation
coalescence and Ostwald ripening [23]
As mentioned above a graft copolymer of the ABn type was
synthesized by grafting several alkyl groups on an inulin (polyfruc-
tose) chain The polymeric surfactant (INUTECregSP1) consists of a
linear polyfructose chain (the stabilizing A chain) and several alkyl
groups (the B chains) that provide multi-anchor attachment to the oil
droplets This polymeric surfactant produces enhanced steric stabili-
zation both in water and high electrolyte concentrations as will be
discussed later
For water-in-oil (WO) emulsions an AndashBndashA block copolymer of
poly (12-hydroxystearic acid) (PHS) (the A chains) and poly (ethylene
oxide) (PEO) (the B chain) PHSndashPEOndashPHS is commercially available
(Arlacel P135 UNIQEMA) The PEO chain (that is soluble in the water
droplets) forms the anchor chain whereas the PHS chains form the
stabilizing chains PHS is highly soluble in most hydrocarbon solvents
and is strongly solvated by its molecules The structure of the PHSndash
PEOndashPHS block copolymer is schematically shown in Fig 22The conformation of the polymeric surfactant at the WO interface
is schematically shown in Fig 23
Emulsions of Isopar Mwater and cyclomethiconewater were
prepared using INUTECregSP1 5050 (vv) OW emulsions were
prepared and the emulsi1047297er concentration was varied from 025 to 2
(wv) based on the oil phase 05 (wv) emulsi1047297erwas suf 1047297cient for
stabilization of these 5050 (vv) emulsions [23]
The emulsions were stored at room temperature and 50 degC and
optical micrographs were taken at intervals of time (for a year) in
order to check the stability Emulsions prepared in water were very
stableshowingno change in droplet size distributionover more than a
year period and this indicated absence of coalescence Any weak
1047298occulation that occurred was reversible and the emulsion could be
redispersed by gentle shaking Fig 24 shows an optical micrograph fora dilute 5050 (vv) emulsion that was stored for 15 and 14 weeks at
50 degC
No change in droplet size was observed after storage for more than
1 year at 50 degC indicating absence of coalescence The same result was
obtained when using different oils Emulsions were also stable against
coalescence in the presence of high electrolyte concentrations (up to
4 mol dmminus3 or ~25 NaCl
The above stability in high electrolyte concentrations is not
observed with polymeric surfactants based on polethylene oxide
The high stability observed using INUTECregSP1 is related to its
strong hydration both in water and in electrolyte solutions The
hydration of inulin (the backbone of HMI) could be assessed using
cloud point measurements A comparison was also made with PEO
with two molecular weights namely 4000 and 20000
Solutionsof PEO 4000 and 20000 showeda systematic decrease of
cloud point with increase in NaCl or MgSO4 concentration In contrast
inulin showed no cloud point up to 4 mol dmminus3 NaCl and up to 1 mol
dmminus3 MgSO4
The above results explain the difference between PEO and inulin
With PEO the chains show dehydration when the NaCl concentration
is increased above 2 mol dmminus3 or 05 mol dmminus3 MgSO4 The inulin
chains remain hydrated at much higher electrolyte concentrations It
seems that the linear polyfructose chains remain strongly hydrated athigh temperature and high electrolyte concentrations
The high emulsion stability obtained whenusingINUTECregSP1 can be
accounted for by the following factors (i) The multi-point attachment
of the polymer by several alkyl chains that are grafted on the backbone
(ii) The strong hydration of the polyfructose ldquoloopsrdquo both in water and
high electrolyte concentrations (χ remains below 05 under these
conditions) (iii) Thehigh volumefraction (concentration) of the loops at
the interface (iv) Enhanced steric stabilization this is the case with
multi-point attachment which produces strong elastic interaction
Evidence for the high stability of the liquid 1047297lm between emulsion
droplets when using INUTECregSP1 was obtained by Exerowa et al [24]
using disjoining pressure measurements This is illustrated in Fig 25
which shows a plot of disjoining pressure versus separation distance
between two emulsion droplets at various electrolyte concentrations
The results show that by increasing the capillary pressure a stable
Newton Black Film (NBF) is obtained at a 1047297lm thickness of sim7 nm
The lack of rupture of the 1047297lm at the highest pressure applied of
45times104 Pa indicate the high stability of the 1047297lm in water and in high
electrolyte concentrations (up to 20 mol dmminus3 NaCl)
The lack of rupture of the NBF up to the highest pressure applied
namely 45times104 Pa clearly indicatesthe high stability of the liquid1047297lm
in the presence of high NaCl concentrations (up to 2 mol dm minus3) This
result is consistent with the high emulsion stability obtained at high
electrolyte concentrations and high temperature Emulsions of Isopar
M-in-water are very stable under such conditions and this could be
accounted for by the high stability of the NBF The droplet size of
5050 OW emulsions prepared using 2 INUTEC regSP1 is in the region
of 1ndash10 μ m This corresponds to a capillary pressure of ~3times104 Pa for
the 1 μ m drops and ~3times103 Pa for the 10 μ m drops These capillarypressures are lower than those to which the NBF have been sub-
jected to and this clearly indicates the high stability obtained against
coalescence in these emulsions
61 WO emulsions stabilized with PHS ndashPEOndashPHS block copolymer
WO emulsions (the oil being Isopar M) were prepared using PHSndash
PEOndashPHS block copolymer at high water volume fractions (N07) The
emulsions have a narrow droplet size distribution with a z -average
radius of 183 nm [25] They also remained 1047298uid up to high water
volume fractions (N06) This could be illustrated from viscosityndash
volume fraction curves as is shown in Fig 26
Fig 20 Energyndashdistance curves for sterically stabilized systems
Fig 21 Variation of Gmin with δR
291T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1219
The effective volume fraction ϕeff of the emulsions (the core
droplets plus the adsorbed layer) could be calculated from the relative
viscosity and using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth21THORN
Where η r is the relative viscosity ϕp is the maximum packing fraction(sim07) and [ η ] is the intrinsic viscosity that is equal to 25 for hard-
spheres
The calculations based on Eq (22) are shown in Fig 27 (square
symbols) From the effective volume fraction ϕeff and the core volume
fraction ϕ the adsorbed layer thickness could be calculated This was
found to be in the region of 10 nm at ϕ =04 and it decreased with
increase in ϕ
The WO emulsions prepared using the PHSndashPEOndashPHS block
copolymer remained stable both at room temperature and 50 degC
This is consistent with the structure of the block copolymer the B
chain (PEO) is soluble in water and it forms a very strong anchor at the
WO interface The PHS chains (the A chains) provide effective steric
stabilization since the chains are highly soluble in Isopar M and are
strongly solvated by its molecules
7 Suspensions stabilised using polymeric surfactants
There are generally two procedures for preparation of solidliquid
dispersions
(i) Condensation methods build-up of particles from mole-
cular units ie nucleation and growth A special procedure
is the preparation of latexes by emulsion or dispersion
polymerization
(ii) Dispersion methods in this case one starts with preformed
large particles or crystals which are dispersed in the liquid by
using a surfactant (wetting agent) with subsequent breaking up
of the large particles by milling (comminution) to achieve the
desirable particle size distribution A dispersing agent (usuallya
polymeric surfactant) is used for the dispersion process and
subsequent stabilization of the resulting suspension
There are generally two procedures for preparation of latexes
(i) Emulsion polymerization the monomers that are essentially
insoluble in the aqueous medium are emulsi1047297ed using a
surfactant and an initiator is added while heating the system
to produce the polymer particles that are stabilized electro-
statically (when using ionic surfactants) or sterically (when
using non-ionic surfactants)
(ii) Dispersion polymerization the monomers are dissolved in a
solvent in which the resulting polymer particlesare insoluble A
protective colloid (normally a block or graft copolymer) is
added to prevent 1047298occulation of the resulting polymers
particles that are produced on addition of an initiator This
method is usually applied for the production of non-aqueous
latex dispersions and is sometimes referred to as Non-Aqueous
Dispersion Polymerization (NAD)
Surfactants play a crucial role in the process of latex preparation
since they determine the stabilizing ef 1047297ciency and the effectiveness of
the surface active agent ultimately determines the number of particles
Fig 22 Schematic representation of the structure of PHSndashPEOndashPHS block copolymer
Fig 23 Conformation of PHSndash
PEOndash
PHS polymeric surfactant at the WO interface
292 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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and their size The effectiveness of any surface active agent in
stabilizing the particles is the dominant factor and the number of
micelles formed is relatively unimportant In the NAD process themonomer normally an acrylic is dissolved in a non-aqueous solvent
normally an aliphatic hydrocarbon and an oil soluble initiator and a
stabilizer (to protect the resulting particles from1047298occulation) is added
to the reaction mixture The most successful stabilizers used in NAD
are block and graft copolymers Preformed graft stabilizers based
on poly(12-hydroxy stearic acid) (PHS) are simple to prepare and
effective in NAD polymerization
Dispersion methods are used for the preparation of suspensions of
preformed particles The role of surfactants (or polymers) in the
dispersion process can be analyzed in terms of the three processesinvolved (i) Wetting of the powder by the liquid (ii) Breaking of the
aggregates and agglomerates (iii) Comminution of the resulting
particles and their subsequent stabilization All these processes are
affected by surfactants or polymers which adsorb on the powder
surface thus aiding the wetting of the powder break-up of the
aggregates and agglomerates and subsequent reduction of particle
size by wet milling
71 Polymeric surfactants in emulsion polymerization
Recently the graft copolymer of hydrophobically modi1047297ed inulin
(INUTECreg SP1) has been used in emulsion polymerization of styrene
methyl methacrylate butyl acrylate and several other monomers [26]All lattices were prepared by emulsion polymerisation using potas-
sium persulphate as initiator The z -average particle size was
determined by photon correlation spectroscopy (PCS) and electron
micrographs were also taken
Fig 24 Optical micrographs of OW emulsions stabilized with INUTEC regSP1 stored at
50 degC for 15 weeks (a) and 14 weeks (b)
Fig 25 Variation of disjoining pressure with equivalent 1047297lm thickness at various NaCl concentrations
Fig 26 Viscosityndashvolume fraction for WO emulsion stabilized with PHSndashPEOndashPHS
block copolymer experimental data calculated using Eq (22)
293T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Emulsion polymerization of styrene or methylmethacrylate showed
an optimum weight ratio of (INUTEC)Monomer of 00033 for PS and
0001 for PMMA particles The (initiator)(Monomer) ratio was
kept constant at 000125 The monomer conversion was higher than
85 in all cases Latex dispersions of PS reaching 50 and of PMMA
reaching 40 could be obtained using such low concentration of
INUTECregSP1 Fig 27 shows the variation of particle diameter with
monomer concentration
The stability of the latexes was determined by determining the
critical coagulation concentration (CCC) using CaCl2 The CCC was low
(00175ndash005 mol dmminus3) but this was higher than that for the latex
prepared without surfactant Post addition of INUTEC regSP1 resulted in
a large increase in the CCC as is illustrated in Fig 28 which shows log
W minus log C curves (where W is the ratio between the fast 1047298occulation
rate constant to the slow 1047298occulation rate constant referred to as the
stability ratio) at various additions of INUTECregSP1
As with the emulsions the high stability of the latex when using
INUTECregSP1 is due to the strong adsorption of the polymeric
surfactant on the latex particles and formation of strongly hydrated
loops and tails of polyfructose that provide effective steric stabiliza-
tion Evidence for the strong repulsion produced when using
INUTECregSP1 was obtained from Atomic Force Microscopy investiga-
tions [27] whereby the force between hydrophobic glass spheres and
hydrophobic glass plate both containing an adsorbed layer of
INUTECregSP1 was measured as a function of distance of separation
both in water and in the presence of various Na2SO4 concentrations
The results are shown in Figs 29 and 30
Fig 27 Electron micrographs of the latexes
Fig 28 In1047298uence of post addition of INUTECreg
SP1 on the latex stability
Fig 29 Forcendashdistance curves between hydrophobised glass surfaces containing
adsorbed INUTECreg
SP1 in water
294 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
295T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
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radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1219
The effective volume fraction ϕeff of the emulsions (the core
droplets plus the adsorbed layer) could be calculated from the relative
viscosity and using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth21THORN
Where η r is the relative viscosity ϕp is the maximum packing fraction(sim07) and [ η ] is the intrinsic viscosity that is equal to 25 for hard-
spheres
The calculations based on Eq (22) are shown in Fig 27 (square
symbols) From the effective volume fraction ϕeff and the core volume
fraction ϕ the adsorbed layer thickness could be calculated This was
found to be in the region of 10 nm at ϕ =04 and it decreased with
increase in ϕ
The WO emulsions prepared using the PHSndashPEOndashPHS block
copolymer remained stable both at room temperature and 50 degC
This is consistent with the structure of the block copolymer the B
chain (PEO) is soluble in water and it forms a very strong anchor at the
WO interface The PHS chains (the A chains) provide effective steric
stabilization since the chains are highly soluble in Isopar M and are
strongly solvated by its molecules
7 Suspensions stabilised using polymeric surfactants
There are generally two procedures for preparation of solidliquid
dispersions
(i) Condensation methods build-up of particles from mole-
cular units ie nucleation and growth A special procedure
is the preparation of latexes by emulsion or dispersion
polymerization
(ii) Dispersion methods in this case one starts with preformed
large particles or crystals which are dispersed in the liquid by
using a surfactant (wetting agent) with subsequent breaking up
of the large particles by milling (comminution) to achieve the
desirable particle size distribution A dispersing agent (usuallya
polymeric surfactant) is used for the dispersion process and
subsequent stabilization of the resulting suspension
There are generally two procedures for preparation of latexes
(i) Emulsion polymerization the monomers that are essentially
insoluble in the aqueous medium are emulsi1047297ed using a
surfactant and an initiator is added while heating the system
to produce the polymer particles that are stabilized electro-
statically (when using ionic surfactants) or sterically (when
using non-ionic surfactants)
(ii) Dispersion polymerization the monomers are dissolved in a
solvent in which the resulting polymer particlesare insoluble A
protective colloid (normally a block or graft copolymer) is
added to prevent 1047298occulation of the resulting polymers
particles that are produced on addition of an initiator This
method is usually applied for the production of non-aqueous
latex dispersions and is sometimes referred to as Non-Aqueous
Dispersion Polymerization (NAD)
Surfactants play a crucial role in the process of latex preparation
since they determine the stabilizing ef 1047297ciency and the effectiveness of
the surface active agent ultimately determines the number of particles
Fig 22 Schematic representation of the structure of PHSndashPEOndashPHS block copolymer
Fig 23 Conformation of PHSndash
PEOndash
PHS polymeric surfactant at the WO interface
292 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
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and their size The effectiveness of any surface active agent in
stabilizing the particles is the dominant factor and the number of
micelles formed is relatively unimportant In the NAD process themonomer normally an acrylic is dissolved in a non-aqueous solvent
normally an aliphatic hydrocarbon and an oil soluble initiator and a
stabilizer (to protect the resulting particles from1047298occulation) is added
to the reaction mixture The most successful stabilizers used in NAD
are block and graft copolymers Preformed graft stabilizers based
on poly(12-hydroxy stearic acid) (PHS) are simple to prepare and
effective in NAD polymerization
Dispersion methods are used for the preparation of suspensions of
preformed particles The role of surfactants (or polymers) in the
dispersion process can be analyzed in terms of the three processesinvolved (i) Wetting of the powder by the liquid (ii) Breaking of the
aggregates and agglomerates (iii) Comminution of the resulting
particles and their subsequent stabilization All these processes are
affected by surfactants or polymers which adsorb on the powder
surface thus aiding the wetting of the powder break-up of the
aggregates and agglomerates and subsequent reduction of particle
size by wet milling
71 Polymeric surfactants in emulsion polymerization
Recently the graft copolymer of hydrophobically modi1047297ed inulin
(INUTECreg SP1) has been used in emulsion polymerization of styrene
methyl methacrylate butyl acrylate and several other monomers [26]All lattices were prepared by emulsion polymerisation using potas-
sium persulphate as initiator The z -average particle size was
determined by photon correlation spectroscopy (PCS) and electron
micrographs were also taken
Fig 24 Optical micrographs of OW emulsions stabilized with INUTEC regSP1 stored at
50 degC for 15 weeks (a) and 14 weeks (b)
Fig 25 Variation of disjoining pressure with equivalent 1047297lm thickness at various NaCl concentrations
Fig 26 Viscosityndashvolume fraction for WO emulsion stabilized with PHSndashPEOndashPHS
block copolymer experimental data calculated using Eq (22)
293T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
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Emulsion polymerization of styrene or methylmethacrylate showed
an optimum weight ratio of (INUTEC)Monomer of 00033 for PS and
0001 for PMMA particles The (initiator)(Monomer) ratio was
kept constant at 000125 The monomer conversion was higher than
85 in all cases Latex dispersions of PS reaching 50 and of PMMA
reaching 40 could be obtained using such low concentration of
INUTECregSP1 Fig 27 shows the variation of particle diameter with
monomer concentration
The stability of the latexes was determined by determining the
critical coagulation concentration (CCC) using CaCl2 The CCC was low
(00175ndash005 mol dmminus3) but this was higher than that for the latex
prepared without surfactant Post addition of INUTEC regSP1 resulted in
a large increase in the CCC as is illustrated in Fig 28 which shows log
W minus log C curves (where W is the ratio between the fast 1047298occulation
rate constant to the slow 1047298occulation rate constant referred to as the
stability ratio) at various additions of INUTECregSP1
As with the emulsions the high stability of the latex when using
INUTECregSP1 is due to the strong adsorption of the polymeric
surfactant on the latex particles and formation of strongly hydrated
loops and tails of polyfructose that provide effective steric stabiliza-
tion Evidence for the strong repulsion produced when using
INUTECregSP1 was obtained from Atomic Force Microscopy investiga-
tions [27] whereby the force between hydrophobic glass spheres and
hydrophobic glass plate both containing an adsorbed layer of
INUTECregSP1 was measured as a function of distance of separation
both in water and in the presence of various Na2SO4 concentrations
The results are shown in Figs 29 and 30
Fig 27 Electron micrographs of the latexes
Fig 28 In1047298uence of post addition of INUTECreg
SP1 on the latex stability
Fig 29 Forcendashdistance curves between hydrophobised glass surfaces containing
adsorbed INUTECreg
SP1 in water
294 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
295T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
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radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1319
and their size The effectiveness of any surface active agent in
stabilizing the particles is the dominant factor and the number of
micelles formed is relatively unimportant In the NAD process themonomer normally an acrylic is dissolved in a non-aqueous solvent
normally an aliphatic hydrocarbon and an oil soluble initiator and a
stabilizer (to protect the resulting particles from1047298occulation) is added
to the reaction mixture The most successful stabilizers used in NAD
are block and graft copolymers Preformed graft stabilizers based
on poly(12-hydroxy stearic acid) (PHS) are simple to prepare and
effective in NAD polymerization
Dispersion methods are used for the preparation of suspensions of
preformed particles The role of surfactants (or polymers) in the
dispersion process can be analyzed in terms of the three processesinvolved (i) Wetting of the powder by the liquid (ii) Breaking of the
aggregates and agglomerates (iii) Comminution of the resulting
particles and their subsequent stabilization All these processes are
affected by surfactants or polymers which adsorb on the powder
surface thus aiding the wetting of the powder break-up of the
aggregates and agglomerates and subsequent reduction of particle
size by wet milling
71 Polymeric surfactants in emulsion polymerization
Recently the graft copolymer of hydrophobically modi1047297ed inulin
(INUTECreg SP1) has been used in emulsion polymerization of styrene
methyl methacrylate butyl acrylate and several other monomers [26]All lattices were prepared by emulsion polymerisation using potas-
sium persulphate as initiator The z -average particle size was
determined by photon correlation spectroscopy (PCS) and electron
micrographs were also taken
Fig 24 Optical micrographs of OW emulsions stabilized with INUTEC regSP1 stored at
50 degC for 15 weeks (a) and 14 weeks (b)
Fig 25 Variation of disjoining pressure with equivalent 1047297lm thickness at various NaCl concentrations
Fig 26 Viscosityndashvolume fraction for WO emulsion stabilized with PHSndashPEOndashPHS
block copolymer experimental data calculated using Eq (22)
293T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
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Emulsion polymerization of styrene or methylmethacrylate showed
an optimum weight ratio of (INUTEC)Monomer of 00033 for PS and
0001 for PMMA particles The (initiator)(Monomer) ratio was
kept constant at 000125 The monomer conversion was higher than
85 in all cases Latex dispersions of PS reaching 50 and of PMMA
reaching 40 could be obtained using such low concentration of
INUTECregSP1 Fig 27 shows the variation of particle diameter with
monomer concentration
The stability of the latexes was determined by determining the
critical coagulation concentration (CCC) using CaCl2 The CCC was low
(00175ndash005 mol dmminus3) but this was higher than that for the latex
prepared without surfactant Post addition of INUTEC regSP1 resulted in
a large increase in the CCC as is illustrated in Fig 28 which shows log
W minus log C curves (where W is the ratio between the fast 1047298occulation
rate constant to the slow 1047298occulation rate constant referred to as the
stability ratio) at various additions of INUTECregSP1
As with the emulsions the high stability of the latex when using
INUTECregSP1 is due to the strong adsorption of the polymeric
surfactant on the latex particles and formation of strongly hydrated
loops and tails of polyfructose that provide effective steric stabiliza-
tion Evidence for the strong repulsion produced when using
INUTECregSP1 was obtained from Atomic Force Microscopy investiga-
tions [27] whereby the force between hydrophobic glass spheres and
hydrophobic glass plate both containing an adsorbed layer of
INUTECregSP1 was measured as a function of distance of separation
both in water and in the presence of various Na2SO4 concentrations
The results are shown in Figs 29 and 30
Fig 27 Electron micrographs of the latexes
Fig 28 In1047298uence of post addition of INUTECreg
SP1 on the latex stability
Fig 29 Forcendashdistance curves between hydrophobised glass surfaces containing
adsorbed INUTECreg
SP1 in water
294 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
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711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
295T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
297T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
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With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1419
Emulsion polymerization of styrene or methylmethacrylate showed
an optimum weight ratio of (INUTEC)Monomer of 00033 for PS and
0001 for PMMA particles The (initiator)(Monomer) ratio was
kept constant at 000125 The monomer conversion was higher than
85 in all cases Latex dispersions of PS reaching 50 and of PMMA
reaching 40 could be obtained using such low concentration of
INUTECregSP1 Fig 27 shows the variation of particle diameter with
monomer concentration
The stability of the latexes was determined by determining the
critical coagulation concentration (CCC) using CaCl2 The CCC was low
(00175ndash005 mol dmminus3) but this was higher than that for the latex
prepared without surfactant Post addition of INUTEC regSP1 resulted in
a large increase in the CCC as is illustrated in Fig 28 which shows log
W minus log C curves (where W is the ratio between the fast 1047298occulation
rate constant to the slow 1047298occulation rate constant referred to as the
stability ratio) at various additions of INUTECregSP1
As with the emulsions the high stability of the latex when using
INUTECregSP1 is due to the strong adsorption of the polymeric
surfactant on the latex particles and formation of strongly hydrated
loops and tails of polyfructose that provide effective steric stabiliza-
tion Evidence for the strong repulsion produced when using
INUTECregSP1 was obtained from Atomic Force Microscopy investiga-
tions [27] whereby the force between hydrophobic glass spheres and
hydrophobic glass plate both containing an adsorbed layer of
INUTECregSP1 was measured as a function of distance of separation
both in water and in the presence of various Na2SO4 concentrations
The results are shown in Figs 29 and 30
Fig 27 Electron micrographs of the latexes
Fig 28 In1047298uence of post addition of INUTECreg
SP1 on the latex stability
Fig 29 Forcendashdistance curves between hydrophobised glass surfaces containing
adsorbed INUTECreg
SP1 in water
294 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1519
711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
295T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1619
layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
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Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
297T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1819
radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1919
With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1519
711 Dispersion polymerization
This method is usually applied for the preparation of non-aqueous
latex dispersions and hence it is referred to as NAD Themethod has also
been adapted to prepare aqueouslatex dispersions by using an alcoholndash
water mixture In the NAD process the monomer normally an acrylic
is dissolved in a non-aqueous solvent normally an aliphatic hydro-
carbon and an oil soluble initiator and a stabilizer (to protect the
resulting particles from 1047298occulation) is added to the reaction mixture
The most successful stabilizers used in NAD are block and graft copo-
lymers Preformed graft stabilizers based on poly(12-hydroxy stearic
acid) (PHS) are simple to prepare and effective in NAD polymerization
Commercial 12-hydroxystearic acid contains 8ndash15 palmitic and
stearic acids which limits the molecular weight during polymerization
to an average of 1500ndash2000 This oligomer may be converted to a
lsquorsquomacromonomer rsquorsquo by reacting the carboxylic group with glycidyl
methacrylate The macromonomer is then copolymerized with an
equal weight of methyl methacrylate (MMA) or similar monomer to
give a lsquorsquocombrsquorsquo graft copolymer with an average molecular weight of 10000ndash20000 The graft copolymer contains on average 5ndash10 PHS
chains pendent from a polymeric anchor backbone of PMMA This a
graft copolymer can stabilize latex particles of various monomers The
major limitation of the monomer composition is that the polymer
produced should be insoluble in the medium used
NAD polymerization is carried in two steps (i) Seed stage the
diluent portion of the monomer portion of dispersant and initiator
(azo or peroxy type) are heated to form an initial low-concentration
1047297ne dispersion (ii) Growth stage the remaining monomer together
with more dispersant and initiator are then fed over the course of
several hours to complete the growth of the particles A small amount
of transfer agent is usually added to control the molecular weight
Excellent control of particle size is achieved by proper choice of the
designed dispersant and correct distribution of dispersant between
the seed and growth stages NAD acrylic polymers are applied in
automotive thermosetting polymers and hydroxy monomers may be
included in the monomer blend used
72 Polymeric surfactants for stabilisation of preformed latex dispersions
For this purpose polystyrene (PS) latexes were prepared using the
surfactant-free emulsion polymerisation Two latexes with z -average
diameter of 427 and 867 (as measured using Photon Correlation
Spectroscopy PCS) that are reasonably monodisperse were prepared
[28] Two polymeric surfactants namely Hypermer CG-6 and Atlox
4913 (UNIQEMA UK) were used Both are graft (lsquorsquocombrsquorsquo) type
consisting of polymethylmethacrylatepolymethacrylic acid (PMMA
PMA) backbone with methoxy-capped polyethylene oxide (PEO) side
chains (M =750 Da) Hypermer CG-6 is the same graft copolymer asAtlox 4913 but it contains higher proportion of methacrylic acid in the
backbone The average molecular weight of the polymer is ~5000 Da
Fig 31 shows a typical adsorption isotherm of Atlox 4913 on the two
latexes
Similar results were obtained for Hypermer CG-6 but the plateau
adsorption was lower (12 mg mminus2 compared with 15 mg mminus2 for
Atlox 4913) It is likely that the backbone of Hypermer CG-6 that
contains more PMA is more polar and hence less strongly adsorbed
The amount of adsorption was independent of particle size
The in1047298uence of temperature on adsorption is shown in Fig 32The
amount of adsorption increases with increase of temperature This is
due to the poorer solvency of the medium for the PEO chains The PEO
chains become less hydrated at higher temperature and the reduction
of solubility of the polymer enhances adsorptionThe adsorbed layer thickness of the graft copolymer on the latexes
was determined using rheological measurements Steady state (shear
stress σ minusshear rate γ ) measurements were carried out and the results
were 1047297tted to the Bingham equation to obtain the yield value σ β and
the high shear viscosity η of the suspension
σ = σ β + ηγ eth22THORN
As an illustration Fig 33 shows a plot of σ β versus volume fraction
ϕ of the latex for Atlox 4913 Similar results were obtained for latexes
stabilised using Hypermer CG-6
At any given volume fraction the smaller latex has higherσ β when
compared to the larger latex This is dueto thehigher ratio of adsorbed
Fig 30 Forcendashdistance curves for hydrophobised glass surfaces containing adsorbed
INUTECregSP1 at various Na2SO4 concentrations
Fig 31 Adsorption isotherms of Atlox 4913 on the two latexes at 25 degC
Fig 32 Effect of temperature on adsorption of Atlox 4913 on PS
295T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1619
layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1719
Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
297T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1819
radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1919
With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1619
layer thickness to particle radius ΔR for the smaller latex The
effective volume fraction of the latex ϕeff is related to the core volume
fraction ϕ by the equation
eff = 1 + Δ
R
3
eth23THORN
As discussed before ϕeff can be calculated from the relative
viscosity η r using the DoughertyndashKrieger equation
η r = 1minuseff
p
minus η frac12 p
eth24THORN
where ϕp is the maximum packing fraction
The maximum packing fraction ϕp can be calculated using the
following empirical Eq (28)
η 1=2r minus1
= 1
p
η 1=2
minus1
+ 125 eth25THORN
The results showed a gradual decrease of adsorbed layer thickness
Δ with increase of the volume fraction ϕ For the latex with diameter
D of 867 nm and Atlox 4913 Δ decreased from 175 nm at ϕ=036 to
65 at ϕ =057 For Hypermer CG-6 with the same latex Δ decreased
from 118 nm at ϕ =049 to 65 at ϕ =057 The reduction of Δ with
increase in ϕ may be due to overlap andor compression of the
adsorbed layers as the particles come close to each other at higher
volume fraction of the latex
The stability of the latexes was determined using viscoelastic
measurements For this purpose dynamic (oscillatory) measurements
were used to obtain the storage modulus G
the elastic modulus Gprime
and the viscous modulus GPrime as a function of strain amplitude γ o and
frequency ω (rad sminus1) The method relies on application of a sinusoidal
strain or stress and the resulting stress or strain is measured
simultaneously For a viscoelastic system the strain and stress sine
waves oscillate with the same frequency but out of phase From the
time shift Δt and ω one can obtain the phase angle shift δ
The ratio of the maximum stress σ o to the maximum strain γ ogives the complex modulus |G|
jGj = σ oγ o
eth26THORN
|G| can be resolved into two components Storage (elastic) modulus Gprime
the real component of the complex modulus loss (viscous) modulusGPrime
the imaginary component of the complex modulus The complex
modulus can be resolved into Gprime and GPrime using vector analysis and the
phase angle shift δ
G V = jGj cosδ eth27THORN
GW = jGj sinδ eth28THORN
Gprime is measured as a function of electrolyte concentration andor
temperature to assess the latex stability As an illustration Fig 34shows the variation of Gprime with temperature for latex stabilised with
Atlox 4913 in the absence of any added electrolyte and in the presence
of 01 02 and 03 mol dmminus3 Na2SO4 In the absence of electrolyte Gprime
showed no change with temperature up to 65 degC
In the presence of 01 mol dmminus3 Na2SO4 Gprime remained constant up
to 40 degC above which Gprime increased with further increase of
temperature This temperature is denoted as the critical 1047298occulation
temperature (CFT) The CFT decreases with increase in electrolyte
concentration reaching ~30 degC in 02 and 03 mol dmminus3 Na2SO4 This
reduction in CFT with increase in electrolyte concentration is due to
the reduction in solvency of the PEO chains with increase in
electrolyte concentrations The latex stabilised with Hypermer CG-6
gave relatively higher CFT values when compared with that stabilised
using Atlox 4913
73 Use of polymeric surfactants for preparation and stabilisation of
nano-emulsions
Nano-emulsions are systems that cover the size range 20ndash200 nm
[29ndash31] They can be transparent translucent or turbid depending on
the droplet radius and refractive index difference between the
droplets and the continuous phase This can be understood from
consideration of the dependence of light scattering (turbidity) on the
above two factors For droplets with a radius that is less than (120) of
the wave length of the light the turbidity τ is given by the following
equation
τ = KN oV 2 eth29THORN
Where K is an optical constant that is related to the difference in
refractive index between the droplets np and the medium no and N o is
the number of droplets each with a volume V
It is clear from Eq (30) that τ decreases with decrease of K ie
smaller (npminusno) decrease of N o and decrease of V Thus to produce a
transparent nano-emulsion one has to decrease the difference
between the refractive index of the droplets and the medium (ie
try to match the two refractive indices) If such matching is not
possible then one has to reduce the droplet size (by high pressure
homogenization) to values below 50 nm It is also necessary to use a
nano-emulsion with low oil volume fraction (in the region of 02)
Fig 34 Variationof Gprime
withtemperature in water andat various Na2SO4 concentrations
Fig 33 Variation of yield stress with latex volume fraction for Atlox 4913
296 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1719
Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
297T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1819
radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1919
With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1719
Nano-emulsions are only kinetically stable They have to distin-
guished from microemulsions (that cover the size range 5ndash50 nm)
which are mostly transparent and thermodynamically stable The longterm physical stability of nano-emulsions (with no apparent 1047298occula-
tion or coalescence) make them unique and they are sometimes
referred to as ldquoApproaching Thermodynamic Stabilityrdquo The inherently
high colloid stabilityof nano-emulsions canbe well understood from a
consideration of their steric stabilisation (when using nonionic
surfactants andor polymers) and how this is affected by the ratio of
theadsorbedlayer thickness to droplet radius as was discussedbefore
Unless adequately prepared (to control the droplet size distribu-
tion) and stabilised against Ostwald ripening (that occurswhen the oil
hassome 1047297nite solubility in the continuous medium) nano-emulsions
may show an increase in the droplet size and an initially transparent
system may become turbid on storage
The attraction of nano-emulsions for application in Personal Careand Cosmetics as well as in Health care is due to the following
advantages (i) The very small droplet size causes a large reduction in
the gravity force and the Brownian motion may be suf 1047297cient for
overcoming gravity mdash This means that no creaming or sedimentation
occurs on storage (ii) The small droplet size also prevents any
1047298occulation of the droplets Weak 1047298occulation is prevented and this
enables the system to remain dispersed with no separation (iii) The
small droplets also prevent their coalescence since these droplets are
non-deformable and hence surface 1047298uctuations are prevented In
addition the signi1047297cant surfactant 1047297lm thickness (relative to droplet
Fig 35 Silicone oil in water nano-emulsions stabilized with INUTECregSP1
Fig 36 r 3
versus t for nano-emulsions based on hydrocarbon oils
297T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1819
radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1919
With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1819
radius) prevents any thinning or disruption of the liquid 1047297lm between
the droplets
The production of small droplets (submicron) requires application
of high energy The process of emulsi1047297cation is generally inef 1047297cient
Simple calculations show that the mechanical energy required for
emulsi1047297cation exceeds the interfacial energy by several orders of
magnitude For example to produce a nano-emulsion atϕ=01 with an
average radius R of 200 nm using a surfactant that gives an interfacial
tension γ =10 mNm
minus1
the net increase in surface free energy is Aγ = 3ϕγ R =15times104 Jmminus3 The mechanical energy required in a
homogenizer is 15times107 Jmminus3 ie an ef 1047297ciency of 01 The rest of
the energy (999) is dissipated as heat
The intensity of the process or the effectiveness in making small
droplets is often governed by the net power density (ε (t ))
p = e t eth THORNdt eth30THORN
where t is the time during which emulsi1047297cation occurs
Break up of droplets will only occur at high ε values which means
that the energy dissipated at low ε levels is wasted Batch processes
are generally less ef 1047297cient than continuous processes This shows why
with a stirrer in a large vessel most of the energy applies at low
intensity is dissipated as heat In a homogenizer p is simply equal to
the homogenizer pressureSeveral procedures may be applied to enhance the ef 1047297ciency of
emulsi1047297cation when producing nano-emulsions (i) One should
optimise the ef 1047297ciency of agitation by increasing ε and decreasing
dissipation time (ii)The nano-emulsion is preferably prepared at high
volume faction of the disperse phase and diluted afterwards However
very high ϕ values may result in coalescence during emulsi1047297cation
(iii) Add more surfactant whereby creating a smaller γ eff and possibly
diminishing recoalescence (iv) Use surfactant mixture that show
more reduction in γ the individual components (v) If possible
dissolve the surfactant in the disperse phase rather than the
continuous phase mdash this often leads to smaller droplets (vi) It may
be useful to emulsify in steps of increasing intensity particularly with
nano-emulsions having highly viscous disperse phase
The high kinetic stability of nano-emulsions can be explained fromconsideration of the energyndashdistance curves for sterically stabilized
dispersions shown in Fig 21 It can be seen from Fig 21 that the depth
of the minimum decrease with increasing δR With nano-emulsions
having a radius in the region of 50 nm andan adsorbed layer thickness
of say 10 nm the value of δR is 02 This high value (when compared
with the situation with macroemulsions where δR is at least an order
of magnitude lower) results in a very shallow minimum (which could
be less than kT ) This situation results in very high stability with no
1047298occulation (weak or strong) In addition the very small size of the
droplets and the dense adsorbed layers ensures lack of deformation of
the interface lack of thinning and disruption of the liquid 1047297lm
between the droplets and hence coalescence is also prevented
One of the main problems with Nanoemulsions is Ostwald
ripening which results from the difference in solubility between
small and large droplets The difference in chemical potential of
dispersed phase droplets between different sized droplets as given by
Lord Kelvin [32]
s r eth THORN = s infineth THORNexp2γ V m
rRT
eth31THORN
where s(r ) is the solubility surrounding a particle of radius r c (infin) is
the bulk phase solubility and V m is the molar volume of the dispersed
phase
The quantity (2γ V mRT ) is termed the characteristic length It has
an order of ~ 1 nm or less indicating that the difference in solubility of
a 1 μ m droplet is of the order of 01 or less
Theoretically Ostwald ripening should lead to condensation of all
droplets into a single drop (ie phase separation) This does not occur
in practice since the rate of growth decreases with increase of droplet
size
For two droplets of radii r 1 and r 2 (where r 1br 2)
RT
V mln
s r 1eth THORN
s r 2eth THORN
= 2γ
1
r 1minus
1
r 2
eth32THORN
Eq (33) shows that the larger the difference between r 1 and r 2 the
higher the rate of Ostwald ripening
Ostwald ripening can be quantitatively assessed from plots of thecube of the radius versus time t [33ndash35]
r 3 = 8
9
s infineth THORNγ V mD
ρRT
eth33THORN
where D is the diffusion coef 1047297cient of the disperse phase in the
continuous phase and ρ is its density
Ostwald ripening can be reduced by incorporation of a second
component which is insoluble in the continuous phase (eg squalane)
In this case signi1047297cant partitioning between different droplets occurs
with the component having low solubility in the continuous phase
expected to be concentrated in the smaller droplets During Ostwald
ripening in two component disperse phase system equilibrium is
established when the difference in chemical potential between
different size droplets (which results from curvature effects) is
balanced by the difference in chemical potential resulting from
partitioning of the two components If the secondary component has
zero solubility in the continuous phase the size distribution will not
deviate from the initial one (the growth rate is equal to zero) In the
case of limited solubility of the secondarycomponent the distribution
is the same as governed by Eq (33) ie a mixture growth rate is
obtained which is still lower than that of the more soluble component
Another method for reducing Ostwald ripening depends on
modi1047297cation of the interfacial 1047297lm at the OW interface According
to Eq (33) reduction in γ results in reduction of Ostwald ripening
However this alone is not suf 1047297cient since one has to reduce γ by
several orders of magnitude Walstra [3637] suggested that by using
surfactants which are strongly adsorbed at the OW interface (ie
polymeric surfactants) and which do not desorb during ripening therate could be signi1047297cantly reduced An increase in the surface
dilational modulus ε and decrease in γ would be observed for the
shrinking drops The difference in ε between the droplets would
balance the difference in capillary pressure (ie curvature effects)
To achieve the above effect it is useful to use AndashBndashA block
copolymers that are soluble in the oil phase and insoluble in the
continuous phase A strongly adsorbed polymeric surfactant that has
limited solubility in the aqueous phase can also be used eg
hydrophobically modi1047297ed inulin INUTECregSP1 This is illustrated in
Fig 35 which shows plots of r 3 versus time for 20 vv silicone oil-in-
water emulsions at two concentrations of INUTECregSP1 (16 top
curve and 24 bottom curve) [38] The concentration of INUTECregSP1
is much lower than that required when using non-ionic surfactants
The rate of Ostwald ripening is 11times10minus
29 and 24times10minus
30 m3sminus
1 at16 and 24 INUTECregSP1 respectively These rates are ~3 orders of
magnitude lower than those obtained using a nonionic surfactant
Addition of 5 glycerol was found to decrease the rate of Ostwald
ripening in some nano-emulsions which may be due to the lower oil
solubility in the waterndashglycerol mixture
Various nano-emulsions with hydrocarbon oils of different
solubility were prepared using INUTECregSP1 Fig 36 shows plots of r 3
versus t for nano-emulsions of the hydrocarbon oils that were stored
at 50 degC It can be seen that both parraf 1047297num liquidum with low and
high viscosity give almost a zero-slope indicating absence of Ostwald-
ripening in this case This is not surprising since both oils have very
low solubility and the hydrophobically modi1047297ed inulin INUTECregSP1
strongly adsorbs at the interface giving high elasticity that reduces
both Ostwald ripening and coalescence
298 T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1919
With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299
8182019 Polymeric Surfactants in Disperse System
httpslidepdfcomreaderfullpolymeric-surfactants-in-disperse-system 1919
With the more soluble hydrocarbon oils namely isohexadecane
there is an increase in r 3 with time giving a rate of Ostwald ripening of
41times10minus27 m3 sminus1 The rate for this oil is almost three orders of a
magnitude lower than that obtained with a non-ionic surfactant
namely laureth-4 (C12-alkylchain with 4 mol ethylene-oxide) This
clearly shows the effectiveness of INUTECregSP1 in reducing Ostwald-
ripening This reduction can be attributed to the enhancement of the
Gibbs-dilational elasticity which results from the multi-point attach-
ment of the polymeric surfactant with several alkyl-groups to the oil-droplets This results in a reduction of the molecular diffusion of the
oil from the smaller to the larger droplets
References
[1] Tadros Tharwat F Applied Surfactants Germany Wiley-VCH 2005[2] Tadros ThF In Goddard ED Gruber JV editors Principles of Polymer Science and
Technology in Cosmetics and Personal Care NY Marcel Dekker 1999[3] Tadros Tharwat In Holmberg K editor Novel Surfactants NY Marcel Dekker
2003[4] JW Gibbs lsquorsquoScienti1047297c Papersrsquorsquo Longman Green London (1906) M Volmer Kinetik
der Phase Buildung Steinkopf Dresden (1939)[5] Piirma I Polymeric Surfactants Surfactant Science Series No42NY Marcel
Dekker 1992[6] Stevens CV Meriggi A Peristerpoulou M Christov PP Booten K Levecke B et al
Biomacromolecules 200121256[7] Hirst EL McGilvary DI Percival EG J Chem Soc 19501297
[8] Suzuki M In Suzuki M Chatterton NJ editors Science and Technologyof FructansBoca Raton Fl CRC Press 1993 p 21
[9] Flory PJ Principles of Polymer Chemistry NY Cornell University Press 1953[10] Tadros ThF In Buscall R Corner T Stageman editors Polymer Colloids Applied
SciencesLondon Elsevier 1985 p 105[11] Silberberg A J Chem Phys 1968482835[12] CA Hoeve J Polym Sci 197030 361 1971 34 1[13] Roe RJ J Chem Phys 1974604192[14] Scheutjens JMHM Fleer GJ J Phys Chem 1979831919
[15] Scheutjens JMHM Fleer GJ J Phys Chem 198084178[16] Scheutjens JMHM Fleer GJ Adv Colloid Interface Sci 198216341[17] GJ Fleer MA Cohen-Stuart JMHM Scheutjens T Cosgrove and B Vincent
London Chapman and Hall 1993[18] Obey TM Grif 1047297ths PC In Goddard ED Gruber JV editors Principles of Polymer
Science and Technology in Cosmetics and Personal Care NY Marcel Dekker1999
[19] Garvey MJ Tadros ThF Vincent B J Colloid Interface Sci 19744957[20] van den Boomgaard Th King TA Tadros ThF Tang H Vincent B J Colloid Interface
Sci 19786168[21] Napper DH Polymeric Stabilization of Dispersions London Academic Press1983
[22] Flory PJ Krigbaum WR J Chem Phys 1950181086[23] Tadros ThF Vandamme A Levecke B Booten K Stevens CV Adv Colloid InterfaceSci 2004108ndash109207
[24] Exerowa D Gotchev G Kolarev T Khristov Khr Levecke B Tadros T Langmuir2007231711
[25] Tadros ThF Dederen C Taelman MC Cosmet Toilet 199711275[26] Nestor J Esquena J Solans C Levecke B Booten K Tadros ThF Langmuir
2005214837[27] Nestor J Esquena J Solans C Luckham PF Levecke B Tadros ThF J Colloid Interface
Sci 2007311430[28] Liang W Bognolo G Tadros ThF Langmuir 1995112899[29] Nakajima H Tomomossa S Okabe M First Emulsion Conference Paris 1993[30] Nakajima H In Solans C Konieda H editors Industrial Applications of
Microemulsions Marcel Dekker 1997[31] Tadros Tharwat Izquierdo P Esquena J Solans C Adv Colloid Interface Sci
2004108ndash109303[32] Thompson (Lord Kelvin) W Phil Mag 187142448[33] Lifshitz IM Slesov VV Sov Phys 195935331[34] Wagner C Z Electroche 193535581
[35] Kabalnov AS Schukin ED Adv Colloid Interface Sci 19923869[36] Walstra P In Becher P editor Encyclopedia of Emulsion Technology NY Marcel
Dekker 1983[37] Walstra P Smoulders PEA In Binks BP editor Modern Aspects of Emulsion
Science Cambridge The Royal Society of Chemistry 1998[38] Tadros T Vandekerckhove E Vandamme A Levecke B Booten K Cosmet Toilet
2005120(No2)45
299T Tadros Advances in Colloid and Interface Science 147 ndash148 (2009) 281ndash 299