Water-Induced Dispersion/Flocculation of Colloidal Suspensionsin Nonpolar Media

CHRISTOPHE A. MALBREL AND P. SOMASUNDARAN I

Langmuir Center for Colloids and Interfaces, Henry Krumb School of Mines, Columbia University,New York, New York J(}()27

Received January 24,1989; accepted March 31, 1989

Colloidal dispersions in apolar media are used in a variety of technological applications, in most ofwhich water is present and plays a major role in determining the behavior of the dispersion. In this workthe effect of water on the conoidal stability of a suspension of alumina in cyclohexane in the presenceof a commonly used stabilizer, the Aerosol OT, is investigated. A succession of flocculated and dispersedstates was observed as the amount of water added to the suspension 'Ya5 increased. Based on an analogybetween the adsorption of water in this system and the adsorption of a gas on a solid substrate, amethodology is developed to help predict the suspension behavior in apolar media in the presence ofwater. e 1989 Academic PI-. Ioc

INTRODUCfION

Stability of colloidal particles in liquids oflow dielectric constant is a subject of increasinginterest, due to its widening range of techno-logical applications. The processing of highperformance ceramics and magnetic tape ( I )and the manufacturing of certain paints andinks (2) require, at one point or another inthe process, the control of the suspension sta-bility. Dispersion of fine coal particles in anonpolar phase has also been considered forcoal cleaning (3). However, whether a stablesuspension or a rapid flocculation for efficientsolid / liquid separation is desired, the mech-anisms governing stabilization in nonpolarmedia are poorly understood, partly due tothe lack of data obtained under carefully con-trolled experimental conditions.

In most of the applications mentionedabove, water is present in the dispersion. Itmay be introduced into the system as adsorbeQwater on the solid particles or as water of hy-dration of the chemicals used or it can be pres-ent as a separate liquid phase. Some studieshave reported the effects of water on the sus-

1 To whom all correspondence should be addressed.

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pension stability ( 4-6 ) and the role it plays indetermining suspension stabilities. Dependingon the nature of the colloid and the amountof water present in the dispersion, water caneither flocculate a suspension or help stabilizeit. The objective of this work is to investigatesystematically the effect of water on a colloidalsuspension of alumina in cyclohexane stabi-lized by an anionic surfactant, Aerosol OT(AOT). A model describing the behavior ofsuspensions in nonpolar media in the presenceof water is formulated based on an analogybetween the adsorption of water from micellarsolution and vapor adsorption.

EXPERIMENTAL SECTION

Materials

The alumina used in the present study waspurchased from Union Carbide Corporationas Linde Alumina Polishing Powder Type A.X-ray diffraction and chemical analysis of thepowder show the mineral to be a we" crystal-lized corundum (alumina type a) of high pu-rity (>99% AI2O3)' Morphologically, thepowder is constituted of p.m-size aggregatescomposed of smaller particles (between 200and 500 nm). Prior to the suspension prepa-

405SUSPENSIONS IN NONPOLAR MEDIA

titrated against hexadecyltrimethyl ammo-nium bromide in chloroform with dimidiumbromide disulfine blue as the end-point indi-cator (8). Water concentration was measuredusing a Karl FISher Coulometer. When re-quired. the solubilization of water was deter-mined by turbidity measurements, a techniquesensitive enough to distinguish micellar solu-tions from W /0 emulsions.

ration, the powder was subjected to a shortgrinding step (5 min) in a mortar to break upthese aggregates and increase the concentra-tion of primary particles in the suspension.Nitrogen adsorption gave a specific surfacearea of 13.4 m2/g for this powder.

Cyclohexane, obtained from Fisher Scien-tific Co., was of certified ACS grade. The sol-vent was stored on Molecular Sieve 4A toavoid contamination by water. The water usedwas triply distilled, of 10-6 mhos conductivity.

The surfactant Aerosol OT (sodium bis-(2-ethylhexyl)-sulfosuccinate), obtained fromFisher Scientific Co., was purified following aprocedure described in the literature (7 ). Thedry surfactant was stored in a desiccator usingP20S as desiccant. Prior to its use, the surfac-tant was vacuum desiccated overnight anddissolved in cyclohexane and the solution wasstored for a week on dehydrated MolecularSieve 4A to remove traces of water.

RESULTS AND DISCUSSION

In order to investigate the effect of water onthe colloidal stability, all experiments wereperformed at constant surfactant concentra-tions. Figure I shows the adsorption isothermof AOT on alumina in cyclohexane. Areas of60 and 80 A 2 for AOT molecules adsorbed at

the water/xylene and water/isooctane inter-faces, respectively, have been reported in theliterature ( 4, 9). Using these values, the surfacecoverages corresponding to the adsorptionisotherm plateau (3.2 X 10-5 mole/g adsorp-tion density) were estimated to be 1.14 and0.86. It is hence reasonable to assume that theplateau reached by the surfactant adsorptionon alumina corresponds to a monolayer cov-erage. All subsequent experiments conductedwith water were performed under the aboveconditions. No significant change in surfactantadsorption density was observed when waterwas added to the system.

Sample Preparation andExperimental Procedure

The samples were prepared using the fol-lowing procedure: ( 1 ) desiccation of the alu-mina powder at 200°C for 6 h followed by acooling period (2 h) at 25°C in a vacuum des-iccator; (2) preparation of the solution byad-dition of a known amount of water to a cy-clohexane solution of surfactant; (3) additionof 15 ml of the solution to 1 g of alumina ina graduated test tube; and ( 4 ) conditioning ofthe sample (tumbling) at room temperaturefor 24 h prior to settling experiments.

The stability of the dispersion was measuredby optically monitoring the settling of the up-per interface of the suspension that is allowedto settle in a 15 cm3 graduated cylinder of 1cm diameter. The suspension settling rate wasobtained by calculating the initial slope of theplot of the upper interface position versus time.After a 24-h sedimentation period, the sus-pension was centrifuged and both the AOTand water residual concentrations were mea-sured. The AOT was analyzed by a two-phasetitration technique in which the surfactant was

406 MALBREL AND SOMASUNDARAN

In Fig. 2a, the stability of the suspension isreported in terms of settling rate as a functionof residual water concentration at two differentsurfactant concentrations (8.5 and 26 X 10-3mole/liter). Figure 2b shows the correspond-ing water adsorption isotherms. As the waterconcentration in the system is increased, thesuspensions exhibit a succession of flocculatedand stable states. At low water concentration,the settling occurs rapidly. It can be seen thatthe onset of stabilization corresponds to asharp increase in the amount of water ad-sorbed on alumina, suggesting that the ad-

sorption of water plays a critical role in thestabilization phenomenon. It is generally ac-cepted that the stabilization by Aerosol OT ofoxide particles is due to the development ofelectrostatic repulsive forces between particleswhen the dissociation of adsorbed surfactantsand the subsequent desorption of the anionslead to the formation of charges at the solidiliquid interface ( 10). The results presentedhere show that, even though a monolayer ofthe surfactant is adsorbed at the interface inall cases, stabilization takes place only whentrace amounts of water are added to the sus-pension. This observation is in agreement withthe conclusion of McGown, ParfItt, and Willis( 4) on the role played by water in charge de-velopment at the solidi AOT adsorbed layerinterface. Thus, it can be concluded that waterplays a major role in the dissociation of thesurfactant ions.

At higher water concentrations, Fig. 2ashows a sharp increase in the suspension set-tling rate at both surfactant concentrationsstudied. But as the surfactant concentrationincreases, so does the water concentration atwhich the flocculation takes place. In theseranges of water concentrations, no relationshipcan be clearly established a priori between thesuspension stability and the water adsorption.

The shape of the water adsorption isothermis similar to that of the one obtained for vaporadsorption on solid substrates. An analogy be-tween the two adsorption phenomena can beused to rationalize the water adsorption data.Vapor adsorption on a solid substrate is afunction of the vapor pressure, P, in contactwith the solid. An increase in the vapor pres-sure increases the adsorption gradually untilthe vapor pressure reaches values at which thevapor starts to condense on the solid, leadingto a sharp increase in the amount of vaporadsorbed. By analogy, water adsorption fromthe AOT Icyclohexane solution is controlledby the water concentration in the solution,[H2O]. The adsorption of a gas is ultimatelylimited by its saturating vapor pressure, Po.Similarly, it is possible to interpret the wateradsorption as being controlled by a critical

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FIG. 2. (a) Effecl of water addition on the suspensionsettling rate. Increase in surfactant concentration shiftsthe suspension destabilization towards higher water con-centrations. (b) Adsorption of water on alumina from theAOT /cyclohexane solution. Increase in surfactant con-centration expands the domain of water concentration inwhich adSOrption takes place. 8: AOT = 8.5 X 10-3 mole/liter; l:.; AOT = 26 X 10-3 mole/liter.

Jouma/ td"ColJoid aJId ,~ Science, Vol. 133, No. 2, Dc.1emIJe. 1989

SUSPENSIONS IN NONPOLAR MEDIA 407

water concentration at which a phase change I~.J»

in solution is observed. In the AOT /cyclo-hexane solution, a phase change is observed ~ 10.~

as the water concentration is increased at ..which the solution goes from a clear, stable ~

micellar solution to a turbid. unstable W /0 ; I.~emulsion. This critical concentration is re- ~

ferred to as the critical emulsion concentration ~0.10

(C.E.C.) and is shown in Fig. 3 as the limit ti:;between the two domains of solution behavior. '"

Results presented in this figure were obtainedat various surfactant concentrations by mon-itoring the sharp change in the turbidity of thesolution as water is gradually added to it. Gasadsorption varies as a function of temperature:as the temperature is increased, so is the sat-urating pressure of the gas, Po. Classically, gasadsorption isotherms are plotted as a functionof relative vapor pressure of the gas, P/ Po, tocompensate the effect of temperature on theadsorption. Assuming that the surfactant con-centration in solution is playing the role oftemperature in the gas adsorption, an analo- :gous normalization of the water adsorption ~isotherm is possible and is shown in Fig. 4, in . ..-

which the data are plotted as a function of

normalized water concentration, [H2O]/ FIG. 4. (a) Normalized suspension settling rate dataC.E.C. The good superimposition of the two showing the superimposition of the two data sets shown

in Figure 2a. (b) Normalized water adsorption data. Thetwo isotherms are also wen superimposed by this nor-maliZAtion procedure. (8: AOT = 8.5 X lO -3 and C.E.C.= lSO x lO-3 mole/liter; ~ AOT = 26 X lO-3 and C.E.C.= 540 X lO-3 mole/liter.

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FIG. 3. AOT IWater/Cyclohexane phase diagramshowing the change of critical emulsion concentration(C.E.C.) as a function of surfactant concentration. Thepoints reported on the diagram represent the concentra-tions at which the turbidity measurements were performed(.: optically clear; +: turbid).

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water adsorption isotherms (Fig. 4b) indicatesthat the C.E.C. is indeed a controlling param-eter of the adsorption of water on the solid. Itjustifies the use of the normalization procedurefor interpreting the settling rate data. Figure4a shows the results of the normalization ofthe two sets of settling rate data. Again the twocurves coincide, suggesting that the floccula-tion phenomenon is also controlled by theC.E.C. of the solution. However, from the fig-ure, it can be seen that the flocculation is notdirectly due to the water condensing on themineral surface since water condensation

408 MALBREL AND SOMASUNDARAN

ter that can be dissolved in the micellar s0-lution increases. The water concentration atwhich water "condensation" takes place onthe surface of the colloid is shifted towardhigher water concentration and so is the waterconcentration at which flocculation takesplace. The observed destabilization may bedue to a decrease in the magnitude of the elec-trostatic repulsive forces between the particlesor to a capillary bridging phenomenon. Thenormalization procedure proposed in this noteallows the comparison of data obtained underdifferent experimental conditions and shouldtherefore be useful for the determination ofthe mechanism controlling the suspensionstability in the presence of the water.

starts above (H2OJ/C.E.C. = 0.70, whereasthe suspension flocculation occurs between 0.2and 0.4.

Two mechanisms have been put forward inthe literature to explain the suspension floc-culation at high water concentrations. Mc-Gown et al. postulated a decrease in the mag-nitude of the electrostatic repulsive forces be-tween particles as the amount of adsorbedwater increases (4). On the other hand, Kan-dori et al. proposed a capillary bridging mech-anism due to a layer of water "binding" theparticles together (6). The results presentedhere do not help to choose between the twomechanisms proposed. However, the normal-ization procedure proposed is thought to pro-vide a means to compare results obtained un-der different conditions, which is necessary tounderstand the flocculation phenomenon athigh water concentrations.

ACKNOWLEDGMENTS

The authors acknowledge the financial support of theNSF (MSM-86-17 183 andCBT-86-15524 »and the NewYorlc Mining and Mineral Resources Research Institute.

CONCLUSIONS

REFERENCESA systematic investigation of the effect ofwater on the colloidal stability in apolar mediahas demonstrated the critical role played bywater in stabilization phenomena.

Addition of trace amounts of water has beenshown to control stabilization when an ionicsurfactant such as Aerosol OT is used as sta-bilizer. At high water concentrations, floccu-lation was observed and it was established thatits onset varies with the surfactant concentra-tion in solution. An increase in surfactantconcentration in solution expands the domainof stability of the suspension. An analogy withgas adsorption was found to be useful for de-scribing the adsorption of water on aluminain the presence of AOT. This analogy was alsoused to interpret the stability data and to de-velop a model for the suspension behavior.

The effect of surfactant concentration onthe stability suggests that the suspension be-havior is controlled by the solvation power ofthe solution for water. When the surfactantconcentration is increased, the amount of wa-

I. "Interfacial Phenomena in the New and EmergingTecIlnologjes," (Proceedings of the NSF Workshopheld at the Univ~ty of Colorado, Boulder, Col-orado, May 1986)(W. B. Krantz and D. T. Wasan,Eds.), HITEX PubI., 1987.

2. McKay, R. B., in "Interfacial Phenomena in ApolarMedia" (M. F. Eicke and G. D. Parfitt, Eds.), Sur-factant Science Series, Vol. 21, p. 361, Dekker,New York, 1987.

3. Capes, C. E., and Gernlain, R. J., in "Physical OeaningofCoaI" (Y. A. Liu, Ed.), Dekker, New York, p.293 (1982).

4. McGown, D. N. L., Parfitt, G. D., and Willis, E., J.Colloid Sci. 20,650 (1967).

5. Kitahara, A., Karasawa, S., and Yamada, H., J. Col-loid Interface Sci. 2S,490 (1967).

6. Kandori, K., ~ma, A., Kon-no, K., and Kitahara,A., Bull. Chern. Soc. JapanS?, 1777 (1984).

7. Kitahara, A., J. Phys. Chern. 69, 2788 (1965).8. Reid, V. W., Longman, G. F., and Heinherth, E.,

TensideS, 90 (1968).9. Maitra, A., and Patanjali, P. K., in "Surfactants in

Solution" (K. L. Mittai and P. BothoreL Eds.),Vol. 5, p. 581, Plenum, New York, 1986.

10. Novotny, V., Colloids Surfaces 2, 373 (1981).

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