Reprinted from "Surfactant Adsorption and Surface Solubilization",R. Sharma, ed. ACS Symposium Series 615, p. 104-137, 1995

Chapter 7

Spectroscopic Characterization of Surfactantand Polymer Solloids ~t. Solid-LiquidInterfates ...~

P. Somasundaran, S. Krisbnakumar, and Joy T. Kunjappu

lAnglnuir Center for Colloids and Interfaces, Henry Krumb sChoolof Mines, Columbia University, New York, NY 10027

Adsorbed ~ aM polymer layers playa major role in several keyindustrial operations such as 6~L:'Jlation/dispersion,ceramic processing.lubrication flotation and detergency. Much work has been done tounderstand the physico-cl1emica1 interactions that govern the fonnationof colloidal layers adsorbed on surfaces (solloids). The structure ofthese layen plays a aitical role in detennining interfacial properties andinformation on such structures. par1icularly at microscopic andmolecular levels, can be helpful for controlling the behavior of thesystems. We have recently adapted fluorescence, electron spinresonance, Fourier transform infrared, and excited-state resonanceRaman spectroscopic tecMiques to obtain infonnation onmicrostructure of adsorbed layers on a molecular level. In-situclwaaeriution tecMiques that have been used recently for examiningthe adsorbed layers at the solid-liquid interface are reviewed here.Etrects of surfactant and polymer structure, pH, and their synergisticeffects on the evolution and properties of the adsorbed layer arediscussed.

Adsorption of surfactants and polymers at the solid-liquid interface is a keyphenomena in many important processes such as flocculation, flotation, dispersion,and stabilization (J). These applications are a sequel to the modification of the solidsurface by,be adsorbates in terms of the electrical and allied processes. Optimumperfonnance of these processes is possible by a detailed underStahding of theadsorbates at the solid-liquid interface and the related stNCnM'e-performancerelationship of these species. - --

In this article, we diSQ1ss some of the advanced techniques and methodologiesthat we have used recently for investigating the microstructure of the adsorbedmonomeric and polymeric aggregates at the solid-liquid interface, which are known

0097-61 S619S,()615-Q 1 04S 1 S.so,o(C) 1995 Amerian Chemical Society

SO~UNDARA.'l ET AI.. Surfactant and Polymer SoUoids 105

"..

106 SURFACTANT ADSORP'nON AND SURFACE SOLUBn.IZA'nO~'

P. - n*exp( -0)/01(1)

..~ -I-c,)- I.cOICXP [-k.t + n(ecp (-k.t)-I»)

(2)

n - [P)/[Au] - [P] N/ ((5] - [5..)(3)

AE - hy - gBH.(4)

7. SOMASUNDARA.~ Er AL Surfactant and Polymer SoUoids 10

~

spin-probing technique, a molecule with a spin is extemally added to the system,whereas in spin labeling a spin bearing moiety forms a part of the molecule throughcovalent bonding. The spin trapping technique is mainly used for the identification ofradicals produced thennally, photochemically or radiolytically by trapping the radicalthrough chemica! re"ictions with a spin tra,p Elike butyl nitroxide) and converting theradical to.a 6'ee radical which can be examined by ESR. This technique bas been usedwidely to study micelJization of different \urfactants in both aqueous~ non-aqueousmedia (8),(9),(JO). This has also been used to study the exchange kinetics andsolubilization sites in water-in-oil microemulsions (J J) . ESR studies as applied tomicellar SYstems rely on the sensitivity of a free radical probe to respond to itsmicroenvironment.

Information on micropolarity and microviscosity can be obtained by measuringthe hyperfine splitting constant A~ and the rotational correlation time 't'. The latter isthe time required for a complete rotation of the nitrox.ide radical about its axis. Itsvalue can be defined as the time required for the nitroxide to rotate through an angleof one radian. The rotational correlation time ('t') measured from the ESR spectrum isreflective of the probe mobility and can be used to monitor the changes inmicroviscosity of the adsorbed layer brought about as a result of adsorption followedby close packing of molecules in the adsorbed layer. The hypemne splitting constant(A~) that can also be measured from the spectrum, changes as a function of theenvironment polarity and hence yields information on the formation of differentrnicrodomains at the interface as a result of adsorption. In the case of slow anisotropicmotion the obser-.°ed changes in the spectrum can be quantified by using the concept 01order parameter S which can be calculated from the spectrum using the followingequation

c..t1/-\.4.(meaJ).L)s. where C. (1.4S-O.019(,(n-A,(meas») gmlSS(5)

(Ai,.2(A\(mear).G)

and Ad and AI are the hypemne splittings measured from the spectra. The orderparameter is usually a parameter of molecular motion and varies between 0 (low order)and l(high order).

Infrared Spectroscopy (IR). The vibrational frequencies of bonds in a moleculecorrespond to the infrared region of the electromagnetic spectrum. Different groupsin a molecule possess characteristic absorption frequencies which serve as theiridentification marks. Moreover, these frequencies are related to the bond strength.bond energy and other related parameters. In the case of a diatomic molecule, thestretching frequecy v is expressed as,

(6:v - //(2&") .r(klp)

where k is the force constant, c the velocity of light. and .u. the reduced mass which i!equal to(mL+mz)/ml~ mL aJxi ~ being the masses of the atoms forming the bond (12)

108 SURFACfANT ADSORPrlON AND SURFACE SOLUBWZATION

A prerequisite for the absorption of infrared radiation by a molcalle is that theconcerned V1oration should cause a change in its dipole moment. Both the stretchingand bending V1orations are useful for the identification of the bonding characteristics.The ftequency positions and the frequency intensities help in the qualitative detectionand the quantitative estimation respectively.

The common IR sampling methods such as KBr penetting. nujol mulling, etc.cannot be applied to an aqueous environment. The analysis of solid samples has beenaccelerated by the use of attenuated total reflectance technique (ATR). In this mode,the powdered sample or the slurry is smeared on an internal reflection element (IRE).The evanescent field of tho IR light undergoing multiple internal reflections samples thematerial adhering onto the IRE and gets attenuated collecting infonnation on thevibrational aspectS ofme material. Equations are available which relate the adsorptiondensity (I) of surfactants quantitatively to the parameters of IRE and IR lightexemplified as follows (J 3),

r - [(A/(Vcot9) - &c.d.}}'_lQOOs <2dId.}~ (7)

where A - integrated absorbance (cmO'), 1- IRP.length, t - IRE thickness, £ - molarabsorpbvity of surfactant (V(cm2.mol», ~ = surfactant bulk concentration (mol/l), d,- depth of penetration, d. - effective depth, and e - incident angle of light on IRE

(degrees).

Raman Spectroscopy. Raman spectroscopy is essentially a scattering techniquegiving infonnation on the V1orational modes of a molecule. Those vibrations causing apolarizability change of a molecule are Raman active. Raman spectroscopy has anedge over infrared spectroscopy since it is ideal in an aqueous environment andversatile in its ease of sample handling.

The Raman pr~ (melastic scattering) is inherently weak in sensitivity dueto its low cross section (-I~. An inelastic impact of the molecule with the lightquantum results in transferring the ..ibrational energy from the light quantum to themolecule (Stokes process) or from the molecule to the light quantum (anti-Stokesprocess). The electric field of the light induces in the molecule a dipole moment ~.which is related to the field strength E and polarizability a. of the molecule accordingto the equation

~'=a£ (8)

Th~ona1 dependence of the polarizability is described by a tensor whichbecomes symmetrical for a non-crural molecule. Quantum mechanical treatmentprovides a good estimate of the intensities of the Stokes and If!ti-$tqJces lines, thoughthe classical picture does not provide a resonable estimate.

The earlier Raman studies have been carried out under continuous wave light.With the advent of lasers, both continuous wave and pulsed lasers are used forcoUecting Raman spectra. To enhance the intensity of Raman lines, the excitation canbe localized to a naJTOW absorption band (Resonance Raman Scattering). The intensityproblem is overcome in a few cases by the Surface Enhanced Raman Spectroscopy

10-'50S/ALUMINA0.1 ... NoCI . DH 6.5 ~

I '1Z.,-.

Ir10-10 CMC-z0~Go=0'"Q-0&

W

..'""-'~C/)-'

on10-1'

10-12

..~

~ .-10-'3 y HMC

10-14~. ""'~~.&-i-J.""~1 .' . "...,1 "".&oW10-$ 10-4 10-3 10-2

RESIDUAL DODECYLSU1.FATE, molls/lilt'

Figure 1. Adsorption isothenn of sodium dodecylsulfate (SDS) on alumina atpH 6.S in 10-1 kmoVm1 NaCI.

1.5

1 :.~. ~.~. ~~~.~~~.~. ~:1.~!J.~.~!Y"'~-:~7':'~~~6;t: - ,

--,I")""4

.5

~

0 ~.I-..L-&..L1.1.1,IJ:-_.l-L-A ~-.1--L..L.L.&.uA1- ~ ~0 10.5 10.4 10-3 10-2

RESIDUAL DODECYLSULFATE. moles/liler

7. SOMASUNDARAN £TALSwfactant and Pol",..,. Solloidr

111

..'

parameter is fairly constant throughout most of the Region m and above and henceseems to be independent of the surface coverage.

Infonnation on miao..;scosity of the adsorbed layer is obtained by studying theexclmer (sgted dimu) forming capabilities of suitable fluorescent molecules like 1,3-

dinaphthyi propane. The exclmer, which is a complex of a ground state and an excitedstate monomer, bua charaCteristic emission frequency. The intramolecular excimerformation is a ~tive function of the microviscosity of its local envirorunent. Thisproperty, expressed as the ratio of the monomer to excimer yield (I.lJJ for 1,3-dinaphthyi propane, is determined (Figure 3) for the solution and for the msorbed layerfor the various regions of the adsorption isotherm (21). These are then compared tothe I.'I. values ofDNP in mi.xtures of ethanol arKi glycerol of known viscosities. Basedon the I.II. values of DNP for ethanol-glycerol miXtUres, a microviscosity value ofabout 100 cP is obtained for the adsorbed layer compared to a value of 8 cP formicelles. A higher value of microvi!COsity in the sonoid as reported by DNP isindicative of a highly condensed surfactant assembly.

A kinetic analysis based Qn the relation of the decay profiles of pyrene in theadsorbed IaYel' for different regions of the alumina/dodecylsulfate adsorption isothennyielded the aggregation numbers marked in Figure 4 for dodecyl sulfate soUoids (20).These results yield a piCture of the evolution of the adsorbed layer. The aggregates inRegion n appear to be of relatively uniform size while in Region m there is a markedgro~h in the aggregate size. In Region n, the surface is not fuDy covered and somepositive sites are stiJl available for further adsorption. Since the aggregation number isfairly constant in Region n, funher adsorption in this region can be considered to occurby the formatiOn of more aggregates but of the same size. The transition from Region11 to 111 corresponds to the isoelectric point of the solid, and adsorption in region mis proposed to occur through the groWth of existing aggregates rather than theformation of new ones due to lack of positive adsorption sites. This is possible by thehydrophobic interaction between the hydrocarbon tails of the already adsorbedsurfactant molecules and the adsorbing ones. The new molecules adsorbing at the solid-liquid interface can be expected to orient with their ionic heads towards the water. assupported by the hydrophobicity studies(22). since the solid panicles possess a netnegative charge under these conditions. A schematic representation of the adsorptionby lateral interactions is given in Figure 5

These StUdies were complemeted by using ESR to further investigate changesin micro..iscosity within the adsorbed layer using three isomeric stable free radicals 5-,12- and 16- doxyl stearic acids as the spin labels. These spin labels ""ere co-adsorbedindi..idually on the alumina along with the main adsorbate. sodium dodecylsulfate. andthe main regions of the isotherm were investigated. The hyperfine splitting constantsof 16-doxyl stearic acid measured in dodecyl sulfate solloids (15.00) arc indicative ofa less polar enWOM\ent in comparison to its vaJue for Water (16.0G) and SDS micelles

(15.6Q). Similarly microviscosities were estimated from or measurements and calibratedagainSt or measured in ethanol-glycerol mi.~res (Figure 6). These give reasonably highvalues for the solloids in reference to the values for water. Three differentmicroviscosities were Obtained using different probes indicating that the nitroxidegroup in each case experienced a different viscosity within the solloid. The~eobservations may be explained by assuming a model for the adsorption of the probe In

2 SURFACIANT ADSORPnON AND SURFACE SOLUBn.IZATiON

,.' ..

6MONOMER/EXCIMER RATIO OF DNP INADSORBED LAYER .1M NoCI. pH 6.5

100 cP 6 6 ~~ 6 6 6

.." 3

.!2. _1~ J~O~~~!~L!)__-

o I ... . 1 ... of ... . ...,1 ." . 1 ,.. . 10.'. 10-'3 10-'2 'O." 10.10 'O.,

OOOECYLSULFATE AOSORPTION.mOI.~/cm2

Figure 3. Monomer to excirner ratio (I.IIJ of'dinaphthyl propane (DNP) inSOS-alumina slurries as a function of'SOS adsorption density. The viscositiesrefer to those of' ethanol-glycerol mixtures which give a similar 1.11. ratio f'orONP. (Reproduced with permission f'rom ref. 20. Copyright 1987. AcademicPress)

~,.~

7. SOMASUNDARA.~ ET ALSurfllctQnl GIld Polymer SoOoidr -m-.

..'

-50S/ALUMINAO.1M NoCl, pH 6.5 , D -~~+~ o(:J

25';- .. m.19~ j m u

166 ~

128

t-'.II,

49

66

/

N 10.9e~~to

"0e. 10-10

%0~Go~0cn 10-nc4I&l~4"-..J 10-12:;)cn-J>-UL&J

g 10.13c C1S0S ONLY

~SOS WITH

PYRENE10-14_~ ~~_& ~

10-5 10-4 10-3 10-2

RESIDUAL OOOECYLSULFATE. moils/liter

Figure 4. Surfactant aggregation numbers detenninined for various adsorptiondensities (average number is indicated). (Reproduced \Jtith pennission from ref:20. Copyright 1987, Academjc Press)

114 SURFACTANT ADSORP'nON AND SURFACE SOLUBnJZA'nON

~£GION INO AGGREGATION

!..'

A£GION INUMBER 0'AGGREGATESIHCREASES-~O'I3OMOI.ECUUS~R AGGREeATE

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':00u

-~

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OI.lY

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-J0~kJU>-

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w.,;

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AL

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Sutflldllnl

and Poly"..,. S

olloids

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c.2...' -g

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oo

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c.-~

IU

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oo.r. V

Ie -

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UU

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>

..00-o..cu u ....=.~

.~ ~

~ e '=

115

SURFACTANT ADSORP'nON AND SURFACE SOLUBn.lZA.T10N116

..'

which the carboxylate group is bound to the aJumina surface. Such a model wouldrequire greater mobility of the mtroxide moiety near the SDS/H1O interface (as in the16-doxyl stearic acid case) and less mobility of the 12-doxyl and 5-doxyl probes(2J),{24). This work is the first reported indication of variations in miaoviscositywithin a surfactant solloid as estimated by any known technique.

The aJumina/dodecyisulfate system has aJso been probed by excited stateresonance RAman Spectroscopy using Tris(2,2'-bipyridyl) ruthenium (II) chloride[Ru(bPYh ~ as a reporter molecule(25). It has been shown that ruthenium POlypyrldyicomplexes serve as excellent photophysical probes for biopolymers like nucleic acids.The excited state ofRu(bpY)J1. shows strong resonance enhanced Raman transitionswhen probed at 355 nIn. FUI'thermore. it has been shown that binding of this ion to clayparticles results in substantia] changes in the ground-state transitions of the excitedstate-resonance Raman spectrum. For these reasons this probe was chosen to study thesolloids formed at the alumina-water interface with excited-state resonance Ramanspectroscopy. The third hannonlc of a Nd-YAG laser excited and scattered theRu(bPYh 10 molecules at a pulse ~ergy of 5 mJ, pulse width of 6 ns. and a wavelengthof354.S nm. The excited stat~Raman SpectNm ofRu(bpyh1oconsisting of 14 lines (7lines each from the ground and the excited states) was calibrated using an authentic

spectrum.Raman spectra ofRu(bPYh~ above the CMC show fi'equency shifts as well as

intensity changes as compared to its spectrum in water. These transitions can beattributed to the perturbation of the excited state by the SDS micelles. Excited stateresonance Raman spectrum of this probe in various regions of adsorption isothenn forthe aJumina/SDS SYStem is shown in Figure 7. The spectrum ofRu(bpy),1- on aluminain the absence of SDS is very much similar to its spectrum in water both in tenns offrequencies and relative intensities. This trend is continued into region ll, where thesoUoid aggregation process begins. In regions III and IV, the Raman spectrum showssignificant changes. The fi'equency shifts are more pronounced than in the case ofmicelles. A plot of change in wave numbers for some of the lines in the four differentregions of the adsorption isotherm is shown in Figure 8. The changes in Ramanftequency and intensity assume substantial significance in the transition regions n andill on~oards only. This could be due to the change of net charge on the alumina surfacefrom positive to negative. The favorable net negative charge enhances the adsorptionof the probe at the solid-liquid interface. Accordingly, no adsorption ofRu(bPY)J'-- wasobserved when the supernatants were analyzed in Region I and Region II or in theabsence of SDS. These results indicate that adsorption of Ru(bPY)J1- onto aluminabecomes significant only close to the isoelectric point. The transitions at 1213, 1286and 142S"O1R°' show significant increments and these trends clearly suggest the probeadsorption on to the solloids. Also it was seen that the 1286 peak shifted to 1281 inthe presence of soUoids while it was practically uochanged in the sodium dodecylsulfatemicelles. It may be speculated that Ru(bpy),1. may be sensing aitferent environmentswithin the SDS micelles and the alumina/SDS solloids. Implicit in these results is alsothe potential of the time resolved b11an spectroscopy as a powerful diagnostic tool toexplore the solid - liQuid interface.

7. SOMASUNDARA.'f ET ALSurfactant a.nd PoI,r1WP' SoOoidr U7

.'

-----

,....-c:)

..c..

<-

..u-..cu-C

H

u>

-0

uc:

1100 1300 1500 1700

Wavenumber (cm-1)

Figure 7. Resonance Raman spectrum afRu(bPYh2. for various regions of thealumina/50S adsorption system. (Adapted from ref. 25)

118

..~ .""'-

7. SOMASUNDARAN ET AL Surfactant and Polymer Solloids 11.9

Isomeric Surfactants at aD Interface (26). Studies of changes in the position of the~lfonate and the methyl groups on the aromatic ring of alkylxylenesulfona.te showeda marked effect of ~ch changes on the micellization and adsorption at thealumina/water interface. ..ftJorescence spectroscopy was used to probe themicro~ of the adsorbed layers in these systems. Adsorption isothenns for 4-(4-

undecyl)-2.~thylbenzenesu1fonate (para-I). 4-( 4-undecyl)-2.S-dimethy~~gJIfonate (para-2) and S-(4-undecyl}-2.4-dimethylbenzenesulfonate (Meta) on alumin&from water and the I;/II values along various regions of the isotherm are,hown inFigure 9 and 10 respectively. At low adsorption densities the value of the polarityparameter. I;/II . is - 0.6 and once the soUoids form the value goes up to about 1. Itcan also be seen that there is no measurable difference among the polarities of theadsorbed layers of the three surfactants mdicating that these layers in all three cases arestructurally similar ~th minimum water penetration. Average aggregation numbers(determined using fluorescence decay) of the solloids increase &om 17 to 76 withincrease in adsorption in all the three cases (Figure 11). The aggregation numbers ofthe two para.xylene sulfonates are similar throughout the range studied. However. at

higher adsorption densities, the aggregation number of the metaxylenesulfonate is lowerthan those of the paraxylenesulfonates. This suggests higher steric hindrance to thepacking of the surfactant molecules in the solloids of the metaxylenesulfonate. Basedon this e-.idence, the effect of change of functional groups on the aromatic ring of the

alkylxylenesulfonates on adsorption can be attributed to the steric constra.ints to thepacking of surfactant molecules in their aggregates.

..~

Conformation of Aerosol-OT at Alumina-Cyclohexane Interface (2i),(28).Colloidal dispersions in non-aqueous media have a number of technologicalapplications, but water is present in most of these cases and plays a major role indetermining the dispersion behavior. The adsorption of Aerosol-OT , sodium bis(2-ethylhexyl) sulfosuccinate, on alumina was investigated using Infrared spectroscopy(29). Figure 12 shows the IR spectrum of AOT adsorbed on alumina in cyclohexanealong with the spectra for alumina and AOT. The spectrum does not show any newabsorption bands or shifts in the existing bands on alumina or AOT due to theadsorption. This suggests that the adsorption proceeds via weak physical interaction.The effect of water on the stability of a colloidal suspension of alumina in cyclohexanein the presence of surfactant, Aerosol-OT is shown in Figure 13. A succession of

flocculated, dispersed and flocculated states are observed as the amount of water addedto the suspensions is increased. ESR studies were conducted using 7-doxyl stearic acidas a probe to get structural information on the adsorbed layer.

Figure 14 shows some of the ~ obtained at different water concentrationswith 7-doxyl stearic acid coadsorbed with a full monolayer of Aerosol-OT on thealumina surface. The changes observed in the ESR line shape correspond to an increasein the probe mobility consistent with a decrease in the ordering of the probeenvironment. The calculated order parameter (eqn. 5) plotted as a function of waterconcentration is shown in Figure 15. It can be seen that as more water is added to thesuspension the order parameter decreased until it reached a constant value of 0.75.These results are interpreted in terms of an increase in the probe lateral diffusion ~~the adsorbed mactant layer. Such an increase in the probe lateral diffusion is realistic,

1.20

..'

SwflldDnl GIld PoiymIr Solloids7. SOMASUNDARAN ET AI... 111

a.~. 78 """"'/8; v _w'-O

..82 ,p

,.' 17& Para!

0 It.la"

c PoraZ

N

-0

X

N.U

0Z

>-...

U)ZWC

Z0

...Q..~0U)C~

--

../..~

.0.10. "-" "

~ I.m 10.CD tCD.m I=~

RESIDUAL SULFONATE tONt. k.ol/~~x 105

Figure 11. Aggregation numbers detem1ined at different adsorption densitiesshown along the isothenn. (Adapted from ref. 26)

...."

A..

~

~~.

of0"

.Q-<

Wavenumber (cm.')

Figure 12. Infrared spectrum of (a) pure alumina (b) pure AOT and (c) AOTadsorbed on alumina. (Adapted from ref. 29)

1(Q.tn

IO.tn.

t.m

.,

w SUR.J'ACfANT ADSORPnON AND SURFACE SOLUBD..IZATION

,.' ..--

Figure 13. Effect of water on the alumina suspension stability at two differentsurfactant concentrations; solid symbols [ACT] - 8.5 xl0.3 MIl and hollowsymbols [ACT) = 26.5 x lQ-1 MIl (dotted line): The corresponding water

adsorption densities are also shown (solid line). (Adapted from ref. 28)

7. SOMASUNDARAN ET AL SIIIfad4nt and PDlymD' Solloids11.3

,..

Wo z 0.2

~ ---

Wo = 1.95

Wa = 7.25

wo=11.1

Figure 14. Effect of water on the ESR spectra of7-doxyl stearic acidcoadsorbed with a monolayer of Aerosol-OT at the aIumina/cyclohexaneinterface. (Adapted from ref. 28)

..'

----

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c..~0

~

c5

... (AO1]. 5 x '0.3 mole/I'.,,

, £..- ... .

~OO100 200

Re.ldual Water ConcentraUon x 103, mole/l

Figure 15 .Effect of Water on the order Parameter, S, calculated &om the ESRSpectra of 7 -doxyl stearic acid coadSOrbed with Aerosol OT at the

aJumina/cyclohexane interface.(Adapted &om ref: 28)

7. SOMASUNDARAN ET AL Surfactant and Polymer SoUoids usconsidering the struCtural reorganization of the complex adsorbed layer when water ispresent at the interface. At low water concentrations. water molecules bind thecarboxylic groups of the stearic acid molecules and the polar groups of Aerosol OT.directly to the hydroxyl groups of the alumina surface. This binding limits the ability ofthe probe to move \I.Qthin the adsorbed layer and is consistent with a model of localizedadsorption where the adsorbed molecules have limited degrees of freedom. As thewater adsorp~n density increases, the carboxylic groups of the probe moleculesinteract with Water molea1les not bound directly to the hydroxyl groups o~ mineralsurface. Similar interactions between the polar groups of Aerosol OT and watermoleoJles are most likely, as a result of which the adsorbed layer can become looselybound to the miaefa1 surface. The molecular diffi1sion limited by the binding of thesurfactant molecules at low water concentrations thus increases markedly as wateradsorbs on particles. --

"..

..~

Oleate 00 Dolomite and Francolite. FTIR spectroscopy in the ATR mode has beenemployed to investigate the surface precipitation phenomenon during the adsorptionof oleate on Francolite and Dolomite. Near total flotation of the individual mineralscould be achieved with oleate collector, a reagent used to induce surfacehydrophObicity. Based on that, the flotation of binary mi.~res attempted underrestricted conditions of pH deemed to proOYide excellent selectiOYity based on the datafor individual minerals. It was conjectured that the loss in selectiOYity of flotation of theabove mixed minerals could be due to the alteration of the surface properties of theminerals due to the interactions between dissolved mineral species and the oleateresulting in surface and bulk precipitation. .o\n FTIR experiment was performed in aHarrick's ATR liquid prism assembly \llith dolomite/oleate system. Dolomite shows astrong C=O stretching frequency of the carbonate group at 1488 crn"'. Figure 16 showsthe FrIR Spectra of dolomite mixed \llith oleate at various concentrations of the latter.The dolomite peak is seen to diminish with increasing oleate concentration a.,d a newpeak characteristic of the oleate appears at a lower frequency. The decre..1Se in thedolomite peak is attributed to direct oleate adsorption, formation ot' Ca and ~fg oleateprecipitates on the mineral surface and/or to the masking of carbonate groups on themineral surface. This observation was in agreement \llith the results from depletionisotherms of the oleate and the dependence on the concentrations of the dissolved Ca!-and Mg~. species at the interface and in solution (30).

Mixed Surfactants on Alumina. Most of the practical situations related to industrialprocesses such as enhanced oil recovery deal ~ith mixed surfactant systems (31)arising either as contaminants or as deliberately added components to tune the surfaceproperties. !-.fi."'(ed surfactants are considerably less expensive and readily av3.ilable thanpure components. Moreover, in mi."'(ed systems comprising ionic and non-ionicsurfactants, the problem of precipitation is abated and greater salt tolerance is achieved.

While the solution properties of mixed surfactant systems have been addressedrecently (32), the nature of aggregation ofmi."'(ed suractants on solids has been onlyscantly investigated. We have attempted recently to elucidate the microstructuralpropenies of adsorbed layers of mixed surfactants on solids like alumina, silica andkaolinite by spectroscopic techniques to supplement the information gathered from

U6 SURF A cr ANT ADS 0 RP'n 0 N AND S1JU ACE SO LUB nJZA. TI 0 N

a) Oleate Conc.(M)....:t;

.1;\

1- 0 (do1~ita alone)2- 10-5

3,.- .10-4.~

'I --J~

11\10\

~~~

If /I~

.! I . . . I . - . , .. - I .

1600 1400 1200 1000-1

Vaveft\8ber (c.a )

b)

1600 1200

(C8-1)

100,0. .1400

V'avenuaber

Figure 16. ATR spectra of dolomite slurry at different concentrations ofoleate~ (a) low concentration region, (b) high concentration region.

7. SO~~UNDARAN ET AI- Surfactant and Pol.)'mu Salloids U7

adsorption. microcalorlmetry and allied studies (33). The microstructures of sodiumdodecyisulfate and dodecyloxyheptaethoxyl ethyl alcohol (C1~OJ layers on aluminawere probed with pyrene. The polarity parameter measured at various points on theisothernt is shown in Figure 17. It is seen that the polarity parameter is nearly aconstant along the isotherm. while the value in dodecylsulfate micelles is lower thanthat obtained for i&.. It (he alumina-water interface. This throws light on the compactnature of the adsorbed ~yers. Pyrene probing of the ~EO~ (nonYlbenzenederivative) aBNegates on silica showed that no hydrophobic aggregates w~e formedat the solid-liquid interface. -

,..

..~Polymer/Solid Systems...Polymer Confonnation in the Adsorbed State. Polymers can exist in differentconformations troth in solution and in the adsorbed state. The adsorption of polymericmaterials onto solid surfaces can be quite different from the adsorptiort'"'t)f smallmolecules in that the polymer adsorption is greatly influenced by the multifunctionalgroups that it possesses (34). This stems from the widely varying sizes andconfigurations available for the polymer. In addition. macromolecules usually possessmany functional groups each having a potential to adsorb on one or more givensurfaces.

Among the pol,'tTneric materials, POlyelectrOlytes are the most imponant becauseof their panicipation in many biological processes and their utility in processes likedispersion. flocculation. adhesion and rheology. Polyacrylic acid (p.-\A) is chosen hereas a model polyelectrolyte to gain insight into the adsorption behavior ofpolyelectrolytes in general. Such an understanding is imponant in acquiring effectivecontrol over processes of colloidal stabilization and flocculation.

Polyacrylic acid can exist in different conformations depending on the solvent,pH. and ionic strength conditions (35). Such a flexibility also intluences its adsorptioncharaCteristics on solids and in turn affects the subsequent suspension behavior. Usinga fluorescent labelled pol~er and by monitoring the e.'<tent of ex,::mer formation it wassho\\,n that the polymer at the interface could have a stretched or coiled conformationat the interface depending on the pH. The rationale behind the use of this technique isthe observation that the e."(tent of excimer formation which depends on the interactionof an e.~cited state pyrene pendant group of the pol~er with another pyrene group inthe ground state and therefore depends on the pol~er contonnation. This may beunderstood by examining Figure18 which shows that at low pH. there is a betterprobability for intramolecular e.xcimer formation between pyrene groups resulting froma favorable coiled conformation. Similarly a low probability for the excimer formationat high pH may be understood as a consequence of the repulsion between the highlyionized carboxylate groups in the pol~er and the subsequent stretching of the polymerchain This difference is reflected in the nature of their fluorescence spectra as seen inFigure 19 where at low pH the intensity due to the excimer emission is larger than athigher pH.

We have also performed a detailed investigation on the flocculation behaviorof alumina particles with and without added polymer under fi.xed and shifted pHconditions. The polymer conformation was shown to be a controlling factor of the

128

..'

'J

7. SO~IASUNDAR.'-~ ET AI- SurflJct4nl and PolYmlr Solloids 129

...' .-Dapral at Solid-Liquid Interface. HydrophobicaUy modified polymers have beenincreasingly used nowadays to increase the viscosity and elasticity of solutions as theycan undergo interesting intramoleaJlar and intermolecular association by hydrophobic --interactions. These polymers are also promising as steric stabilizers as well u8oca11ating agentS for h:Ydroplu"Hc and hydrophobic surfaces in both aqueous and non-aqueous media. We have studied the stabilization effects ofDapraJ GE 202 (maleicanhydride «-olefin copolymer with both hydrophobic and hydrophilic side chains)which is a h:YdroPhobicaily modified comb-type polymer whose structUre is as indicatedin Figure 22. The observed changes in gjspension Stability cannot be explained in tenDSof the corresponding changes in electrokinetic propenies (37). Fluorescence studieswere conducted using free pyrene as the probe and the polarity parameter measured atdifferent levels ofDaprai adsorption (Figure 23). At low concentrations the I/lt valuefor pyrene is similar to that in water but with increase in concentration this increasesreaching upto a value of 1 at a Dapral concentration of 500 ppm. These suggest theformation of aggregates at the interface with the creation of hydrophobic microdomainsinto which the p:OTene panitions preferentially. Such an aggregation can explain thecontinued adsorption ofDapral e\'en after the re\"ersa1 of surface charge. The tormationof such hydrophobic aggregates would now render the surfaces hydrophilic since theethylene oxide chains will be dangling into solution and stabilization of the suspensionin this case is attributed to a combination of electroStatic repulsion and sterichindrance of the adsorbed molecules

Fluor~ce Spectroscopy was also used to detect the orientation of DapraJ onalumina panicles in non-aqueous media(38). 7-dimethylamino-4-methyl coumarinwhich is a relatively hydrophilic molecule was used to probe the adsorbed layer ofD.o\PRAL at the alumina/toluene interface. The spectrum for the probe in the ab~ofDAPRAL is similar to that obtained in water solution (ma."<imwn emission at 470 nm,Figure 24). This is attn"buted to the hydrophilicity of the bare alumina surface. With anincrease in polymer adsorption, the maximum wavelength shifts towards the shoner

wavelength range reaching the value for hydrocarbon solvents (390 nm) at 500 mg/1 ofD.-\PR.o\L (Figure 25). This obser.'ation gjppons the mechanism of polymer adsorptionthrough the interaction between the ethylene oxide chains and the hydroxyl groups onthe alumina ~ace \liith the hydrocarbon side chains dangling ~owards the solution andthe alumina panicles eventually fully covered to become towly hydrophobic. This is

supponed by the fact that the alumina suspensions are stabilized signjficantly by theadsorption ofDAPRAL.

130

..~

'7. SOMASUNDARAN ~ AL SwftJdlult and Polymer Solloids 131

~) ..40v 'pH .- -

.~ ~-

b) "'9" OH- - 1-'"0 """'=--~~--A --

. . . .Clil.d'sllwll..

. . . .'a~0j\4t41 lOlu'..',.. .

V\.- - i .~~~:::::::;:-

~Iqh pH

~-A-~-+ + + .

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10. pH

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~

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-~:~ ~.J~ '- -- -, -- -,,-

portio", t'~O"dc4/o4'o'b.4 ta~o"4c4/od'o'bt4

Figure 21. Schematic representationof the adsorption process of pyrene-labeledpolyacrylic acid on alumina A) At low pH polymer is coiled in solution whichleads to B) adsorption in coiled fonn. C) Subsequent raising of the pH causessome expansion of the polymer D) Polymer at high pH in solution is extendedand binds E) strongly to the surface in this confonnation F) Subsequentlowering of pH does not allow for sufficient intrastrand interactions for coilingto occur. (Adapted from ref. 36)

SURl'ACfANT ADSORPTION AND SURl'ACE SOLUBn.IZAnON

.---

Figure 22. Molecular structure ofDAPRAL.

1.1

'$ 0.9

0.1

'U

I~?;-.5-0~

In Solution0.7

0.6Q

0 000.5 -. . .It

0 1 10 100 1000

DAPRAL Conc.e:\1ra1ion, ppm

Figure 23. Aggregation ofDapral in solution and at the alumina-water interface(pH 8):(a) in solution. (b) at the interface. (Reproduced with permission fromref. 3~pyright 1994, Elsevier)

7. SOMASUNDARAN rr ALSurfac:t4nt and Pol.)'mC7' Sol1oids

133

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440

420

400~"_~",.,.-0 200 ~ 600 100 1000

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SURFACTANT ADSORFTION AND SURFACE SOLUBWZA'nON134

PolymerISurf'actaDt/Solid System

Polymer-Surfactant Auregates at .0 lDterface. Investigations on polymer-surfactant aggregates are of importance 'Owing to the use of these mixed systems inareas related to enhanced oil recovery, conoidal stability, flotation and flocculation.The confonnation of polyethylene oxide (PEO) mixed with sodium dodecysulfate atalunUna-Wata' interface was measJred using pyrene-labeled polymer. Figure 26 showsthe VI. values of~ PyPEO on alumina ~ preadsotbed with SOS. Interestingly,the confonnation of the co-adsorbed PEa was markedly influenced as a result of itsinteraction with SDS (39). Thus PEa could be force-adsorbed onto alumina. Thisexperiment demonstrates the potential of fluorescence spectroscopy to investigate theconfonnational aspects of polymers and mactants adsorbed together at a solid-liquidinterface.

Concluding Remarks~ .A new insight has been obtained ~n the structure and evolution of surfactant andPOIyma' aggregates at solid-liquid interfaces both in aqueous and non-aqueous mediaby the application of spectroscopic and classical methods. These findings have beencorrelated \1fith the perfonnance parameters of the irxfividual systems. The aggregationnwnber of sodium dodecylsulfate on alumina was detemtined at various points of thesoUoid ~.olution and the variation of the micro-viscosity within the soUoid layer wasdetermined along the isotherm using ESR and Raman spectroscopy. Fluorescencespectroscopic studies also indicate that steric hindrance to packing in the adsorbedlayers controls the solloid formation of the alkyl xylenesulfonates. Also inftaredspectroscopy has been used to study the adsorption of surfactants such as oleate on~ surfaces. Pyrene probing revealed the soUoidal structure of mixed surfactants.The effect of pH dependent conformational equilibria of polyacrylic acid on theadsorption process and its implications to flocculation have been explored to suggestreasonable conformational structures for the polymer soUoid. The spectroscopictechniques were also useful in studying adsorption processes in non-aqueous media.Thus, the role of water in controlling the dispersion properties of alumina dispersed incyclohexane by increasing the lateral diffi.1sion of the adsoroed surfactant molecules wasdepicted using ESR technique. A rare insight has been gained on the stabilization ofsolids by the hydrophobically modified polymer Dapral in aqueous and non-aqueousmedia. Fur1hermore, spectroscopic methods were adopted to interrogate morecomplex systems consisting of surfactants and polymers. It is clear that there is avariety o~hniques now becoming available to examine structures insitu at levelssmaller than ever before and one can obtain valuable equilibriun\, and kineticinformation on a molecular level by adapting these techniques suitab1Y.

. -Acknowledgments. We ~ish to acknowledge the financial support from NationalScience Foundation. Department of Energy, Unilever Research and Nalco.

Surfadant and Polymer SolIoids7. SOMASUNDARAN Er AI- 135

~ ---

.." .-

Figure 26. PEa conformation on the alumina surface preadsorbed with SDSexpressed as coiling index. (Reproduced with permission from ref. 39.Copyright 1990, John Wtley 8c. Sons)

136 SURFACfANT ADSORP'110N AND SURFACE SOLUBn.lZA110N

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~

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7. SOMASUNDAM.~ ErA!- Surfactant and PolymIJO Solloids 137

..~

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RECEIVED JuIv 27.1995