1285Reprinted from LANGMUIR, 1992, 8.Copyright @ 1992 by the American Chemical Society and reprinted by permission of the copyright owner.

Effect of Water on the Dispersion of Colloidal Alumina inCyclohexane Solutions of Aerosol OT

C. A. Malbrel and P. Somasundaran.

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

Received May 8, 1991. In Final Form: February 10, 1992

Colloidal dispersions in nonpolar media are used in a variety of technological applications, and in mostof these applications, water is present and plays a major role in determining the dispersion behavior. Theeffect of water on the stability of colloidal suspensions of alumina in cyclohexane in the presence of asurfactant, Aerosol OT, is discussed here. A succession of flocculated, dispersed, and again flocculatedstates are observed as the amount of water added to the suspension is increased. Adsorption studies showcolloidal stability to be controlled by the activity of water in solution which is best described by the ratioWo - (H~)/(AOT]. Tests using electron spin resonance (&9R) spectroscopy detected an increase inlateral diffusion of the surfactant molecules at the interface as more water was added to the system. Afingering mechanism in which the water phase forms bridges between particles is proposed for the water-induced flocculation in nonpolar media.

solid/liquid interface (for both electrostatic and stericstabilization, the addition of surfactant and/or polymerto the suspension is necessary to impart dispersion stabilityin nonpolar media).

There are several studies reporting the effects of wateron suspensions stabilized by surfactant addition.8-12 De-pending on the nature of the colloid and the amount ofwater present in the dispersion, water can either flocculatea suspension or stabilize it.

Since water does govern colloidal stability by controllingthe interfacial characteristics of the adsorbed surfactantlayer, it becomes necessary to study in detail the adsorptionprocess of both surfactant and water to elucidate the mech-anisms involved in determining the effect of water. Forthe present study, alumina stabilized in cyclohexane byAerosolOT (sodium bis(2-ethylhexyl) sulfosuccinate) wasselected as the colloidal system.

In addition, electron spin resonance spectroscopy wasused to follow in situ changes in the organization of theadsorbed layer. Fundamentally, this technique is basedon the property that a free electron placed in a magneticfield shows a typical resonance energy absorption spectrumsensitive to the electron environment. Developed pri-marily for microenvironmenta1studies of biological mem-branes and membrane-mimetic systems such as micelles,ESR probing has been recently adapted to the study ofsurfactant adsorbed layers at the solid/liquid interfaceusing stable free radicals incorporated into the surfactant-adsorbed layer.13

Introd uction

For many processes, performance is critically depend-ent on the dispersion quality of colloidal suspensions innonpolar media. For instance, the development of high-performance structural ceramics is currently hamperedby the inability to eliminate flaw populations generatedby incomplete and nonhomogeneous dispersions of par-ticles during powder processing prior to firing.l Magnetictape manufacturing involves adhesion of well-dispersedsuspensions of magnetic particles on a polymeric 1tIm.Again the dispersion quality is a major parameter tomonitor since it controls the final storage density of thetape.2 Liquid inks in reprographic technologies,3 paints,.cosmetics, 5 dry cleaning,8 and oil recovery from tar sand7are other industrial applications the development of whichlies in a better understanding and control of the colloidalinteractions in nonpolar media.

In all the applications mentioned above, water is presentin the nonaqueous dispersions. It is introduced as an ad-sorbed phase on the colloid, as water of hydration of thechemicals used or it can also be present as a separate liquidphase. Because of its ubiquitous nature, it is difficult toremove water from any practical system and it is evenmore difficult to avoid contamination by water.

Water has no direct stabilizing or flocculating effect initself: it does not create charges on the particle surface,nor does it form a steric barrier to prevent flocculation.Water affects the suspension stability by changing thecharacteristics of the layer of additives adsorbed at the

(1) Bleier, A. In Ult1'a8truetwe Procu.i", of CeramiC8, GlM8e8, andComPOlite8; Beach, L L, UlriCh, D. R., Ed..; J. Wiley &: Sob8: NewYork, 1984; p 391.

(2) Budaon, G. F.; Racbavan, S.; Roylance, D. A. In Surface and Col-loid Science in Computer TechnoiOlY; Mittal. K. L., Ed.; Plenum Press:~ York, 1987; p 61.

(3) Duff, J. M.; Wong, R. W.; Croucher, M. D. In Surface and ColloidScience in Computer TechrIOlo,)'; Mittal, K. L., Ed.; Plenum Pr-= NewYork, 1987; p 385.

(4) McKay, R. B. In Interfacial Pheraonlerao in Apolar Media; Sur-f8daDt Science s.n. 21; Ricke. B. F.. Parfitt, G. D.. Eds.; MarcelDekker: New York, 1987; p 361.

(5) Fu, C.1n Surf octant in Coanletics; Surfactant Science Seri. Vol1&: Rierer. M. M.. P.d.; Marcel Dekker: New YOI'k, 1985; p ~1.

(6) Wentz, M. In Deterrency: Theory and Techraology; SurfactantScience Seriel2O; Cutler, W. G., KiI8a. E., Eda.; Marcel Dekker: NewYork, 1987; p 459.

(7) Marlow. B. J.; S~ty, G. C.; Bugbee, R. J.; Mahajen, O. P. ColloidsSurf. 1987, 24,~.

Materials and Methods

Materials- The alumina used for the present study waspurchased from Union Carbide Corp. as Linde Alumina PolishingPowder Type A. Morphologically, the powder was constitutedof micrometer-size aggregates composed of smaller particles

(8) McGown, D. N. L.; Parfitt, G. D.; Willis, E. J. Colloid Sri. IH?,20, 650.

(9) Kitahara, A.; Karuawa, S.; Yamada, H. J. Colloid Interface Sci.1967,25,490.

(10) Kitahara, A.; Tamura, T.; Mataumura, S. Chem. Lett. 1979, 1127.(II) KaDdori, K.; Kitahara, A.; KOD-DO, K. J. Colloid Interface Sci.

11M, 99, 455.(12) KaDdori, K.; Kazama, A.; KOn-DO, K.; Kitahara, K. Bull. Chem.

Soc. Jpn. 1t84, 57,1777.(13) (a) Chandar, P.; Somasundaran, P.; Watenoan, K. C.; Turro, N.

J. J. Phy.. Chem. 1187,91,150. (b) Malbrei, C. A.; Somasundaran, P.;Turro, N. J. J. Colloid Interface Sci. 1990,137,600.

MA.~_'7A.~~/Q9/9AnQ_19Q~en') nn/n A 1~ &_-:-~""'__;_1 a ;_.

1286 Langmuir, Vol. 8, No.5, 1992 Malbrel and So1na8undaran

~ 4O""..,c.",."..!~,.0E

.-°30-

M40

30

20

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.-0 g- L . g4( 0 2 4 a 8 10

AOT Residual COncenb'atlon x 103, mole/I

Figure 1. Adsorption isotherm of Aeroeol OT on alumina incyclohexane (solid line) and related suspension stability asestimated from settling rate measurements (dotted line).

Q 2000. a.

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or; 1500-M

~..c 1000

c!

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J.~ 0 100 200 ~ ~

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Figure 2. Effect of surfactant solution concentration on thewater adsorption isotherm: (e) [AOT]oq = 3.85 x 10-3 mol/L;(4) [AOT]eq = 8.5 x 10-3 moVL; (8) [AOT]eq - 26.5 x 1Q-3 mol/L.

(between 200 and 500 nm) with a nitrogen surface area of 14m2/g. The absence of hysteresis on the adsorption/desorptionisotherm 8uggested this material to be essentially nonporou8.Stearic acid adsorption from cyclohexane 80lutions gave a surfacearea of 12.7 m2/g.

Aerosol OT is the moat commonly used anionic surfactant innonpolar media. Ita solution behavior is well documented14 andit bas 8hown significant effect on the 8uspension 8tability in thepresence of water.8-12 It was purchased from Fisher ScientificCo. Before use, the 8urfactant was purified following a proceduredescribed elsewhere.15

7. Doxy18tearic acid was selected as ESR probe because of itsgeneral resemblance with a surfactant molecule and because thenitroxide position on the seventh carbon of the stearic acid alkylchain would locate it inside the surfactant adsorbed layer (mono-layer thickne88 equivalent to a 9- to II-carbon alkyl chain). Itwas purchased from Aldrich Chemical Co., Inc., Milwaukee, WI,and used as received without further purii1C8tion.

Cyclohexane, of 8pectroscopic grade, was obtained from FisherScientific Co. It was selected for this study because of its weakinteractions with oxide 8urfaces, which allows a possible residualadsorption of the solvent during surfactant adsorption to beignored.1. When required, the solvent was 8tored on MolecularSieve 4A to avoid contamination by water. The water added wastriply distilled, of 10-. Ucl conductivity.

Methoda. Sample Preparation. Alumina samples wereprepared by desiccating it at 200 °C for 6 h followed by coolingfor 2 h at 25 °C in a vacuum desiccator. An alumina sample of1 g was added to 15 mL of 8urfactant solution in cyclohexanecontai11ing a known amount of water. This suspension wasconditioned for 12 h prior to the settling experiments.

For the samples used in ESR experiments, 10 cm3 of a cy-clohexane 80lution containing the ESR probe (2.5 X 1(}"" mol/L)was added to the dry powder and the samples were conditionedfor 24 h. The samples were then centrifuged, the supernatantwas removed, and the sediment was freeze-dried for 24 h to removeany reeidualsolution. The probe molecules preadsorbed on themineral by this proCedure were able to record any change in theirenvironment in the interfacial region.

Aaalytieal Techniques. Aerosol OT was analyzed by a two-phase titration technique with hexadecyltrimethylammoniumbromide as the titrant in chloroform and dimidium bromide di-su1fine blue as the end-point indicator}7 Water concentrationswere measured by Karl Fisher titration.

AdlOrptiou at the Solid/Liquid Interface. Surfactant andwater adsorption densities at the alumina! cyclohexane interfacewere determined by measuring the residual concentrations afteradsorption.

Stabnity Experiments. The stability of the suspension wasmeasured by monitoring optically the descent of the upperinterface in a 15 cm3 graduated cylinder of 1 cm diameter. The8uspension settling rate was obtained from the initial slope ofthe plot of the upper interface position versus time.

EBB Experiments. All experiments were performed with anffiM Bruker Modell00D X-band spectrometer equipped witha 9-GHz microwave frequency. The solution or the slurry to beanalyzed was introduced in an ESR quartz capillary tube andplaced in the resonance cavity of the spectrometer. The initialsolution, the sediment, and the supernatant were analyzed foreach experiment to check that the information obtained origi-nated from probes adsorbed at the interface. All experimentswere performed at room temperature.

Results and DiscussionSurfactant Adsorption and Suspension Stability.

The adsorption isotherm of Aerosol OT is plotted in Figure1. It can be seen from an examination of the isothermthat the affinity of the surfactant for the surface is high

since the maximum adsorption is reached almost beforeany residual concentration could be detected in thesupernatant.

By use of the specific surface area estimated by stearicacid adsorption (12.7 m2/g) and a maximum adsorptiondensity of AOTbetween 3.2 and 3.6 X l~mo1/g,aparkingarea ofO.58-{).66 nm2 was obtained for the AOT molecule.This estimate agrees well with the values published in theliterature for AOT molecules adsorbed at the water/xy-lene8 and water/isooctane interfaces18 (0.6 and 0.8 nm2/molecule, respectively). On the basis of this calculation,a model in which AOT adsorption is limited to a mono-layer is the most reasonable.

The suspension stability, as estimated from settling ratemeasurements, is also plotted in Figure 1 as a function ofresidual surfactant concentration. It is clear from thisfigure that the suspension is stabilized only when the sur-factant completely covers the mineral surface.

Water Adsorption from AOT/CyclohexaneSolution. For each water adsorption isotherm, the sur-factant concentration was kept constant within 10%.Throughout this study, the surfactant concentrations usedwere in the range of dilute micellar solutions (between 2and 60 X 10-3 mol/L). Typical isotherms are shown atdifferent surfactant concentrations in Figure 2.

After the initial sharp increase in adsorption correspond-ing to the adsorption of the f1f8t water molecules directlyon the mineral sUrface, the water adsorption density

(18) (a) Maitra, A. N.; Eicke, H. F.J. P#ays.Chem.I981,85, 2687. (b)Maitra, A. N.; Vasta, G.; Eicke, H. F. J. Colloid Interface Sci. IM3,93,383.

(14) Ricke, H. F. In Topics in Current Chemistry; Spinpr-Veria&:Berlin. 1990; Vol. 87, p 85.

(15) MaIbrel, C. A.; Somuundaran, P. J. Colloid Interface Sci. I"',133, 4()4.

.(16) Suda, Y.; Morimoto, T. Limgmuir 1985, 1, 544.(17) Reid, V. W.;i.oncman, G. F.; Heinherth, E. Ten.side 1168,5,00.

f/).==~

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Dispersion of Alumina in Cyclohexane Langmuir, Vol. 8, No.5, 1992 1287

0.0 0.2 0.4 0..H~/C.E.C.

0.8 1.0

0 10 20¥/AOT Mol. R.ao. Wo

Figure 4. Water adsorption data of Figure 2 plotted 88 a functionof normalized water concentration, HtO/CEC and molar ratio Wo= H2O/AOT: (e) [AOT]oq = 3.85 x 10""3 mol/L; (.) [AOT)oq-8.5 x 10-3 mol/L; (8) [AOT]oq = 26.5 x 10""3 mol/L.

~ 40

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0 5 10 15H20/AOT Molar Ratio, Wo

Figure 5. Effect of water on Aerosol OTadsorption. The sur-factant adsorption density and the corresponding per(:ent of solidsleft in suspension after 8 hof settling (to estimate the suspensionstability) are plotted as a function of the molar ratio Wo = HzO/AOT. The adsorption data plotted are the maximum adsorptiondensities measured under given conditions (they would corre-spond to the plateau of the isotherm shown in Figure 1).

1.0 10.0 Im.oAOT Concentration x 103, mole/I

Figure 3. AOT/waur/cyclohexane phase diagram showing thechange in critical emulsion concentration (CEC) as a funtion ofsurfactant concentration. The points reported on the diagramrepresent the concentrations at which turbidity measurementswere performed (e, optically clear; +, turbid).

increased more gradually with the residual water con-centration in solution. As even more water was added tothe solution, another marked ('.baDge in the slope of theadsorption isotherm was observed and the amount of ad-sorbed water rose sharply. This rapid increase suggesteda catastrophic multilayer buildup attributed to a phaseseparation of a water-rich phase initiated by the surface.

Figure 2 shows water adsorption isotherms obtained atdifferent surfactant concentrations. As the surfactantconcentration, and thus the solubility of water in solution,increased, the range of water concentration for adsorptionexpanded. The critical water concentration correspond-ing to surface condensation (i.e. the catastrophic multi-layer buildup) shifted toward higher water concentrations,but overall, the isotherm shape remained unchangedthroughout the range of surfactant concentrations inves-tigated.

The shape of the water adsorption isotherm is similarto the one obtained for water vapor adsorption on thesame alumina. An analogy between the two adsorptionphenomena can be used to understand the water adsorp-tion data obtained in solution.15 The adsorption of a gasis limited by its saturated vapor pressure, Po. Similarly,water adsorption can be interpreted as being controlledby a critical water concentration at which a phase changein solution occurs. In the AOT/cyclohexane solution, aphase change is observed as the water concentration isincreased. The solution goes from a clear stable micellarsolution to a turbid unstable W /0 emulsion. This criticalconcentration can be referred to as the critical emulsionconcentration (CEC) and is shown in Figure 3 as the limitbetween the two domains of solution behavior (the datapresented in this figure were obtained at various surfac-tant concentrations by monitoring turbidity changes ofthe solution as water was gradually added to it). Clas-sically, gas adsorption isotherms are plotted as a functionof relative vapor pressure of the gas, P/Po. A similarnormalization of the water adsorption isotherm is possible.In Figure 4, adsorption data are plotted as a function ofthe normalized water concentration, [H2O]/CEC.

The excellent superimposition of the data obtained atdifferent surfactant concentration provides an additional,quantitative proof for the fact that water adsorption onalumina is controlled by water solubility in the solution.

The successful use of normalized water concentrations[H20]/CEC demonstrates that water adsorption from amicellar surfactant solution in nonpolar media is analogousto the adsorption of a liquid solute of limited solubility .19

However, its utility as a thermodynamic quantity is limited.A physically more m~ parameter is the ratio[H20]/[AOT] (Wo) which is directly related to the activityof water in the surfactant solution.20 Figure 4 shows alsowater adsorption data plotted as a function of WOo

Effect of Water on the S w-factant Adsorption. Thesurfactant adsorptiOil8 reported above were performed atlow water concentration (below 10 X 10-3 mol/L). Whenthe amount of water present in the solution was increased,the adsorption density corresponding to the plateau ofthe surfactant adsorption isotherm varied as shown inFigure 5. Despite the scattering, it is clear that the sur-factant adsorption density does decrease as the relativeamount of water present in solution is increased.

The decrease in surfactant adsorption density as thesurface becomes more hydrated can be interpreted as dueto an increase in the area required for an Aerosol OT

(19) Oecik, J. Adlorption; Ellis Horwood, Ltd.: Chicbeeter, EDIIaDd(J. Wiley &; SoDS diltrib.), 1982.

(20) Malbre). C. A. Effect of Water on the Dilperlion of CoUoida1Alumina in Cyclobeune SolutioDS of AeroeoI QT. D. Eng. Sci. Thelia,Columbia University, 1991.

1288 Langmuir, Vol. 8, No.5, 1992 Malbrel and Som48undaran

f'"~M 2000 I~

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FicU1'8 6. Effect of water on the alumina suspension stabilityat two different surfactant concentrations (dotted line, solidsymbols): (8) [AOT)., = 8.5 X 10-" mo1/L; (A.) [AOTJoq = 26.5X 10-" moVL. Also shown are the two corresponding wateradsorption isotherms (solid line. empty symbols).

molecule to adsorb on the surface. This is supported bythe evidence obtained in water and oil (W 10) microemul-lion studies in which it was observed that the interfacialarea of Aerosol OT molecules increased with Wo- On thebasis of these results. the following structure can beproposed for the interface: the highly hydrophilic aluminasurface interacts primarily with a layer of water on top ofwhich a layer of surfactant is adsorbed; as more wateradsorbs. a thick layer of water develops between thealumina surface and the layer of surfactant molecules, thehydrophilic ionic groups of which interact with the watermolecules.

Effect of Water on the Suspension Stability, InFigure 6. the suspension stability is reported in tenns ofsettling rate as a function of residual water concentrationat two different surfactant concentrations (8.5 and 26 X10-3 mol/L). As the water concentration in the systemwas increased, the suspension ezhibited a succession offlocculated and stable states.

At very low water concentrations, settling OCCUR rapidly.The onset of stabilization corresponds to a sharp increasein the amount of water adsorbed on alumina, suggestingthat the adsorption of water plays a critical role in thestabilization phenomenon. It is generally accepted thatthe stabilization of oxide particles by Aerosol OT is dueto the development of electrostatic repulsive forces be-tween particles when dissociation of adsorbed surfactantsand subsequent desorption of the anions lead to thegeneration of charges at the solid/liquid interface.21 Theresults presented here show that even though a surfactantmonolayer was adsorbed at the interface in all cases.stabilization took place only when trace amounts of waterwere present in the system (this observation is in agreementwith the conclusion of McGown, Parfitt. and Willis8 onthe role played by water in the charge development at thesolid/Aerosol OT adsorbed layer interface). The logicalconsequence of this observation is that water must beplaying a critical role in ion dissociation in apolar media.

At higher water concentrations, Figure 6 shows a sharpincrease in the suspension settling rate at both the sur-factant concentrations studied. However. as the surfac-tant concentration was increased, 80 was the waterconcentration at which the flocculation takes place. Byadding water andlor surfactant to the suspension, it wasalso possible to show that the changes observed werereversible.

When the stability data are superimposed on theadsorption data and plotted as a function of Wo (Figure 7),

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the data obtained at different surfactant concentrationscoincide but it is clear that the flocculation induced bywater (taking place for 5 < Wo < 8) is not related to watercondensation on the mineral surface which occurs onlyaround Wo = 15 (see sharp increase in water adsorptiondescribed in Figure 4 and taking place at Wo = 15)..

Structural Information Obtained by ESR Spec-troscopy. The data presented here were obtained by using7 -doxyl stearic acid in which the ESR-sensitive nitroxideis attached to the seventh carbon of the stearic acid alkylchain..

Figure 8 shows the spectrum obtained with the probeadsorbed at the alumina/cyclohexane interface in theabsence of the surfactant.. The spectrum obtained ischaracteristic of the probe moving in a highly constrictedenvironment.. In the absence of surfactant or any otheradditive, the only thing that can hinder the probe motionis the alumina surface; i.e.. the stearic acid molecule alreadyknown to adsorb by its carboxylic group must also beanchored to the surface by the nitroxide group.. Theresulting flat conformation of the molecule at the oxide/cyclohexane interface is consistent with the results of Ca-denhead and co-workers on the organization of a probemonolayer at the water-air interface.22

Figure 8 shows also the spectrum obtained in thepresence of coadsorbed surfactant.. The values of AIobtained for 7 doxylstearic acid were used to compare the

(22) Cadenhead. D. A.; Kellner. B. M. J.; MulJer-Landau, F. Biochim.Biophy" Acto 1175. 382. 253.(21) Novotny, V. Colloidl Surf. IMl, 2, 373.

~

120 IM-008

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Figure 1. Settling rate (solid symbols) and water adsorptiondata (empty symbols) of Figure 6 88 a function of the relativemolar ratio Wo - HsO/ AOT: (8, 0) (AOT]~ = 8.5 X 1(}-8 mol/L;(A, A) [AOT]oq = 26.5 X 1(}-8 mol/L.

Dilperaion of Alumina in Cyclohexane Langmuir, Vol. 8, No.5, 1992 1289

~ ,.- .0.0.2

..-

A Wo = 1.95~

0- 100 200 ~

AMId- W.. Concentration x 103, mole!,

Figure 10. Effect of water on the order parameter, S, calculatedfrom the ~R spectra of 7 -doxyl stearic acid coadsorbed withAerosol OT at the alumina/ cycloheune interface. Experimentswere performed at two different surfactant concentrati0D8: (e)[AOT]oq = 5 x 10"3 moVL and (..) [AOT]oq - 24.5 x 10"3 moVL.

1000. I

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.

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Figure 11. Data of Figure 10 expressed in terms of the molarratio Wo: [AOT]eq = 5 X lo-a mol/L. The corresponding wateradsorption and settliDl rate data of Figure 7 are also plotted inthe figure for comparison.

Firure 9. Effect of water on the ~R spectra of 7-doxyl stearicacid coadsorbed with a monolayer of Aerosol OT at the aluminalcyclohexane interface. Notice the change in the position of theinner peaks as more water is added to the system.

spectra obtained in the absence and in the presence ofcoadsorbed surfactant. The values estimated (69 and 65G for the probe adsorbed alone and coadsorbed withAerosol OT, respectively) are sigDificantiy different,suggesting that, despite the slow motion of the probe, itis possible to distinguish between the probe interactingdirectly with the mineral surface (when no surfactant iscoadsorbed with the probe) and the probe hindered in itsmotion by surfactant molecules surrounding it.

Figure 9 shows some of the spectra obtained at differentwater concentrations with 7 -doxylstearic acid coadsorbedwith a full surfactant monolayer on the alumina surface.The changes observed in the ESR lineshape correspondto an increase in the probe mobility consistent with adecrease in ordering of the probe environment. Toquantify these changes, the concept of order parameter,developed to interpret ESR spectra obtained in liquidcrystals and biological systems, can be used. In the caseof doxyl stearic acid, the order parameter S is experi-mentally determined from a spectrum using the parameterdescribed in Figure 9. The expression used for S is thefollowing:23

S - 1.66 ~ - (A1(meaa) + C)

AI + 2(A1(meas) + C)

where

(high order). The calculated order parameter S is plottedin Figure 10 as a function of the equilibrium waterconcentration in solution. It can be seen that. as morewater was added to the suspension (and as a result. morewater adsorbed on alumina). the order parameter de-creased until it reached a value of about 0.75.

When the results are plotted as a function of Wo andcompared with stability and adsorption data (Figure 11).it is clear that changes in the probe environment correlatewell with suapension stability: at low Wo (low relative waterconcentration). the order parameter decreases as morewater adsorbs on allJmina. The onset of t1occulationmarked by an increase in settling rate corresponds to thewater concentration beyond which no further change inthe order parameter is detected.

Interpretation of the ESR Data. The mobility ofthe probe adsorbed at the alumiDa/cyclohexane interfacemay be characterized by three types of motion: (1) therotational motion about the principal axis of the stearicacid molecule; (2) the motion of the stearic acid alkyl chainitself; (3) the lateral diffusion of the probe within the ad-sorbed layer.

Computer simulation of ESR spectra has been per-formed to determine the effect of specific types of motionon the lineshape of the Spectrum.23 ESR lineshapesobtained for randomly oriented probes tumbling prefer-entially along the z axis of the nitroxide (which is the casefor any doxylstearlc acid probe) are identical, suggesting

C = (1.45 - O.O19(A.- A1(meas»J. gauss

The order parameter is usually used as a parameter ofmolecular motion. S varies between 0 (low order) and 1

(23) Griffith, O. H.; JOlt. P. C. In Spin Labelling 1: Theory andApplicatiom; Berliner, L. J., Ed.; Academic ~ New York, 1979; p454.

1290 Langmuir, Vol. 8, No.5, 1992 Malbrel and Somalundaran

that the probe is insensitive to changes in the character-istics of its rotation about its principal axis. In other words,the increase in probe mobility observed in the presentstudy is not due to modifications in the probe rotationalmotion.

With increasing surfactant adsorption densities (15, 30,and 34.5 x 10-6 moVg Al20a) , the order parametersobtained at low constant water concentrations remainconstant at 0.90 :I: 0.03. Therefore, the decrease in sur-factant adsorption density is not responsible for thedecrease in the value obtained for the order parameter aswater was added to the solution.

By elimination, the only remaining motion for explainingthe increase in probe mobility is an increase in probe lateraldiffusion within the surfactant adsorbed layer. Such anincrease in the probe lateral diffusion through the ad-sorbed layer is ~ealistic, considering the structural orga-nization of the complex adsorbed layer when water ispresent at the interface:

At low water concentration, water molecules bind thecarboxylic groups of the stearic acid molecules and thepolar groups of Aerosol OT, directly to the hydroxyl groupsof the alumina surface. This limits the ability of the probeto move within the adsorbed layer and is consistent witha model of localized adsorption where the adsorbedmolecules have limited degrees of freedom.

As the water adsorption density increases, the carbox-ylic groups of the probe molecules interact with watermolecules not directly bound to the hydroxyl groups ofthe mineral surface. Similar interactions between the polargroups of Aerosol OT and water molecules are most likelyand, as a result, the adsorbed layer becomes loosely boundto the mineral surface. The molecular diffusion, limitedby the binding of the surfactant molecules at low waterconcentrations, thus increases markedly as water adsorbson the particles.

molecule changes as in W /0 microemulsions. The inter-facial area occupied by each surfactant molecule progres-sively increases, limiting the number of surfactant mol-ecules that can adsorb at the interface. In addition, thesurfactant molecule becomes progressively more inde-pendent from the mineral surface and the surface hy-droxyls. This progressive increase in surfactant mobilitylevels off once the surfactant adsorbed layer is totallyseparated from the surface and a second layer of watermolecules forms between the surface hydroxyls and thepolar groups of the adsorbed surfactant layer.

The excellent correlation between the onset of floccu-lation by water and the leveling of the order parameterstrongly COnIlrms that water-induced flocculation is pri-marily related to a modification of the adsorbed layerstructure. In 1981, Horn and Israelachvili reported theeffect of water on the structural forces between two micassurfaces CQated with a monolayer of hexadecyltrimethyl-ammonium bromide (CT AB) in a nonpolar liquid, octam-ethylcyclotetrasiloxane. ~ They observed that the presenceof water in the system induces very strong forces attractingthe two surfaces together once the distance between thetwo surfaces falls below 7 to 10 om. This phenomenon,observed at high water concentration, did not occur in thedry system and was reversible (upon the removal ofwater,the strong attraction disappeared). Christenson, whosubsequently studied this phenomenon in more details,attributed it to capillary condensation or phase separationof bulk water between the two surfaces.25

The increase in lateral diffusion observed by ESRspectroscopy COnIums Christenson's intuition that "thebridge [responsible for flocculation] forms by surfacediffusion and a spontaneously thickening film on thesurfaces."25

The spontaneous thickening of the adsorbed layer bysurface diffusion may be viewed as "rmgers" extendinginto bulk solution toward approaching particles. Thisphenomenon differs from the classical description ofcapillary bridging in which particles adhere to one anotheronly after contact. Here, the attractive force bringing theparticles together acts before contact of the two approach-ing surfaces. This phenomenon is therefore proposed asa fingering mechanism.

Acknowledgment. We wish to thank Professor N. J.Tullo for allowing us to use his ESR spectrometer.Financial support of the National Science Foundation(MSM-86-17183, CBT -86-15524, and CBT -89-21570) andof the New York Mining and Mineral Resources ResearchInstitute is acknowledged.

Registry No. AOT, 577-11-7; AlsOs, 1344-28-1; HsO, 7732-18-5; cycloheune, 110-82-7.

(24) Horn, R. G.; Israelaehvili, J. N. J. Chern; Phy,. 1981, 75, 1400.(25) (a) ChristeDlOn, H. K. J. Cf/lloid Interface &i.I985,I04, 234. (b)

Christenson, H. K.; 810m, C. E. J. Chern. Phyt. 1187, 86, 419.

ConclusionsWater adsorption from surfactant solutions in nonpo-

lar media is characteristic of a liquid of limited solubilitycontrolled by its activity in the solution. It is best described88 a function of the ratio [H2O]/[AOT] (wo).

From the data presented here, the following model isproposed to explain the effect of water on the colloidalstability in nonpolar media.

At very low water concentrations, the surfactant mol-ecules adsorbed at the alumina! cyclohexane interface donot dissociate and the absence of charge on the mineralsurface allows the particles in the suspension to flocculate.

At low water concentrations, the surfactant moleculesare partially dissociated at the interface and surfacecharges are generated. Colloidal stability is in this casethe result of the strong electrostatic repulsive forcesovercoming the van der Waa1s attractive forces betweenparticles.

Upon hydration, the conformation of the surfactant