Reprinted from

COLWIDSANDSUREACESAN IN1EflNAT1CIW. ~

A: PHYSICOCHEMICAL ANDENGINEERING ASPECTS

Colloids and SurfacesA: Physicochemical and Engineering Aspects 117 (1996)227-233

Adsorption of Aerosol-aT on graphite from aqueousand non-aqueous media

S. Krishnakumar, P. Somasundaran *

Langmuir Center for Colwids & Interfaces, Henry Krumb School, Columbia University, New York, NYl0027, USA

ELSEVIER

COllOIDS AND SURFACESA: PHYSICOCHEMICAL AND ENGINEERING ASPECTS

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COLLOIDSANDSURFACES AColloids and Surfaces

A: Physicochemical and Engineering Aspects 117 (1996)227-233ELSEVIER

Adsorption of Aerosol-aT on graphite from aqueousand non-aqueous media

S. Krishnakumar, P. Somasundaran *Langmuir Center for Colloids & lnterfaces, Henry Krumb School, Columbia University, New York, NYl0017, USA

Received 13 October 1995; accepted 14 March 1996

Abstract

Efficient dispersion of carbonaceous particles in polar and non-polar liquids is important in the processing and useof paints, pigments, printing inks and tertiary oil. In this study we examined the adsorption of an anionic surfactant,Aerosol-OT (AOT), on graphite particles in cyclohexane and water and the ensuing changes in their dispersionproperties. The adsorption of AOT on hydrophobic graphite particles in cyclohexane results from the tendency of thepolar head group of the surfactant molecules to partition into hydrophilic domains away from the solvent which ishydrophobic. Fluorescence studies using I-methyl-8-oxyquinolium betaine as probe reveals the presence of "reverse-micelle"-like surfactant aggregates at the interface at all surfactant concentrations. The observed changes in dispersionstability suggest the fonnation of interparticle surfactant aggregates at low surface coverages while at higher coveragesthe aggregates are fonned on individual particles. AOT adsorbs on graphite from aqueous solutions as well, despitethe unfavorable electrostatic repulsion, evidently through hydrophobic interactions. Such adsorption stabilizes theaqueous dispersions by providing increased electrostatic repulsion among the particles. Desorption of preadsorbedAOT from graphite when contacted with different solvents showed a marked solvent effect with maximum desorptionoccurring in solvents of intennediate polarity where the hydrophilic and hydrophobic driving forces are balanced.

Keywords: Adsorption; Aerosol-OT; Graphite; Non-aqueous

. Introduction While electrostatic attraction and chemicalbonding are the most frequently encounteredmechanisms in the adsorption of surfactants onpolar surfaces from aqueous solutions, it has beensuggested that in organic solvents electrostaticforces do not playa pronounced role in adsorption[7,8]. Acid-base interactions between the soluteand the substrate have been suggested to be thedominant adsorption mechanism in these systems.This mechanism essentially consists of dipolarinteractions involving an electron or protontransfer between the solute and substrate moleculesresulting in the formation of an acid-base adductat the interface. This can be understood easily forpolar adsorbents but fails to explain adsorption

Efficient dispersion of colloidal particles in non-aqueous liquids is important in many technologiessuch as reprography and high performance ceram-ics [1-4J. Adsorption of surf act ants and polymershas been employed in the past to modify particlesurfaces and to change their dispersion and rheo-logical properties'[5,6]. The effect of these surfacemodifiers depends on their electrostatic and struc-tural properties which also determine their adsorp-tion behavior and the molecular orientation at theinterfaces.

* Corresponding author.

0927-7757/96/$15.0001996 Elsevier Science B. V. All rights reservedP/I 80927-7757(96)03648-5

228 S. KrishnakJImor, P. SomasundaranjCo/loidf Surfaces A: Physicochem. Eng. Aspects 117 (1996) 117-133

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Fig. 1. Molecular structure of: (a) Aerosol-OT; (b) I-methyl.8-oxyquinolium betaine.

The fluorescent probe, I-methyl-8-oxyquinoliumbetaine (Fig. l(b» was obtained from MolecularProbes Inc. and used as received. Initially the UVabsorption of the probe was measured in differentsolvents and found to be similar to that reportedin the literature [12]. The A.uax of absorption wasobserved to shift from 375 nm in non-polarsolvents to about 364 om in polar solvents. Theprobe is insoluble in pure cyclohexane but dissolvesin the presence of AOT micelles. The absorptionspectra of this probe in AOT solutions in cyclohex-ane are characterized by an absorption at 364 nmsignifying a highly polar environment for the probein the interior of the reverse micelles (Fig. 2). Theprobe also displayed a characteristic fluorescenceemission at 550 nm at an excitation wavelength of364 nm in these solutions.

on hydrophobic surfaces such as graphite. In aque-ous systems ionic surfactants are known to aggre-gate at low surface coverages due to lateralinteractions among the adsorbed molecules andeven form bilayers at higher concentrations [9].In non-aqueous solvents adsorption has been pos-tulated to proceed only up to a monolayer as thelateral hydrophobic interactions are very weak inthis case.

In this study we have investigated the adsorptionof anionic Aerosol-OT on a hydrophobic adsor-bent, graphite, in cyclohexane as well as water.The changes in dispersion stability and zeta poten-tia] (in the aqueous case) were monitored atdifferent surface coverages. In addition, we haveused steady state fluorescence spectroscopy tomonitor the formation of molecular aggregates atthe solid-liquid interface.

Fluorescence probing utilizes the photophysicalresponse of a probe molecule which is sensitive tochanges in the structure and composition of itslocal environment [10]. The selection of a suitableprobe depends on the following two criteria: (a) itshould partition selectively and provide informa-tion on a specific microdomain; (b) its presence,even in traces, should not significantly alter theproperties of the original system. For this studywe selected I-methyl-8-oxyquinolium betaine asthe fluorescent probe. This probe has been pre-viously used to study the characteristics ofAerosol-OT reverse micelles in benzene and isknown to partition into the hydrophilic core ofthe reverse micelles [11].

2.2. Methods

Adsorption-desorption was estimated by meas-uring changes in concentration of the surfactant inthe bulk solution upon contact with the solid fora period of 12 h. AOT was analyzed in solutionby the two phase colorimetric titration technique[12]. The stability of dispersions was estimated bymonitoring the rate of descent of the upper inter-face in a homogeneously dispersed suspension. Thesediment volumes were also measured in all casesafter allowing the dispersions to settle for 24 h.Zeta potentials of the aqueous dispersions weremeasured by electrophoresis using the Zetameter3.0 setup. Fluorescence spectra were recorded usinga Photon Technology International PTI-LS spec-trophotometer at an excitation wavelength of364 nm.

2. F~xperimental

2.1. Materials

Graphite particles of size 1 JUD, with a BETsurface area of 15 m2 g-l, were purchased fromSigma Chemicals. Aerosol-OT (Fig. 1 (a), AOT)used in this study was purchased from FisherScientific and purified by dissolving in methanoland recrystallizing by solvent evaporation.Cyclohexane and other organic solvents of spectro-scopic grade were obtained from Fisher Scientificand used without further purification.

S. Krishnakumar, P. Soma.fundaranICo//oids Surfaces A: Physicochem. £ng. Aspects 117 ( 1996) 227-233229

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Wavelength, nmFig. 2. UV adsorption spectra of 1-methyl-oxyquinolium betaine in cyclohexane at different concentrations of AOT: (a) 4 x 10-2 M;(b) 1 x 10-2 M; (c) 4 X 10-3 M; (d) 4 x 10-4 M; (e) 4 x 10-5 M.

ting pattern as shown in Fig. 3. The adsorptionincreases sharply in the initial part suggesting highaffinity of the surfactant for the solid at lowconcentrations. The adsorption then appears toreach a plateau. Calculations based on apparent

2.3. Sample preparation

For the adsorption studies, the graphite wasdried first by heating at 200°C for 6 h followed bycooling to room temperature under vacuum. 1 gof the dried solid was then mixed with 10 ml ofthe surfactant solution and conditioned for 12 h.The particles were then removed by centrifugationand the supernatant analyzed for residual surfac-tant. For the fluorescence studies, surfactant solu-tions containing 10-4 M of the probe are preparedand conditioned with the solid as before.Fluorescence spectra were acquired from the solid-liquid interface as well as from the supernatant.For the desorption studies the surfactant was firstadsorbed on graphite from cyclohexane and thenthe solids separated and redispersed in solvents ofdifferent polarity. Residual concentrations of AOTin the different solvents were then measured after12 h of conditioning.

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3. Results and discussion

3.1. AOT adsorption in cyclohexane

The adsorption isotherm of Aerosol-OT ongraphite from cyclohexane follows a very interes-

Fig. 3. Adsorption isotherm of Aerosol-OT on graphite fromcyclohexane.

230 S. Krishnakumar, P. SomasundaranjCo//ow Surfaces A: Physicochem. Eng. Aspects 117 ( 1996) 127-233

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Fig. 5. Schematic diagram showing the formation of:(a) interparticle surfactant aggregates which cause ftocculation;(b) aggregates with monomers in solution leading toredispersion.

monolayer coverage at this level yield a parkingarea of approximately 1.03 nm2 per molecule ofAOT. This molecular parking area corresponds toan AOT molecule adsorbing flatly at the solid-liquid interface. At a higher concentration, above10 -2 M, there is a further sharp increase in the

adsorbed amount and this reaches about five timesthe initial plateau value at about 3 x 10-2 M. Itshould be noted at this point that the CMC ofAOT in cyclohexane is about (0.8-1) x 10-3 M.Thus this sharp increase in adsorption occursabove the CMC though it does not coincide withthe onset of micellization.

The settling behavior of these dispersions as afunction of the adsorbed AOT amount is also veryinteresting (Fig. 4). The settling rate rises sharplyat first and then decreases as sharply, suggestingrestabilization. At low surface coverages the polarhead group of the flatly adsorbing AOT is exposedto the solvent with which it is not compatible. Insuch a case the AOT molecules, as shown inFig. 5(a), can form interparticle aggregates thatwould effectively create a polar microdomain toshield the head groups from the solvent. Such aninterparticle aggregation can account for theincreased settling rate. As the AOT concentrationis increased the adsorbed molecules are proposed

to form aggregates at the interface as shown inFig. 5(b). This leads to the sharp increase in theadsorption density as well as the decrease insettling rate due to the disappearance of the inter-particle aggregation observed at low surface cover-ages. Formation of such reverse-hemimicelle-likeaggregates at the interface have been reportedearlier for the adsorption of l-decanol on graphi-tized carbon black from non-polar solvents [13].

3.2. Fluorescence studies

Fluorescence studies were conducted by coads-orbing the probe, l-methyl-8-oxyquinolium beta-ine, with the surfactant and measuring thefluorescence response of the probe from the inter-face as well as in the supernatant after adsorption.The fluorescence spectra from the graphit~yclo-hexane interface were of extremely low intensitiesat all concentrations probably due to absorptionof the emitted light by the graphite surface. Fromthese spectra it was therefore not possible to ascer-tain whether the probe is incorporated into theadsorbed surfactant layer at the interface. Howeverwhen the supernatant was analyzed it was foundthat there was no residual probe in any of thecases including those where the residual AOTconcentration is above the bulk CMC value incyclohexane. This observation clearly shows com-plete transfer of the probe to the interface. Fromthe earlier studies it was shown that the probe canbe solubilized only in polar domains and thus

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Krishnaklmlar. P. SomasundoranjColJoiJs Surfacu A: Plrysicochml. &g. Asprct.f 117 ( 1996) 227-233 231

cannot be expected to adsorb at the graphitesurface by itself. Thus the probe must be solubilizedin the polar microdomains created by the adsorbedAOT molecules at the interface - in the inter-particle aggregate at low concentrations and in theadsorbed micelle at higher concentration.

isotherm has a- Langmuirian shape with the onsetof the plateau roughly corresponding to the CMCof AOT in aqueous solutions (8 x 10-4 M). Acalculation of parking area based on monolayercoverage at this adsorption plateau gives a valueof 1.03 nm2 per molecule, which is the same valueas that obtained for the first plateau in the adsorp-tion from cyclohexane. As in the previous case thiscorresponds to a flatly adsorbing molecule at theinterface. Fig. 6(a) also shows the change in zetapotential of graphite with AOT adsorption and itcan be seen that the graphite becomes increasinglynegative with the adsorption of anionic AOT. Thisincreased negative charge also stabilizes the disper-sions as indicated by the decrease in the sedimentvolumes with surfactant adsorption (Fig. 6(b». Itis interesting that the negatively charged surfactantis able to adsorb on graphite which is negativelycharged to start with. This, along with the flatorientation of the AOT molecule, suggests that theadsorption in this case is driven by the interactionsof the hydrophobic chains of the surfactant withthe predominantly hydrophobic graphite surface.The adsorption of ionic surfactants on polar solidsin aqueous solutions has been widely studied andis characterized by a sharp increase in adsorptiondensity at a critical low concentration (hemimicelleconcentration) which corresponds to the formationof molecular aggregates or "solloids" at the inter-face due to lateral interaction among the adsorbedmolecules. However in this case we do not observeany such increase in adsorption density in theconcentration range studied. This suggests that thehydrocarbon chains are firmly anchored on thegraphite surface and are unavailable for solloidformation.

3.3. Adsorption of AOTfrom aqueous solution

Fig. 6(a) shows the adsorption isotherm or AOTon graphite rrom aqueous solutions at pH 6.6. The

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3.4. Desorption into different solvents

Graphite particles with preadsorbed AOT wereredispersed into solvents of different polarity andthe surfactant desorption was monitored. Theresults, as shown in Fig. 7, indicate that the desorp-tion increases with solvent polarity, attains a maxi-mum and then decreases again. A similar behaviorhas been observed for the desorption of AOT fromoxide minerals [14]. This clearly demonstrates thechanging mechanism of adsorption with change in

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Fig. 6. (a) Adsorption isotherm of AOT on graphite in aqueoussolutions [0] and the corresponding change in zeta potential[8]. (b) Effect of AOT adsorption on the sediment volumesof aqueous graphite dispersions.

232 S. Krishnakumar, P. Somasundaran/Colloids Surfaces A: Physicochem. &g. AspeCU 117 (1996) 217-213

solvent polarity. In low dielectric constant solventsthe polar interactions of the surfactant moleculewith the polar sites that may be present on thesubstrate cause adsorption. With increase insolvent polarity the surfactant becomes more com-patible with the solvent and partitions more intothe solvent. In the more polar liquids the hydro-carbon part of the surfactant becomes incompatiblewith the solvent and is driven to the hydrophobicgraphite surface resulting in adsorption. Thesolvents in which the desorption is maximumcorrespond to those with which the surfactant iscompatible on the whole and which exist in themonomolecular form [15].

tion to the interfacial aggregates in preference tothe aggregates in solution. This suggests that themicropolarity of the interfacial aggregates is higherthan that of the ones in solution. However thiscould not be quantified as the graphite surfaceappeared to quench any fluorescence signal origi-nating from the probe on the surface.

The settling rates of the dispersions in cyclohex-ane initially increase with AOT adsorption butlater decrease. The initial increase is attributed tothe formation of interparticle surfactant aggregates.At higher concentrations the adsorbed moleculeaggregates with the excess surfactant in solutionrather than with molecules on other particles, sothe flocculation ceases to occur and the dispersionis restabilized.

Aerosol-OT adsorbs on graphite from aqueoussolutions through hydrophobic interactions despitethe electrostatic repulsive force. The adsorbed AOTstabilizes the dispersions by providing increasedrepulsive force between the particles as evidencedby the change in zeta potential.

The adsorbed AOT does desorb into differentsolvents, but this desorption is dependent on thesolvent polarity. The desorption passes through amaximum with increasing solvent polarity. Ananalysis of this result reveals that at low solventpolarity the adsorption is driven by polar inter-actions while at higher solvent polarities hydro-phobic interactions playa dominant role. Thisobservation is in line with earlier findings onsurfactant adsorption in aqueous media in whichthe adsorption was found to increase with increasein the surfactant chain length. Thus the mechanismof surfactant adsorption on hydrophobic surfacesdepends on the nature of the solvent and the effectof such adsorption on the dispersibility of thesuspensions depends on the orientation of theadsorbed surfactant.

4. Conclusions

Aerosol-OT adsorbs on graphite from cyclohex-ane as well as water, but the mechanisms ofadsorption differ in the two solvents. In cyclohex-ane the adsorption is driven initially by thedifferential polarity between the graphite surfaceand the solvent with respect to the surfactantmolecule. At higher AOT concentrations theadsorption density increases sharply and this ishypothesized to be due to the fomlation of reverse-micelle-like aggregates at the solid-liquid interface.Fluorescence studies using I-methyl-8-oxyquinolium betaine showed the probe to parti-

Ackno,,'iedgments

The authors wish to acknowledge the financialsupport of this work by National Science Foun-dation (NSF-CTS-93-11940) and MMMRI, NewYork.

S. Krishnakumar, P. Somasundaran/Colloidr Surfaces A: Physicochem. Eng. Aspect.f 117 ( 1996) 227-233 233

References [8] R.J. Pugh, in L. Messing, S. Hirano and H. Hauser(Eds.), Ceramic Powder Science III. American ChemicalSociety, Westerville, OH, 1990, p. 375.

[9] P. Somasundaran and D.W. Fuerstenau, J. Phys. Chem.,70 (1966) 90.

[10] P. Chandar, P. Somasundaran and N.J. Turro, J. ColloidInterface Sci., 117 (1987) 31.

[11] M. Ueda and ZA. Schelly, Langmuir, 5 (1989) 1005.[12] V.W. Reid, G.H. Longman and E. Heinhirth, Tenside, 5

(1968) 90.[13] T. Gu and B-Y. Zhu, Colloids Surfaces, 46 (1990) 81.[14] S. Krishnakumar and P. Somasundaran, Langmuir, 10(8)

(1994) 2787.[15] S. Krishnakumar and P. Somasundaran, J. Colloid

Interface Sci., 162 (1994) 425.

[IJ V. Novotony, Colloids Surfaces, 24 (1987) 361.[2J A. Blier, Stability of ceramic suspensions, in L.L. Hench

and D.R. Ulrich (Eds.), Ultrastructure Processing ofCeramics, Glasses and Composites, J. Wiley & Sons,New York, 1984, p.391.

[3J R.B. McKay, Pigment dispersions in apolar media, inH.F. Eicke and G.D. Parfitt (Eds.), Interfacial Phenomenain Apolar Media, Marcel Dekker, New York:, 1987.

[4J F.M. Fowkes and RJ. Pugh, Polymer Adsorption andDispersion Stability, ACS Symp. Ser., 240, AmericanChemical Society, Washington, DC, 1984, p. 331.

[5J P. Somasundaran in P. Somasundaran and R.B. Greves(Eds.), AIChE Symp. Series, 1975, p. 1.

[6J P. Somasundaran, T.W. Healy and D.W. Fuerstenau,J. Phys. Chem~ 68 (1964) 3652

[7J F.M. Fowkes in S. Ross (Ed.), Physics and Chemistry ofInterfaces II, American Chemical Society SpecialPublication, 1971, p. 153.

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