14JOURNAL OF COLLOID AND INTERFAa SClEP«:"E 162. 425-430 (1994)

Aggregation Behavior of Aerosol OT in Nonaqueous Solvents and ItsDesorption-An ESR StudyS. KRISHNAKUMAR AND P. SoMASUNDARANI

Lan.~llir Cmlt'r.IiJr Cni/oid" and IIIlnjacN, Henry. Arumb Sdlt_l/" Ccl/llmbia l. "nir('r.fil,l'. Nt""" York. Nt"", Yorl.: 10017

Received April IS. 1993: 8C(.-epted September 3. 1993

ESR technique is used in this study to monitor the aggregationbehavior of an anionic surfactant Aerosol OT in nonaqueoussolvents by measuring the rotational correlation time, T, of anitroxide labeled fatty acid probe (5-doxyl stearic acid) as afunction of the surfactant concentration. These studies revealthat ,,-hile Aerosol OT can form aggregates in low polarity (re-verse micelles) and high polarity (regular micelles) solvents,they do not aggregate significantly in solvents of intermediatepolarity. Also. \\ater was not found to have any significant effecton the aggregation behavior of this surfactant in nonpolar sol-vents. A good correlation is obtained between the surfactantaggregation tendency and its desorption from alumina into dif-ferent solvents. '1994 A.-mic Pras.11K.

INTRODUCTION

Micelle formation by sodium bis-2-ethylhexylsulfosuc-cinate (Aerosol OT) has been studied widely by several in-vestigators ( 1-4 ). Micelles of amphipathic (having both polarand nonpolar groups) surfactant molecules in nonpolar liq-uids have the inverse structure of micelles in aqueous solu-tions. with the polar heads packed together to form a centralcore sun'Ounded by the hydrocarbon tails ( 5 ) which are oftenreferred to as re\.erse micelles. The presence of trace amountsof \\'3ter is considered to be a prerequisite for the formationof reverse micelles ( 6 ). However, Y u ef al. in a recent paperhave given evidence of water as an antimicellization agentin their study of the aggregation of sodium bis(2-ethylhexyl)phosphate in II-heptane (7).

The main dri\ing force for the formation of reverse mi-celles is the polar interactions among the surfactant moleculesand can be classified by the tendency of the molecules' in-organic ions to donate or accept electrons or to interact bypolarization (8 ). The stability of the reverse micelles is alsodependent on the nature of the solvent. It was found thatamong nonpolar solvents the more apolar the solvent is, themore pronounced is the aggregation tendency of the surfac-tants. Solvents \\ith hydrogen bonding abilities like dioxane

and ethyl acetate have a disaggregation effect on these am-phipathic surfactants (9). The size and shape of these micelleshave also been well doc.umented (10, II). A number of de-tailed analyses have been carried out regarding the size andshape of these aggregates in different solvents and a goodcorrelation has been obtained between the micelle size andthe solubility parameter of the solvent which suggests thatthe latter may be the appropriate parameter for characterizingthe solvent rather than its dielectric constant (12, 13).

Several techniques have been used to study the aggregationbehavior ofsurfactants in nonaqueous media. Vapor pressureosmometry (14). light scattering ( 15), dye solubilization(16), and, more recently, positron annihilation technique( 17) have been used to determine CMC of surfactants innonaqueous media. Spectroscopic techniques like fluores-cence spectroscopy have been previously used by several in-vestigators to determine the CMC, aggregate size, and ag-gregation number of ionic and nonionic surfactants inaqueous and nonaqueous media (18-20). Electron spin res-onance has been used to chardcterize the micropolarity andmicroviscosity of adsorbed surfactant layers (21 ) and micellarsolutions of sulfate surfactants (22, 23). Nitroxide-labeledspin probes have also been used in the past to study theexchange kinetics and solubilization sites in water-in-oil mi-croemulsions (24). By using a suitable probe that partitionsinto the surfactant aggregates it is possible to study the dy-namics of the micellization and the behavior of micellar ag-gregates in solution. The rotational correlation time mea-sured from the ESR spectrum is reflective of the probe mo-bility and can be used to study the formation of aggregatesin solution. The change in probe mobility can often alsoyield useful information on the structure of such aggregates(25). The value of the hypemne splitting constant is depen-dent on the polarity of the probe environment (26) and canbe used to extract valuable information on the probe envi-ronment. The use of spin probing by ESR has the obviousadvantage that in situ information on the different portionsof the micellar interior can be obtained by choosing appro-priate probes.

In this work. the behavior of 5-doxyl stearic acid, a com-monly used ESR probe, was investigated in different organic~To whom connpondence should be addressed

425 0021-9797/94 $6.00Copyrilht e 1994 by Academic ~ Inc.

All rights of ~1M:tioD in any £arm ~.

426 KRISHNAKUMAR AND SOMASUNDARAN

~$.Oliquids. The ESR parameters analyzed in this work are AN,the hyperfine splitting constant, and TD, the rotational cor-relation time. The aggregation behavior of Aerosol OT indifferent nonaqueous solvents and the effect of water on theaggregation df Aerosol OT in cyclohexane is elucidated. Also,a correlation of the desorption of Aerosol OT from aluminasurface into different solvents with the solubility parametersof the solvents is presented.

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EXPERIMENTAL JI.\1 aleria/so The ESR probe, 5-doxyl stearic acid was pur-

chased from Aldrich Chemicals and used as received. Thesurfactant. Aerosol OT was purchased from Fisher Scientificand purified following a procedure detailed in literature (27).All the organic solvents were bought from Fisher Scientificand dried bv storing in molecular sieves when necessary,The alumin~ used in the desorption studies was bought fromUnion Carbide Corporation as Linde A. X-ray diffractionand chemical analysis of the powder have shown it to be awell-cryStallized corundum of high purity (>99% AI2O3)'Morphologically the powder consists of micron-sized aggre-gates composed of smaller particles of 300 nm and the mea-sured nitrogen adsorption BET surface area is 14 m2/g.

.\felhods. The ESR experiments were performed on aMicronow MN8300 X-band spectrometer. All the solutionsamples contained 2 X 10-4 M /Iiter of the probe and therequisite amount of surfactant and were deoxygenated bynitrogen bubbling prior to acquiring the spectrum. The watercontent of the solutions was measured by the Karl Fisherwater titrimeter. The hyperfine splitting conStant AN androtational correlation time T8, TC were calculated from theESR spectrum using procedures detailed in literature (26).

0 CyclohexaneV' Water0 Methanol ~

13.510-7 10-6 '..10-5 10-. 10-3 10-2 10-1

ConcentratiOn AOT. mol/liter

FIG. 2. Diagram illustrating the effect of AOT concentration of .oft; of5-doxyl stearic acid in different solvents.

For the desorption experiments, the Aerosol OT was firstadsorbed onto alumina from cyclohexane solutions of thesurfactant. The solids were then separated by centrifugationand dried thoroughly. The alumina with the preadsorbedsurfactant was then conditioned with the desired solvent.The Aerosol OT was analyzed by a two-phase titration tech-nique where the surfactant is titrated against hexadecyltri-methylammonium bromide with dimidium bromide disul-fine blue as the end-point indicator (28). All experimentswere performed at room temperature (22 :t 2aC).

RESULTS AND DISCUSSION

.4nal)'sis 0.( cO/lpling cons/an/so Figure I shows the hy-perfine splitting constant AN for 5-doxyl stearic acid in dif-ferent solvents as a function of the solvent dielectric constant.The observed variation of AN with solvent polarity is in ac-cordance with previously reported data. with AN varying fromabout 13.7 gauss in cyclohexane (nonpolar) to about 15.8gauss in water. Several relationships have been proposed torelate AN to the dielectric constant of the medium (29. 30).Figure 2 shows the variation of AN as a function of AerosolOT concentration in three different solvents-cyclohexane,methanol. and water. While in the case of cyclohexane agradual increase in AN is observed with AOT concentration,in the case of water there is a sharp decrease in the splittingconstant at a certain AOT concentration (8 X 10-4 M/liter),and with methanol no significant change in AN is obsef\'edin the range of surfactant concentration studied, Both thechange and the rate of change of AN give valuable informationabout the mechanisms bringing about the change. For cy-clohexane the increase in AN corresponds to a transfer of theprobe from the nonpolar solvent to the polar environment

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427AGGREGATION OF AEROSOL OT BY FSR

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order of magnitude increase in correlation times due to theformation of surfactant aggregates. The probe is incorporatedinto the micelle (reverse micelle) by polar (hydrophobicchain) interactions and this restricts its motion, causing

of the reverse micelles. The gradual change is proposed tobe due to premicellar probe-surfactant interactions fromwhich one can infer the existence of premicellar surfactantaggregates in solution. This is in good agreement with thewidely accepted model of reverse micellization (31). Thesharp decrease of AN in water corresponds to the CMC (-8X 10-4 M /liter) and is due to the transfer of the probe fromaqueous environment to the hydrocarbon environment ofthe micelle. However, no change in AN is observed in meth-anol. suggesting the absence of any aggregation or any probe-surfactant interaction.

.4nalysis Qf correlation limes. Figure 3 shows spectra of5-dox:-i stearic acid in premicellar and micellar solutions ofAerosol OT in cyclohexane and water. In both cases linebroadening is observed as a result of micellization. The vari-ation of T8 in cyclohexane, water, methanol, chloroform,and ".ater-methanol mixture as a function of surfactantconcentration is shown in Fig. 4. A sharp increase in therotational correlation time was observed in cyclohexane andwater at a concentration corresponding to the formation ofreverse micelles and micelles. respectively. In the case ofmethanol no significant change in T is observed, corrobo-rating the absence of surfactant aggregation or probe-sur-factant interaction. T8 and TC values of 5-doxyl stearic acidin premicellar and micellar regions forcyclohexane and waterare given in Table I. It can be seen that there is almost an

428 KRISHNAKUMAR AND SOMASUNDARAN

TABLE IRotational Correlation Times of 5-Doxyl Stearic Acidin Micellar and Premicellar Solutions of Aerosol OT

PmnicellarAOT (os)

TC

MicellarAOT (ns)

Solvent T. - T. "'C;".~ '.

Cyclohexane 0.097 0-065 0.935 -J...Water 0.119 M 0.938 l.12

cases spectral splitting may be observed due to probe ex-change between different microenvironments (32). Theprobe motion also becomes more anisotropic with increasein water content as indicated by the increasing value of TC- TB (Fig. 5b). This may be due to the fact that in the swollenmicelle the probe may now have a motion independent ofthe micellar rotation. like lateral diffusion within the aggre-gate.

The desorption of preadsorbed Aerosol OT from aluminainto different solvents is shown in Fig. 6a. It can be seen thatas the solvent polarity (as indicated by the dielectric constant)is increased the desorption increases, goes through a maxi-mum, and then decreases. Note that when the data of Fig.6a are plotted against the solubility parameter of the solventinstead of the dielectric constant, the correlation is better(Fig. 6b). Also, the de~rption data obtained using solventsof unusually high dielectric constants like formamide (t =109) and A "-methyl formam ide (t = 182) fit well into thecurve shown in Fig. 6b when plotted against their solubilityparameter. The forces causing adsorption from organic sol-vents are the same as those which induce micellization-namely dipole interactions between the polar moieties in thesystem. Little and Singleterr)'. in their study on the aggre-gation of dinaphthylsulfonate in organic solvents. postulatedthat as the solubility parameters of the solvent and surfactantapproach each other the surfactant aggregate becomes smallerand in the ideal limit tends to exist as monomers interacting,",'ith the solvent molecules when the solubility parametersmatch exactly ( 12). As the solubility parameter differenceincreases the surfactant forms aggregates the size and shapeof which are such that the effective solubility parameter ofthe aggregate as seen by the solvent is similar to its own

broadening of the ESR spectral lines. The premicellar probe-surfactant interactions in cyclohexane may not be significantenough to produce a measurable change in TB. Also shownin Fig. 4 are the rotation correlation times in chloroformand water-meth.anol mixture. In both cases there is no sig-nificant aggregation at low concentrations. although at higherconcentrations the increase in correlation times suggestssome micelle-aggregate formation.

TB for S-doxyl stearic acid at different surfactant concen-trations in cyclohexane is shown in Fig. Sa as a function ofthe water content of the solutions. At low surfactant con-centrations (premicellar solutions) \..'ater has no effect onthe correlation times. indicating that water does not inducean)' micellization in these systems. However, in micellar so-lutions there is an increase in TB with water concentration.At low water concentration the probe is tightly bound to themicelle and rotates isotropically with the micelle. The sol-ubilization of water in the micelles makes the micelle bulkierand less mobile in general. The probe location in the micellealso changes with the concentration of water and in some

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AGGREGATION OF AEROSOL OT BY ESR 429

value. This takes place by the formation of reverse micellesor regular micelles, depending on the nature of the solvent.In the present case in the low solubility parameter (also lowdielectric constant) region the polar portion of the surfactanthas a tendency to remove itself from the nonpolar solvent.This can be achieved by the formation of reverse micellesor by adsorption onto a polar surface through dipole inter-actions. As the solubilitY parameter is increased the dipoleinteractions between the solvent and surfactant increase,thereby decreasing surfactant-surfactant interaction as well.This is evident by the lack of surfactant aggregation in thesesolvents (ethanol and methanol) as seen from the ESR re-sults. As the solubility parameter is further increased thesolvent-surfactant interaction also decreases similarly, andthe solid-surfactant interaction increases, leading to an in-crease in adsorption again. Based on this argument a solu-bility parameter of about 12-14 can be considered to beappropriate for Aerosol OT. Thus it can be concluded thatin the lower solubility parameter region AOT adsorption onalumina is through dipole interactions with the surface, whilein the higher solubility parameter region the surfactant in-teracts with the solid through its hydrocarbon chains.

2. Electron spin resonance studies reveal that Aerosol OTforms aggregates in solvents with which it has a significantsolubilit), parameter difference.

3. In solvents with solubility parameter similar to that ofthe surfactant it exists possibly as monomers directly inter-acting with the solvent molecules.

4. Solubility parameter is more indicative of the surfac-tant-solvent interaction tendency rather than the solvent di-electric constant.

5. Adsorption in non-polar solvents is controlled by thesame forces responsible for aggregation, namely the dipoleinteractions between the polar moieties in the system.

6. The solubility parameter difference (polarity difference)between the surfactant and the solvent determines the extentof adsorption. The more the difference the more is the ad-sorption.

7. As in aqueous systems. in polar organic solvents hy-drocarbon chain interactions playa significant role in de-termining the adsorption.

ACKNOWLEDGMENT

The authors acknowledge the financial suppon of this work by NationalScience Foundation (CTS-90-II991 ).CONCLUSIONS

I. Measurement of hyperfine splitting constant, AN androtational correlation time, TB in ESR spectrum of 5 doxylstearic acid in Aerosol OT solutions in different solventsshows aggregation of surfactant in solvents of low and highpolarity. There is no significant aggregation in solvents ofintermediate polarity.

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430 KRISHNAKUMAR AND SOMASUNDARAN

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