2786 Reprinted from LANGMUIR, 1994, 10.Copyright @ 1994 by the American Chemical Society and reprinted by pennission of the copyright owner.

Role of Surfactant-Adsorbent Acidity and Solvent Polarityin Adsorption-Desorption of Surfactants from Nonaqueous

Media

S. Krishnakumar and P. Somasundaran'

Langmuir Center for Colloids & Interfaces, Henry Krumb School of Mines,Columbia University, New York, New York 10027

Received March 8, 1993. In Final Form: February 22, 1994@

Adsorption of surfactants at the solid -liquid interface is governed in general by the properties of thesurfactant, solid, and solvent. However, very little is known about the role of these properties in the caseof adsorption from organic media. In this study the effect of adsorbent and surfactant acidities and solventpolarity on the adsorption of surfactants on oxide minerals in nonaqueous solvents has been investigated.The studies conducted using anionic (Aerosol OT) and cationic (dimethyldodecylamine) surfactants on thebasic and acidic oxide minerals (alumina and silica) reveal that for a given surfactant the polarity differentialbetween the adsorbent and the solvent is responsible for the partitioning of the solute to the solid -liquidinterface. Following this, acid-base-type interactions between the solute and the adsorbent can takeplace to enhance the attachment of the former to the particles. The acid-base interactions depend on thenature of the surfactant and the substrate, with an acidic surfactant interacting strongly with a basicadsorbent and vice versa. By using the solvent dielectric constant as an indicator of solvent polarity, itwas found that while polar interactions control the adsorption from solvents of low polarity, hydrocarbonchain interactions with the surface playa major role in determining adsorption from solvents of higherpolarity.

that the surface coverage varied as a function of theirchain length.5 The bulk of the studies have been directedtoward detennining the orientation of surfactants at theinterface. Recent efforts have aimed toward identifyingthe major adsorption mechanisms in nonpolar media.Pugh6 showed in a recent paper that acid-base interac-tions are chiefly responsible for adsorption of solutes fromnonaqueous solvents. In the absence of significantionization, electrostatic mechanisms do not playa majorrole in adsorption processes in solvents of low dielectricconstants. Labib and Williams discuss acid-base inter-actions in tenDS of the donicity of surfaces which is relatedto the electron donor properties of the surface species.7They found that a solid in contact with a liquid of lowerdonicity developed positive surface charges by electrontransfer to the liquid and vice versa.

In the present work the effect of adsorbent-solute-solvent interactions on the adsorption process and thedominant adsorption mechanisms were systematicallyinvestigated using surfactants and solids of differentacidities and solvents of different polarities.

Materials and Methods

The alumina used for this study was Linde Alumina PolishingPowder Type A purchased from Union Carbide Corp. Morpho-logically, the powder was constituted of micrometer-size ag-gregates composed of smaller particles (between 200 and 500nm) with a nitrogen surface area of 14 m2/g. The absence ofhysteresis on the adsorption -desorption isotherm suggested thismaterial to be essentially nonporous. The silica used wasSpherosil-B obtained from Rhone Poulanc, with a nominal sizeof 40-100 JIm and a Brunauer-Emmett-Teller (BET) surfacearea (SA) of 25 m2/g. The pore size was measured to be about400 nm. The other oxide minerals rutile (SA = 2 m2/g) andhematite (SA = 9.2 m2/g) were obtained from Aldrich Chemicals.

Introduction

Nonaqueous colloidal dispersions are being widely usedcurrently in a number of important technological pro-cesses.1 High-performance ceramic processing, for ex-ample, requires a well-dispersed system prior to firing inorder to minimize the flaw population.2 Other technologiesusing dispersions in nonpolar media include magnetic tapemanufacturing,2 electrophoretic image processing,2 re-pographic inking,2 and monodispersed colloid production.3In all these applications it is necessary to have dispersionsthat have long-term stability under adverse conditions ofhigh electric fields, high solid concentration, etc. Typicallythe dispersions are stabilized by the addition of surfaCtantsor polymers that adsorb on the particle surface and provideelectrostatic and/or steric stabilization. To understandthe stabilization mechanisms, it is necessary to have afull knowledge of the nature of the adsorption processsince this does control the amount and orientation of thestabilizing agent at the interface.

There are several factors that control the adsorptionprocess which are reviewed in detail by Parfitt andRochester.. The nature of the adsorbent and the soluteand their mutual interaction plays an important role inadsorption. The structure and orientation of the adsorbedlayer depend on the relative strength of the interactionbetween the adsorbent and solute. The solvent can alsoaffect the adsorption process either by competing withthe solute for adsorption or by weakening the adsorbent-solute interactions.

Adsorption of surfactants from nonaqueous media hasbeen a subject of interest for a long time. Earlier studieson adsorption ofn-fatty acids on silica from hexane showed

. Abstract published in Advance ACS Abstracts, July 15, 1994.(1) Novotny, V. Colloids Surf. 1987,24,361.(2) Blier, A. In Ultrastructure Processing of Ceromics. Glasses and

Composites; Hench, L. L., ffirich, D. R., Eds.; J. Wiley & Sons: NewYork, 1984; p 391.

(3) Esumi, K.; Suzuki, M.; Tano, T.; Torigoe, K; Meguro, K ColloidsSurf. 1891, 55, 9.

(4) Parfitt, G.; Rochester, C. H. In Adsorption {rom solution at thesolid/liquid interface; Parfitt, G., Rochester, C. H., Eds.; AcademicPress: New York, 1983; p 3.

(5) Annistead, C. G.; Tyler,A.J.; Hockey, T.A. TraM. Faraday Soc.1971,67,493.

(6) Pugh, R. J. Ceram. Trans. 1990, 12,375.m Labib. M.; Williams, R. J. Colloid interface Sci. 1984,97,356.

0743-7463/94/2410-2786$04.50/0 @ 1994 American Chemical Society

Langmuir, Vol. 10, No.8, 1994 2787Surfactant -Adsorbent Acidity and Solvent Polarity

Table 1. Dielectric Constant of Relevant Solventsa

53.558.568.6380

cyclohexane 2.02 methanolchloroform 4.8 glycerolI-hexanol 13.3 methanoVwater (wt %)butanol 15.8 50:502-isopropanol 18.3 40:60acetone 20.3 20:80ethanol 24.3 water

4 From the CRC Handbook of Chemistry and Physics.

The concentration of surface hydroxyl groups on several oxideshas been determined previously by several researchers.8

Aerosol OT (sodium bis{2-ethylhexyl) sulfosuccinate) (AOT)is the most commonly used anionic surfactant in nonpolar media.It was purchased from Fisher Scientific Co. Before use, thesurfactant was purified by dissolving it in methanol and filteringoff the undissolved impurities. The excess solvent is driven offby rotary evaporation followed by freeze drying. The waxy solidleft. behind is stored in a dry atmosphere. Cationic dimethyl-dodecylamine (DDA) was obtained from ICN pharmaceutical andused without further purification.

Cyclohexane, of spectroscopic grade, was obtained from FisherScientific Co. It was selected for this study because of its weakinteractions with oxide surfaces, which allows any residualadsorption of the solvent in the prsence of the surfactant to beignored.9 When required, the solvent was stored on molecularsieves 4A to avoid contamination by water. All the other organicsolvents used were purchased from Fisher Scientific and usedafter drying without further purification. A list of solvents usedalong with some of their relevant properties is shown in Table1.

Samples for adsorption studies were prepared by desiccatingthe mineral at 200 DC for 6 h followed by cooling it for 2 h at 25DC in a vacuum desiccator. Dehydroxylation of alumina wasdone by heating it at 900 DC for 72 h and confirmed by thedisappearance of the OH adsorption bands (3800 cm-l) on theIR spectrum. For adsorption tests a 1-g mineral sample wasadded to 15 mL of surfactant solution in the desired solvent andconditioned for 12 h in a glovebox. Samples for the desorptionexperiment were prepared by first adsorbing the surfactant onthe mineral from cyclohexane. Following this, the solids withthe adsorbed AOT were separated by centrifugation, vacuumdried for 12 h, and then conditioned with different solvents for12 h, and the resultant supernatant was analyzed for thesurfactant. Analysis of the anionic and cationic surfactants wasconducted by the two-phase titration technique described in theliterature.lO

Results and Discussion(8) Effect of Solid. All oxide minerals have surface

hydroxyl groups formed by the reaction of the oxygenatoms on the surface with the atmospheric moisture. Thedensity of these surface hydroxyls determines the relativebasicities of minerals which can also be inferred from theisoelectric points (iep) of the minerals in aqueous solutions(the lower the iep, the more acidic is the mineral). Aluminahas a high density of hydroxyls on the surface (10-15OH/nm2) and a high iep (8.5) and is a basic oxide. On theother hand silica has a lower density of hydroxyls (3-4OH/nm2) and a low iep (2.5) and is an acidic oxide. Thedensity of the hydroxyl groups and hence the basicity ofthe surface can also be controlled by dehydroxylation ofthe surface by extended heating. Figure 1 shows theadsorption isotherms of Aerosol OT on alumina and silica,and it is observed that the anionic surfactant has a greateraffinity for the basic oxide than for the acidic oxide. Thesituation is reversed in the case of adsorption of the cationic

dimethyldodecylamine. Dimethyldodecylamine adsorbsmore on acidic silica than on alumina (Figure 2). Cal-culation based on a plateau adsorption of about 3 x 10-6Mlm2 on alumina gives a parking area of about 0.55 nm2per AOT molecule. This is in good agreement with thepublished values for AOT molecules adsorbed at thewater-xylenel1 and water-isooctane12 interfaces, sug-gesting that the AOT adsorbs as a monolayer in anorientation perpendicular to the adsorbent with thehydrocarbon chain extending into the solution. A similarcalculation based on plateau adsorption for the cationicsurfactant gives a parking area of 1.6 nm2 per moleculewhich is much less than the area occupied by a flatlyadsorbing DDA molecule (approximately 4 nm2), sug-gesting that this also adsorbs perpendicular to theadsorbent. Figure 1 also shows the adsorption of AerosolOT on dehydroxylated alumina. Dehydroxylation in-creases the acidity of the surface and hence reduces the

(11) McGown, D. N. L.; Parfitt, G. D.; Willis, E. J. Colloid InterfaceSci. 1967. 20, 650.

(12) Maitra, A. N.; Eicke, H. F. J. Phys. Chern. 1981,85,2687.

(8) Zettlemoyer, A. C.; McCafferty, E. Croat. Chern. Acta 1973,45,173.

(9) Suda, Y.; Morimoto, T. Langmuir 1985, 1, 544.(10) Reid, V. W.; Longman, G. H.; Heinhirth, E. Tenside 1968,5,90.

2788 Langmuir, Vol. 10, No.8, 1994 Krishnakumar and Somasundaran

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Figure 3. Infrared spectra of (a) pure alumina, (b) pure AOT, and (c) AOT adsorbed on alumina.

adsorption of anionic AOT. Thus, it is clear that Aerosol 5-OT adsorbs by interaction with the hydroxyl groups onthe oxide surface. These results also suggest that for agiven surfactant the relative acidity of the oxide surface N Eis one of the major factors that determine the extent of ~ 4interaction. E

(b) Effect of Surfactant. Comparison of Aerosol OT .and dimethyldodecylamine adsorption data from Figures ~ 0 31 and 2 also shows the role of the surfactant properties -on its adsorption on alumina from cyclohexane. For )(

surfactants, the anionic surfactant being a better electron ~acceptor is more acidic than the cationic surfactant which ~ 2can accept protons and can be considered as basic. As can ~be seen the affinity of the acidic Aerosol OT for alumina cis greater and the plateau adsorption larger compared to ~those of basic dimethyldodecylamine. However, the e- 1

adsorption trend is reversed on silica where the basic gamine adsorbs to a much larger extent than the acidic ~Aerosol OT. The infrared spectrum of AOT adsorbed on o.alumina (Figure 3) does not show any new absorption 0bands or any shift in the alumina or AOT bands due toadsorption. This suggests that the interaction is weakand probably of an acid-base type rather than of achemical type.

From these results it can be inferred that the hydrophilicgroups on the surfactants interact with the hydroxylgroups on the oxide surfaces and for a given oxide surfacethe interaction depends mostly on the acid -base characterof the surfactant. In general we observe that the acidicsurface has a greater affinity for the basic solute and viceversa.

(c) Effect of Solvent. The adsorption isotherms ofAerosol OT on alumina from three different solvents,cyclohexane, chloroform, and methanol, are shown inFigure 4. As the polarity of the solvent is increased fromcyclohexane to methanol the adsorption becomes weaker(initial slope of the adsorption isotherm) and lesser ,""

(adsorption density). Figure 5 depicts the desorption of

sResidual Concentration x 10 . mol/l

Figure 4. Effect of solvent on adsorption of Aerosol <Yr onalumina.

Aerosol OT, preadsorbed on alumina from cyclohexane,into solvents of different polarity. It can be seen that asthe solvent polarity is increased the amount of surfactantdesorbed rises sharply above a certain critical value ofthe dielectric constant of the solvent. As the solventpolarity increases the surfactant interacts more with thesolvent with a concomitant reduction in the surfactant-solid interaction. This increase in solvent-surfactantinteraction with increasing polarity is also evident fromthe data for aggregation of AOT in different solvents. 13 Insolvents of low polarities, aggregation was observed and

(13) Peri, J. B. J. Colloid Interface Sci. 1969,29, 6.

Langmuir, Vol. 10, No.8, 1994 2789Surfactant-Adsorbent Acidity and Solvent Polarity

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Dielectric ConstantFigure 6. Desorption VB solvent dielectric constant for differentminerals showing the effect of solid surface acidity on desorp-tion: (0) alumina, (8), silica, (0) hematite, (8) rutile.The relative hydrophobicities of these minerals will be inincreasing order, alumina < hematite < rutile < silica(based on the surface densityofOHgroups). In the regimeof regular micelles, the more the hydrophobicity of themineral the greater is the tendency of the surfactant topartition to the interface, and hence the desorption curvemoves down.

the extent of aggregation decreased as the polaritybfthesolvent was increased due to increased interaction of thesurfactant with the solvent. In addition interaction ofthe solvent with the oxide surface can also be expected toincrease with solvent polarity, and this in turn would leadto a decrease in the oxide-surfactant interaction.

Most interestingly, beyond a certain solvent polaritythe affinity of the surfactant for the oxide surface startsto increase again as indicated by the maximum in thedesorption curve. This occurs in solvents where AerosolOT is known to exist as regular micelles in contrast to thereverse micelles observed in solvents of low dielectricconstant. At higher solvent polarities the hydrocarbonchain of the surfactant is less compatible with the solventand tends to form aggregate structures to remove thehydrocarbon parts from the bulk solvent. This can beaccomplished via micelle formation in solution or adsorp-tion at a relatively less polar interface. Evidently thelatter mechanism is favored in this case.

Studies with different oxides (Figure 6) showed similarresults, with the curves shifting to the right in the lowdielectric constant regime and moving lower in the highdielectric constant regime as the acidity of the oxideincreased. The acidity of the oxides1' increases in theorder alumina < hematite < rutile < silica, and theobserved shifts also follow the same trend. This bringsout the effect of the nature of the solid on the solventinfluence on adsorption. This can be better understoodifwe consider the solvent dielectric constant at which thedesorption rises steeply to be equivalent to the dielectricconstant of the oxide surface. Thus, the apparent dielectricconstant of the oxide surfaces increases in the order silica< rutile < hematite < alumina. It is proposed that thesurfactant partitions favorably to the higher dielectricconstant environment in the solvents where it formsreverse micelles. Thus, in solvents of dielectric constantless than the apparent dielectric constant of the solid, thesurfactant partitions favorably to the interface, and withincreasing dielectric constant it partitions more into thebulk.

A similar explanation based on the relative hydropho-bicities of the surfaces is used for the high dielectricconstant regular micellar regime (beyond the maximum).

Conclusions(1) It is shown that the adsorption of surfactants on

oxide minerals in nonpolar media occurs primarily throughinteractions between the hydroxyl groups on the oxidesurface and the polar moiety of the surfactant molecule.

(2) Adsorption can be considered to be controlled by twofactors: The first one is the surfactant partitioningbetween the bulk and the interface, the extent of whichis controlled by the polarity difference between the solventand the adsorbent. The second one is the degree ofinteraction via the acid-base mechanism between thesurfactant and the adsorbent, the extent of which dependson the acid-base character of the adsorbent and thesurfactant. In general, it is observed that an acidicsurfactant interacts more with a basic adsorbent and viceversa. However, the desorption of the surfactant indicatesthat these interactions are weak and easily reversed.

(3) The solvent can affect the adsorption process byinfluencing the solute aggregation behavior and bycompeting for adsorption sites on the adsorbent. It wasobserved that the affinity of Aerosol OT for a given oxidesurface initially decreased as the solvent polarity wasincreased and then increased again.

(4) In solvents of low dielectric constant the surfactantpartitions to the more hydrophilic solid surface, while insolvents of high polarity it partitions to the relatively morehydrophobic solid surface. While polar interactions controlthe adsorption in low-polarity solvents, hydrophobicinteractions appear to playa significant role in adsorptionfrom high-polarity solvents onto oxide minerals.

(5) In the solvents of medium polarity the surfactantpartitions favorably into the bulk solvent phase as bothparts of the amphipathic molecule are fairly compatiblewith the solvent phase.

Acknowledgment. Financial support from the Na-tional Science Foundation (Grant CTS-90-11991) is ac-knowledged.

(14) Koksal, E.; Ramachandran, R.; Somasundaran, P.; Maltesh, C.Powder Technol. 1990, 62, 253.