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STUDY OF ADSORBED SURFACTANT LAYERS AT THE 80LXD/LIQUXDINTERFACE USING ELECTRON SPIN RBSOHAMCE SPECTROSCOPY

C.A. Malbrel and P. Somasundaran

Lanqmuir center for colloids and Interfaces

Columbia University, New York, NY 10027, USA

ABSTRACT: Conformation of adsorbed molecules at thesolid-liquid interface controls the efficiency of manyinterfacial processes (pigment dispersion, flocculation,lubrication). Electron Spin Resonance spectroscopy isshown here to be a versatile technique forcharacterizing in sit~ surfactant layers adsor~ed onoxide minerals (alumina) in both aqueous and non aqueousmedia. Short-term kinetics of surfactant adsorptionthat could not be followed by traditional methods canalso.be obtained using this technique.

The efficiency of industrial processes such asflotation, enhanced oil recovery, waste water treatment,and lubrication is determined to a large extent by themodification of surface properties of the solidparticles involved by adsorption of surfactant orpolymers(1). Until recently, the characterization ofthese adsorbed phases was limited to the measurement ofmacroscopic properties like adsorption density, zetapotential, heat of adsorption or wettability(~). Suchstudies were helpful for developing an insight into themechanismS controlling the adsorption but they could notprovide direct information on the configuration of themolecules at the interface although this information iscritical in many interfacial phenomena (dispersion,flocculation, wetting). with recent applications ofsp@ctroscopic techniques to the study of interfaces, it

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Electron spin Resonance spectroscopy (ESR) is one ofsuch techniques. Briefly, this technique is based onthe property that a free electron placed in a magneticfield shows a typical resonance enerqy absorptionspectrum sensitive to the electron environment. Freeelectrons are. provided by paramagnetic species such astransition metal ions or free radicals (~). ESR can beused to obtain information on the Microenvironment ofthese species. However, this technique would be oflimited application without the development of stablefree radicals that can be used as probes. Developedprimarily for microenvironmental studies of biologicalmembranes (!) and membrane-mimetic systems such asmicelles (~), the spin probing technique can be usedalso to study adsorbed layers at the solid/liquidinterface (~). The work presented here is based on dataobtained using this technique for studying organizedsurfactant assemblies adsorbed on alumina in bothaqueous and non-aqueous media.

APPLICATION OFESR SPECTROSCOPY TO INTERFACIAL STJ10IES

(a)Most ESR studies providing information on theorganization of molecular assemblies in solution areperformed using stable free radicals. The expe.rimentsreported here have been carried out using n-doxylstearic acid molecules which are stearic acid moleculeswith stable nitroxide radicals tagged at differentlocations alonq the alkyl chain as shown in fig. 1.These probe molecules were selected because structuralsimilarities with the surfactants used (sodium dodecylsulfate (SDS) and Aerosol OT (ACT» allow them to beintegrated to the surfactant aggregates in solution aswell as in the adsorbed state.

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Several pieces of information may be obt'ained. from theESR response of this type of probe (fig. 2):- polarity of the probe environment: this informationcan be obtained by measuring the hyperfine splittingconstant, aN' of the probe from the separation (in gaussunits) between the low field and central lines of thethree line ESR spectrum. This information is importantsince it can reveal the location of the probe, i.e.whether it is situated in aqueous or hydrocarbon media.- viscosity/structural ordering of the probeenvironment: hindrance in the rotational motion of theprobe induces line broadening of the spectrum. Thisproperty can be used to assess the fluidity of the probeenvironment. An isotropic spectrum, characterized by asharp three line spectrum is obtained when the nitroxideis tumbling freely in a fluid of low viscosity.Immobilization of the probe leads to line broadening andto a spectrum that has lost .ost of its details. If themolecule rotates slowly, an intermediate spectrum isobtained.- state of aggregation of the probe molecules: when twoprobe molecules interact, the spectrum obtained ischaracterized by a phenomenon called spin-spinrelaxation. This feature can be used to control thestate of agqregation of molecules at the interface.

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By usinq the homoloqous series of n-doxyl stearic acids(in which the position of the ESR sensitive nitroxideradical is varied along the alkyl chain of the stearicacid molecule), it is possible to obtain a profileinformation, an extremely powerful tool to studyasymmetric entities such as adsorbed layers.Finally, by monitoring changes in the ESR lineshape as afunction of time, it is possible to follow in-sitQkinetics of changes in the probe environment.

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AJNDAMENTALS OF ADSORPTION532

HEMIMICELLES STRUCTURE AT THE ALUMINA/WATER INTERFACE

For this study! samples were prepared by addinq a knownvolume of 10- mole/l 50S (Fluka Chemicals) solutioncontaininq 10-5 mole/l probe (Aldrich) to 0.5q 2Ofalumina (Linde type A, Union carbide Corp., 15m /9surface area). Sufficient HaCl solution was added tobrinq the salt concentration to 0.1 mole/l (totalvolume: 15 ml). HCl was used to bring the solution pHto 6.5. The suspension was then stirred for 12 hours.The ESR spectra were recorded on a IBM Bruker 1000 X-band ESR spectrometer. 50S concentration was analyzedusinq a two phase titration technique where thesurfactant is titrated aqainst hexadecyltrimethylammonium bromide in chloroform with dimidium bromidedisulfiDe blue as end-point indicator.

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Fiqure 3 shows the adsorption isotherms obtained for sosa1one (dashed line) and for SDS in the presence of theprobes (solid line). It can be seen that the shape ofthe isotherms obtained is unchanged. However, signifi-

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cant deviations are observed at low SDSconcentrationswhere the increase in surfactant adsorption isattributed to the synergistic adsorption of both probeand SDS molecules (~). The ESR spectra displayed infig. 3 have been obtained using 16 doxyl stearic acid asa probe. They are distinctly different from the onesobtained with the same probe in 50s micellar solutions.Furthermore, they change ~arkedly as the surfactantadsorption density increases. At low 50s concentration,the spectrum consists only of a broad sinqle peakcharacteristic of spin-spin relaxation occurring whentwo nitroxides interact with each other (spectrum A).This phenomenon is also observed in the absence of co-adsorbed surfactant, suggestinq that the probe moleculesaggregate spontaneously on the surface. As the SDSconcentration is raised to adsorption densities at whichprobe/probe interactions decrease, a sharper,anisotropic spectrum is obtained (spectrum C). Acomparison between spectrum Band C shows that thischange in the ESR lineshape occurs over a relativelynarrow ranqe of surfactant concentration at whichsurfactant aggregates, known as hemimicelles, startforming on the surface (~). The anisotropy of thespectra obtained under these conditions can beattributed to the high local viscosity. When thespectra are compared with the spectra obtained with 16doxyl stearic acid in ethanol/91ycerol mixtures ofvarying viscosity, it is found that the probe completelyimmersed into the SDS hemimicelles gives a spectrumsimilar to the one obtained in a 75-80' 9lycerolsolution (120-165 cP). This value is in fair agreementwith the value obtained independently using dinaphthylpropane excimer fluorescence measurements (90-120 cP)(2) .

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In order to determine whether the microviscosity andhence the flexibility of the surfactant alkyl chainsvary within the adsorbed layer, similar experiments wereconducted using different probes. Figure 4 shows thespectra obtained for the three probes integrated to the80S hemimicelles. It can be seen that the nitroxidesshow a marked decrease in mobility when compared withthe corresponding experiment in micellar solution.However, it was not possible to simulate the spectraobtained with the different probes using any uniqueethanol/methanol mixture. This suggests that the probesdetect a variation in the surfactant aggregatesstructure. Figure 4 shows that spectra obtained with16, 12, and 5 doxy I stearic acid in ethanol/glycerolmixtures of 75, 80, and 91% glycerol respectivelysimulate well the spectra obtained in the SDShemimicelles. Assuming that the carboxylic qroup of the

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probe is bound to the surface, this result indicatesthat the probe has a greater ability to rotate atdistances farther away from the surface, suggesting arelative ordering of the SDS alkyl chains within thehemimicelles. Near the alumina surface, packing of thesurfactant alkyl chains is relatively dense so thatrotational mobility is severely restricted. Furtheraway from the bound polar group of the surfactant, thealkyl chains can spread out leadinq to an increasedflexibility.

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ESR INVESTIGATION OF ADSORPTION IN NON POLAR MEDI~

Nitroxides have been widely used in non-polar media tostudy the structure of reverse micelles (~) and W/Omicroemulsions (~). However, little is known on theirbehavior at the solid/non-polar liquid interface. Wereport here results obtained at the alumiria/cyclohexaneinterface with n-doxyl stearic acids (n=5, 12, and 16)in the presence of a surfactant often used in non polarmedia studi~s, AerosolOT (AOT).

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For these experiments, an AOT (Fisher) solution of knownconcentration was pr,pared in a cyclohexane solution ofthe probe (2.5 10- mole/I). 3q of alumina wereconditioned in 20 cc of surfactant solution for 24hours. The slurry obtained is then circulatedcontinuously through the cavity of the ESR spectrometerto avoid settling of the suspension. Again in thiscase, surfactant concentration was measured by a twophase titration.

Figure 5 shows the adsorption isotherm of ACT on aluminaas well as the corresponding ESR spectra obtained using12 doxyl stearic acid as probe. It can be seen from theshape of the adsorption isotherm that AOT shows a highaffinity for the surface since complete adsorption isalmost reached before any residual surfactantconcentration could be detected in the supernatant.Similarly, ESR spectra of the supernatant showed thatthe probe had also a very high affinity for the surfacesince no residual probe could be detected in thesupernatant at the probe concentration used for thisstudy.

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~igyre ~: Adsorption isotherm of AOTalumina/cyclohexane interface in the presencedoxyl stearic acid and some representative ESRof the probe.

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fUNDAMENTALS OF AIJ~UK}# 1 JUN816 926 8'7E5 P.leS36

The changes in the lineshape observed as the surfactantadsorption density is increased are consistent with theproposed changes in the probe conformation at theinterface (.12):.(1) In the absence of surfactant, the extremelyanisotropic spectrum obtained is characteristic of theprobe in a "frozen" state in which the rotational motionof the nitroxide radical is very low. This low mobilityis probably due to the proximity of the surfacehinderinq the probe motion, a phenomenon which can beexplained by the probe adsorbing with a flatconfiguration on the surface since both carboxylic qroupand the polar nitroxide radical have a high affinity forthe polar alumine surface. It is interesting at thispoint to compare the adsorption behavior of the probe inaqueous solution and in cyclohexane. In water, theprobe bas been found to aggregate on the surface, asindicated by the evidence of nitroxide interactionsthrough spin-spin exchanqe. In cyclohexane, thespectrum obtained in the absence of co-adsorbedsurfactant does not indicate any spin-spin exchanqesuggesting that the probe molecules do not interact witheach other at the interface.(2) The maximum adsorption density obtained at theplateau of the isotherm corresponds to a completecoyerage of the surface by a monolayer of the surfactant(~). Under, these conditions, interactions between theadsorbed probe and the surfactant lead to a very mobilespectrum consistent with a model where the probe is in astretched configuration, pushed up on the surface by thesurfactant monolayer.(3) At intermediate surfactant adsorption densities,the complex spectra obtained can be interpreted ascombinations of the two previously described spectra: afraction of the probe population adsorbed on the surfacelies on the surface in a flat configuration while theother fraction is adsorbed in a stretched configuration,pushed up on the surface by the surfactant mo'leculesadsorbing around the probe. As the surfactantadsorption density is increased, the contribution of themobile fraction would increase. The hypothesis that twopopulations of probe molecules are responsible for thecomplex spectra obtained was confirmed by theobservation that any experimental spectrum could besimulated using a linear combination of the two extremeESR lineshapes.

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§HORT-TERK KINETICS OF SURFACTANT ADSORPrIONl..istccit

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S37C. Ma1b~l and P. Somasundaran

adequately, despite its practical significance. Thissituation is largely due to experimental difficulties inmonitorinq a very fast process such as adsorption ofsurfactant or polymer in a complex medium of solidparticles suspended in a liquid phase (~). Classicalexperimental techniques for kinetics studies based onthe flow of solutions through a packed bed of adsorbent(~) monitor changes in the liquid phase composition.These techniques have slow response times. thatinherently prohibit their use in the study of the veryfirst staqes of surfactant adsorption. ESR has twoimportant advantages over the other techniques thatmakes it a good candidate for short-term kineticsstudies: (1) the technique monitors changes in-situ,directly at the interface where the surfactantadsorption takes place: (2) the instrument response timeis very fast (order of a few milliseconds). In thispaper, we report results obtained by monitoring changesin the 12-doxyl stearic acid conformation as ACT adsorbsat the alumina/cyclohexane interface. For the firsttime, changes occurinq within the first few seconds ofcontact between the surfactant and the mineral surfaceare reported.

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FUNDAMENTALS OF ADSORPnON816 926 8'~ "'.l~

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Fiqure 5 shows the ESR spectra obtained at differentsurfactant adsorption densities. Among other changes,it can be seen that, as the surfactant adsorptiondensity increases, a third peak appears andproqressively increases in intensity. Figure 6 givesthe intensity of the third peak, as obtained from thespectra shown in fig. 5, as a function of surfacec~verage (defined as r/rma¥). By setting the .maqne~icf~eld of the ESR spectrometer at the ~ntens~tycorresponding to this peak, it is possible to followsurfactant adsorption as a function of time after theaddition of the surfactant to the suspension (Fiqure 7).

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When the signal intensity is calibrated using the curvein fig. 6, it can be seen that this increase in signalintensity corresponds to 40% of the total adsorption anddoes not seem to depend on the surfactant concentration(see fig. 8). The second part of the adsorption processis, on the other hand, much slower (1 to 3 hours) andstrongly depends on the surfactant concentration in the

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polar media. The results presented hereversatility of this technique in studyingaspects of surfactant adsorption.

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The use of this technique yields valuable information onthe organization and structure of surfactant assembliesat the solid/liquid interface:(1) In aqueous media, surfactant molecules aggregate onsolid surface. The hemimicelles formed show a qradualincrease in the flexibility of the alkyl groups of thesurfactant molecules as a function of distance from thesurface.(2) At the alumina/cyclohexane interface, the probemolecules are found to adsorb away from each other,suqgesting that contrary to adsorption in aqueoussolutions, no strong interactions take place in non-polar media between adsorbed surfactant molecules. WhenAOT is added to the system, the increase in the probemobility, as suggested by its ESR spectrum, isconsistent with a change in the conformation of theprobe. Without co-adsorbed surfactant, the probeadsorbs flat on the surface. As surfactant is added,interactions between probe and surfactant molecules takeplace, forcing the probe to stand up on the surface.

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The change in probe conformation occurinq duringsurfactant adsorption has been used to follow thekinetics of surfactant adsorption. During the first 5seconds of contact between the mineral and thesurfactant solution, 40~ of the adsorption takes place,independant of the initial surfactant concentration(provided that enough surfactant is added to thesuspension). The second stage of the adsorption is muchslower (of the order of one to three hours) and dependsupon the surfactant concentration in solution.

ACKNOWLE DGMENTS

We wish to thank professor N.J. TUrro for his advice.Financial support of the National science Foundation(MSM-86-17193 and CBT-86-15524) and of the New YorkMininq and Mineral Resources Research Institute isacknowledged.

LITERATURE CITED

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816 926 8?E5 P.IS)11c. Malbrel and P. SoRlaSUDdar3n

Paris, 1985, p. 411. (c) J.M. Cases, P. Levitz, J.E.poirier, and H. Van Damme in ~d~ances in_MineralProcessina, P. Somasundaran ed.. AIME, New York, p. 171.3 - (a) P. Chandar and P. Somasundaran, Lanqmuir, 1987,3, p. 298. (b) X.F. Tjipangandjara, Y.B. Huang, P.Somasundaran, and N.J. Turro, Colloids & Surfaces, in

press.4 - (a) L.J. Berliner, SEin Labelling I: Theorv angAg~licatiQns, Academic Press, New York, 1979. (b) P.F.Delvaux, ~iQIQgical Maanetic Resonanc~, L.J. Berlinerand J.H. Reuben eds., Plenum Press. New York, 1983,p.183.5 - H. Yoshioka, J. Am. Chem. Soc., 101, p. 28 (1979).6 - (a) X.C. Waterman, N.J. Turro, P. Chandar, and P.Somasundaran, J. Phys. Chem., 90, p. 6828 (1986). (b)P. Chandar, P. Somasundaran, X.C. Waterman, and N.J.Turro, J. Phys. Chem., 91, p. 150 (1987).7 - P. Chandar, P. Somasundaran, and N.J. Turro. J.ColI. Interf. Sci., 117, p.31 (1987).8 - H. Yoshioka and S. Kazama, J. ColI. Interf. Sci.,95, p. 240 (1983).9 - A. Barelli and H.F. Eicke, Lanqmuir, 2, p. 780

(1986).10 - C.A. Malbrel, P. Somasundaran, and N.J. Turro,Lanqmuir, 5, p. 490 (1989).11 - C.A. Malbrel and P. Somasundaran, J. ColI. Interf.sci., in press.12 - P. Somasundaran and A. Sivakumar, Colloids &Surfaces, 30, p. 401 (1988).13 - D.T. Grow and J.A. Shaeiwitz, J. ColI. Interf.sci., 86. p. 239 (1982).

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