Post on 05-Apr-2018
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CHM 342 (2-46) Surface Chemistry
(. )Surface Tension
Surface tension holds water droplets on theleaves of the Japanese Root Iris.
How can an insect in the abovepicture walk on the water?
( )
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CHM 342 (2-46) Surface Chemistry
(. )
Floating a tack on water.
The tack doesnt really float on the
water, because it is more dense that thewater and should sink to the bottom
The reason that it doesnt sink is because
of the surface tension of the water
What is density?
What is surface tension?
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CHM 342 (2-46) Surface Chemistry
(. )
A tack on the surface of a glass of water.
Poking the surface of the
water with a toothpick is
not sufficient to disturb
the surface tension
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CHM 342 (2-46) Surface Chemistry
(. )
If you dip the toothpick in a soap solution before you poke the water thenthe soak will disrupt the surface tension.
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CHM 342 (2-46) Surface Chemistry
(. )The origin of surface tension.
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CHM 342 (2-46) Surface Chemistry
(. )
Work required for the Formation of Surface
Separation of liquid
requires work
against cohesion
forces
Surface tension ( or ) - increase in free energy as a result offormation of 1 unit (m2 ,cm2) of surface
Formation of surface of square W results in increase in the free
energy G:
)!!positive!(always0;A
G
Tp,
0AGA
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CHM 342 (2-46) Surface Chemistry
(. )YOUNG-LAPLACE EQUATIONEquation of Capillarity
Pressure inside a drop or bubble is always greater than inthe continuous phase.
The balance between surface tension and external forces(e.g. gravity) dictate the shape of drops and bubbles.
R1
R2
21
R
1
R
1P
- Interfacial Tension
P - Pressure
R - Radius
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CHM 342 (2-46) Surface Chemistry
(. )Surface Tension is a Force!
Force: mg [N, Newton]
Force: 2L
units: [N/m]
L
Soap film
Surface tension is a vector, i.e. has direction
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CHM 342 (2-46) Surface Chemistry
(. )Measurement ofSurface Tension
External force
Surface tension
x
fT Force
Ring tearingx
Ring tearing technique
nce}circumfere{ring2
factor)on(correctif T
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CHM 342 (2-46) Surface Chemistry
(. )SURFACE TENSION MEASUREMENT-- du Nouy ring --
)R2(2wtwt ringtotal
wttotal = total weight , wtring = ring weight
R = ring radius , = surface tension
Still commonly used but values may be as
much as 25% in error.
Adamson, Physical
Chemistry
of Surfaces, 2nd Ed
p. 22 (1976)
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CHM 342 (2-46) Surface Chemistry
(. )SURFACE TENSION MEASUREMENT-- Wilhelmy Plate --
2l
b)(wtwtcos()
platetotal
= surface tension
q = contact angle
wttotal = total weight
wtplate = plate weight
b = buoyancy force
l = length of plate
Normal platinum is used to have q 0 and plate just touches
liquid so buoyancy is small
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CHM 342 (2-46) Surface Chemistry
(. )SURFACE TENSION MEASUREMENT-- Drop Weight Method --
W = 2r
W = weight of droplet
r = radius of droplet = surface tension
Ref. Adamson, Physical Chemistry of Surfaces, 2nd Ed , p. 19 (1976)
9See: http://www.erc.ufl.edu/education/courses/intephen 1/_files/lecture1.ppt
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CHM 342 (2-46) Surface Chemistry
(. )Drop on a Solid Surface
Youngs equation relates interfacial tensions and contact angle
q
LV
SV
Solid, S
Liquid, L
Vapor, V
SL
q cosLVSLSV
Contact angle, q(reflects the degree of wetting)
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CHM 342 (2-46) Surface Chemistry
(. )Wetting Phenomena
q
q > 90oq = 90oq < 90o
q = 0o q =180o
Absolute wettingNo wetting
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CHM 342 (2-46) Surface Chemistry
(. )Wettability of Powders
Solid
q
Liquid
Vapor
LV
SV
SL
q cosLVSLSV
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CHM 342 (2-46) Surface Chemistry
(. )
Micelles are not reallydissolved in water, theyare suspended.
These sort ofsuspensions are called
colloids.
Colloids : Particles ofone substance dispersedthroughout another.
Colloids can be detectedby the scattering of lightthat is passed through thesample, this is called theThyndall Effect.
Colloids
The Tyndall effect is evident as a beam oflight passes through a colloidal dispersion
of water droplets in the atmosphere.
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CHM 342 (2-46) Surface Chemistry
(. )Types of Colloidal Dispersions
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CHM 342 (2-46) Surface Chemistry
(. )
All soaps are detergents;all detergents are
surfactants.
()
Surfactants
Surfactants are molecules that preferentially adsorb at aninterface, i.e. solid/liquid (froth flotation), liquid/gas (foams),
liquid/liquid (emulsions). Significantly alter interfacialfree energy(work needed to
create or expand interface/unit area). Surface free energy of interface minimized by reducing
interfacial area.
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CHM 342 (2-46) Surface Chemistry
(. )
If enough soap is added to water the molecules
arrange themselves into a structure called a micelle
Hydrophilic( lyophobic,water-loving) headcontaining a chargedfunctional group
Hydrophobic( lyophilic,water-fearing ) tail
containing ahydrocarbon chain
Soap Molecules
CHM ( ) S f Ch i
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CHM 342 (2-46) Surface Chemistry
(. )Amphiphilic Surfactants
Amphiphilic surfactants contain a non-polar portion(tail) anda polar portion (head).
S O-Na
+
O
O
S
O
OO
OO
O+Na
-O
Aerosol OT
Sodium dodecylsulfate (SDS)
CHM 342 (2 46) S f Ch i
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CHM 342 (2-46) Surface Chemistry
(. )Classification of Surfactants
Anionic
Cationic
Zwitterionic
Nonionic
N+
Br-
S O-Na
+
O
OSodium dodecylsulfate (SDS)
Cetylpyridinium bromide
O
O
P
O
O O
OCH2CH2N(CH3)3+
O-
Dipalmitoylphosphatidylcholine (lecithin)
OO
OO
OH
Polyoxyethylene(4) lauryl ether (Brij 30)
Soap
CHM 342 (2 46) S f Ch i t
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CHM 342 (2-46) Surface Chemistry
(. )Molecular Architecture
Aerosol OTSodium dodecylsulfate (SDS)
CHM 342 (2 46) S f Ch i t
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CHM 342 (2-46) Surface Chemistry
(. )
4 nm
Unimers (monomers) Normal micelles
spherical
cylindrical
Bilayer lamellaReverse micelles
Inverted hexagonal phase
Surfactant aggregates
CHM 342 (2 46) Surface Chemistry
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CHM 342 (2-46) Surface Chemistry
(. )
If concentration is sufficiently high, surfactants can form aggregatesin aqueous solution micelles.
Typically spheroidal particles of 2.5-6 nm diameter.
McBainLamellarMicelle Hydrocarbon
Layer
WaterLayer
WaterLayer
HartleySpherical
Micelle
+
+
+
+
+
+
+
+
- - - - ---
--
- - -
-
--
-
Micelles
CHM 342 (2 46) Surface Chemistry
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CHM 342 (2-46) Surface Chemistry
(. )
0
2
4
6
8
10
12
14
0 1
Surfactant concentration
CMC
Critical Micelle Concentration
CMC
Below CMC only unimers arepresent
Above CMC there are micelles inequilibrium with unimers
Onset of micellizationobserved by sudden change in
measured properties of solution at characteristic surfactantconcentrationcritical micelle concentration (CMC).
CHM 342 (2 46) Surface Chemistry
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CHM 342 (2-46) Surface Chemistry
(. ) Onset of micellizationobserved by sudden change in measured
properties of solution at characteristic surfactant concentration
critical micelle concentration (CMC).
Critical Micelle Concentration
( From: Klimpel, Intro to ChemicalsUsed in Particle Systems, p. 29, 1997,
CHM 342 (2 46) Surface Chemistry
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CHM 342 (2-46) Surface Chemistry
(. )Solution Properties & CMC
02468
101214
0 1Surfactant concentration
CMCConc.
unimers
micelles
02468
101214
0 1Surfactant concentration
CMC
Osmotic pressure
02468
101214
0 1(Surfactant concentration)1/2
CMC
Molar conductivity1/R
02468
101214
0 1Surfactant concentration
CMC
Isc Light scattering
CHM 342 (2-46) Surface Chemistry
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CHM 342 (2-46) Surface Chemistry
(. )--Driving Force-- Lyophobic (solvent-fearing) groups can perturb solvent structure and
increase free energy of system. Surfactant will concentrate at the
solvent-gas interface to lowerGo.
Gocan also be decreased by aggregation into micellessuch thatlyophobic groups are directed into interior of structure and lyophilicsolvent-loving) groups face solvent.
Decrease in Go for removal of lyophobic groups from solvent contactby micellization may be opposed by:
(i) loss in entropy &
(ii) electrostatic repulsion for charged headgroups
Micellizationis therefore a balance between various forces
AIR
WATER
Micellization Thermodynamics
CHM 342 (2-46) Surface Chemistry
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CHM 342 (2 46) Surface Chemistry
(. )Micellization Thermodynamics
m.[SURF] [SURF]m
Kmic
mmic
mmic
mic][]SURF[ cmc
ccK
)10030(lnlnm
lnm
lnmm
micomic
mcmcRTcmcRTcRT
KRTG
Go
o
Nonionic surfactant
m - number of aggregation
abovecmc: [SURF] cmc
micmic mm][ cccmcco
T
cmcRTcmcRS
T
cmcRTH
cmcRTG
d
lndln
d
lnd
ln
2o
mic
2omic
omic
CHM 342 (2-46) Surface Chemistry
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CHM 342 (2 46) Surface Chemistry
(. )--Temperature and Pressure-- For ionic surfactants there exists a critical temperature above which
solubility rapidly increases (equals CMC) and micelles formKrafft pointor Krafft temperature(TK),
Below TK solubility is low and no micelles are present.
(Klimpel,Intro to Chemicals Used in Particle Systems, p. 30, 1997, Fig 22)
Micelles
CHM 342 (2-46) Surface Chemistry
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CHM 342 (2 46) Surface Chemistry
(. )
surfactantcrystals
TK
Temperature
Surfactants much less effective belowKrafft point, e.g. detergents.
For non-ionic surfactants, increase in temperature may result in
clear solution turning cloudy due to phase separation. This criticaltemperature is thecloud point.
Cloud point transition is generally less sharp than that ofKrafft
point.
Micelles --Temperature and Pressure--
CHM 342 (2-46) Surface Chemistry
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CHM 342 (2 46) Surface Chemistry
(. )Krafft Point, CMT and Cloud Point (~ temp.)
(I) Krafft Point
Cloud
point
(II) CMT and Cloud Point
1
c
T
Micelles +
solution
Solution
CMC
CMT
Phase
Separation
1Tkrafft
CMC
c
T
Micelles +
solution
Solution
Crystals +
solution
Liquid
crystals
I. Above the Kraft point, the solubility is sharply increase due to the formation of micelles.
II. Some surfactants (e.g. polyoxyethylene (POE)-based) dehydrate and phase separateupon temperature elevation (cloud pint).
III. In Pluronic block copolymer, the micelles form as a result of the dehydration of
polyoxypropylene segments (critical micelle temperature or CMT).
CHM 342 (2-46) Surface Chemistry
S H d l ?
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C 3 ( 6) Su ace C e st y
(. )Soap: How does soap clean?
CHM 342 (2-46) Surface Chemistry
S H d l ?
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( ) y
(. ) soap decreases surface tension of water, making it a better wetting agent.
soap converts greasy and oily dirt into micelles that become dispersed in
water.
soap keeps the greasy micelles in suspension and prevents them from
redepositing until they can be washed away. (repulsion of the charges)
Soap: How does soap clean?
CHM 342 (2-46) Surface Chemistry
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( ) y
(. )
Acid-catalyzed and base-catalyzed hydrolysis.
Through the breakdown of esters by a hydrolysis process.
This bond is broken
How are soap produced?
CHM 342 (2-46) Surface Chemistry
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( ) y
(. )
The saponification of a triglyceride.
Soaps are not produced from simple esters such as methyl acetate
but from more complex esters
animal fat
How are soap produced?
CHM 342 (2-46) Surface Chemistry
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(. )
What are all of these
chemicals ?
What do they do ?
What is in a box
of detergent ?
CHM 342 (2-46) Surface Chemistry
Wh t i i b f d t t ?
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(. )What is in a box of detergent ?
CHM 342 (2-46) Surface Chemistry
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(. )
CHM 342 (2-46) Surface Chemistry
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(. )Solubilization
Spontaneous transfer of a compound insoluble in the bulk solvent into
the solution inside the surfactant micelles.
polar compound
Reverse micelles
non-polar compound
Normal micelles
amphiphilic compound
0
2
4
6
8
10
12
14
0 1Surfactant concentration
CMCSolub
ility
Solubility of a poorly solublecompound increases as aresult of solubilization in themicelles.
CHM 342 (2-46) Surface Chemistry
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(. )HLB and Use of Surfactants Amphiphilic surfactants are characterized by the hydrophilic-lipophilic balance (HLB): a relative ratio of polar and non-polargroups in the surfactant.
HLB number, ranging from 0-40, can be assigned to a surfactant,based on emulsification data. Semi-empirical only.
Strongly hydrophilic surfactant, HLB 40
Strongly lyophilic surfactant, HLB 1
oil
water
Coil
Cwater
C6H13COO- C8H17COO
- C10H21COO-
HLB decreases
CHM 342 (2-46) Surface Chemistry
HLB d U f S f
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(. )
HLB ca. 1 to 3.5: AntifoamsHLB ca. 3.5 to 8: Water-in-Oil Emulsifiers
HLB ca. 7 to 9: Wetting and spreading agents
HLB ca. 8 to 16: Oil-in-Water Emulsifiers
HLB ca. 13 to 16: DetergentsHLB ca. 15 to 40: Solubilizers
HLB and Use of Surfactants
Strongly hydrophilic
Strongly Lipophilic
Question:Why antifoaming or water-in-oil emulsifiers use low-HLBsurfactants & why detergents use hi-HLB surfactants?
CHM 342 (2-46) Surface Chemistry
HLB needed for emulsification of the oil phase
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(. )HLB needed for emulsification of the oil phase
* Calculate the required HLB for the oil phase of the following o/w emulsion: cetylalcohol 15 g., white wax 1g. Lanolin 2 g, emulsifier (q.s.), glycerin 5 g. water100 g.
HLB (from reference) Fraction
Cetyl alcohol 15 x 15/18 = 12.5White wax 12 x 1/18 = 0.7
Lanolin 10 x 2/18 = 1.1Total required HLB = 14.3
Surfactant blends are commonly used to obtain desired emulsifyingproperties.
If there are several oil ingredients the required HLB is calculated as asum of their respective required HLB multiplied by the fraction of each.
* What is the HLB of the mixture of 40 % Span 60 (HLB = 4.7) and 60 % Tween 60(HLB = 14.9)?
HLB of mixture: 4.7 x 0.4 + 14.9 x 0.6 = 10.8
* In what proportion should Span 80 (HLB = 4.3) and Tween 80 (HLB = 15.0) bemixed to obtain required HLB of12.0?
4.3.(1-x) + 15.x = 12 x = 0.72 ( 72 % Tween 80 and 28 % Span 80)