Water-Induced Dispersion/Flocculation of Colloidal Suspensions in Nonpolar Media

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Water-Induced Dispersion/Flocculation of Colloidal Suspensions in Nonpolar Media CHRISTOPHE A. MALBREL AND P.

dispersions in apolar media are used in a variety of technological applications, in most of which water is present and plays a major role in determining the behavior of the dispersion.

A succession of flocculated and dispersed states was observed as the amount of water added to the suspension 'Ya5 increased.

in the presence of water. e 1989 Academic PI-.

1 Water-Induced Dispersion/Flocculation of Colloidal Suspensions in 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 of which water is present and plays a major role in determining the behavior of the dispersion. In this work the effect of water on the conoidal stability of a suspension of alumina in cyclohexane in the presence of a commonly used stabilier, the Aerosol OT, is investigated. A succession of flocculated and dispersed states was observed as the amount of water added to the suspension 'Ya5 increased. Based on an analogy between the adsorption of water in this system and the adsorption of a gas on a solid substrate, a methodology is developed to help predict the suspension behavior in apolar media in the presence of water. e 1989 Academic PI-. Ioc INTRODUCfION Stability of colloidal particles in liquids of low dielectric constant is a subject of increasing interest, due to its widening range of technological applications. The processing of high performance ceramics and magnetic tape ( I ) and the manufacturing of certain paints and inks (2) require, at one point or another in the process, the control of the suspension stability. Dispersion of fine coal particles in a nonpolar phase has also been considered for coal cleaning (3). However, whether a stable suspension or a rapid flocculation for efficient solid / liquid separation is desired, the mechanisms governing stabiliation in nonpolar media are poorly understood, partly due to the lack of data obtained under carefully controlled experimental conditions. In most of the applications mentioned above, water is present in the dispersion. It may be introduced into the system as adsorbeq water on the solid particles or as water of hydration of the chemicals used or it can be present as a separate liquid phase. Some studies have reported the effects of water on the sus- 1 To whom all correspondence should be addressed. pension stability ( 4-6 ) and the role it plays in determining suspension stabilities. Depending on the nature of the colloid and the amount of water present in the dispersion, water can either flocculate a suspension or help stabilie it. The objective of this work is to investigate systematically the effect of water on a colloidal suspension of alumina in cyclohexane stabilied by an anionic surfactant, Aerosol OT (AOT). A model describing the behavior of suspensions in nonpolar media in the presence of water is formulated based on an analogy between the adsorption of water from micellar solution and vapor adsorption. EXPERIMENTAL Materials SECTION The alumina used in the present study was purchased from Union Carbide Corporation as Linde Alumina Polishing Powder Type A. X-ray diffraction and chemical analysis of the powder show the mineral to be a we" crystallied corundum (alumina type a) of high purity (>99% AI2O3)' Morphologically, the powder is constituted of p.m-sie aggregates composed of smaller particles (between 200 and 500 nm). Prior to the suspension prepa /89 $ by AC8iomic Press. Inc. AI of UCIion id ony form.--l ".,.. If V.. t J3, No. 2, December t 989

SUSPENSIONS IN NONPOLAR MEDIA 405 ration, the powder was subjected to a short grinding step (5 min) in a mortar to break up these aggregates and increase the concentration of primary particles in the

The solvent was stored on Molecular Sieve 4A to avoid contamination by water. The water used was triply distilled, of 10-6 mhos conductivity.

The dry surfactant was stored in a desiccator using P20S as desiccant.

Sample Preparation and Experimental Procedure The samples were prepared using the following procedure: ( 1 ) desiccation of the alumina powder at 200 C for 6 h followed by a cooling period (2 h) at

a graduated test tube; and ( 4 ) conditioning of the sample (tumbling) at room temperature for 24 h prior to settling experiments.

3 SUSPENSIONS IN NONPOLAR MEDIA 405 ration, the powder was subjected to a short grinding step (5 min) in a mortar to break up these aggregates and increase the concentration of primary particles in the suspension. Nitrogen adsorption gave a specific surface area of 13.4 m2/g for this powder. Cyclohexane, obtained from Fisher Scientific Co., was of certified ACS grade. The solvent was stored on Molecular Sieve 4A to avoid contamination by water. The water used was triply distilled, of 10-6 mhos conductivity. The surfactant Aerosol OT (sodium bis-(2- ethylhexyl)-sulfosuccinate), obtained from Fisher Scientific Co., was purified following a procedure described in the literature (7 ). The dry surfactant was stored in a desiccator using P20S as desiccant. Prior to its use, the surfactant was vacuum desiccated overnight and dissolved in cyclohexane and the solution was stored for a week on dehydrated Molecular Sieve 4A to remove traces of water. Sample Preparation and Experimental Procedure The samples were prepared using the following procedure: ( 1 ) desiccation of the alumina powder at 200 C for 6 h followed by a cooling period (2 h) at 25 C in a vacuum desiccator; (2) preparation of the solution byaddition of a known amount of water to a cyclohexane solution of surfactant; (3) addition of 15 ml of the solution to 1 g of alumina in a graduated test tube; and ( 4 ) conditioning of the sample (tumbling) at room temperature for 24 h prior to settling experiments. The stability of the dispersion was measured by optically monitoring the settling of the upper interface of the suspension that is allowed to settle in a 15 cm3 graduated cylinder of 1 cm diameter. The suspension settling rate was obtained by calculating the initial slope of the plot of the upper interface position versus time. After a 24-h sedimentation period, the suspension was centrifuged and both the AOT and water residual concentrations were measured. The AOT was analyed by a two-phase titration technique in which the surfactant was titrated against hexadecyltrimethyl ammonium bromide in chloroform with dimidium bromide disulfine blue as the end-point indicator (8). Water concentration was measured using a Karl FISher Coulometer. When required. the solubiliation of water was determined by turbidity measurements, a technique sensitive enough to distinguish micellar solutions from W /0 emulsions. RESULTS AND DISCUSSION In order to investigate the effect of water on the colloidal stability, all experiments were performed at constant surfactant concentrations. Figure I shows the adsorption isotherm of AOT on alumina in cyclohexane. Areas of 60 and 80 A 2 for AOT molecules adsorbed at the water/xylene and water/isooctane interfaces, respectively, have been reported in the literature ( 4, 9). Using these values, the surface coverages corresponding to the adsorption isotherm plateau (3.2 X 10-5 mole/g adsorption density) were estimated to be 1.14 and It is hence reasonable to assume that the plateau reached by the surfactant adsorption on alumina corresponds to a monolayer coverage. All subsequent experiments conducted with water were performed under the above conditions. No significant change in surfactant adsorption density was observed when water was added to the system.

Figure 2b shows the corresponding water adsorption isotherms. As the water concentration in the system is increased, the 10 IGI RESIDlML VIITER CDflCENTRRTIDH x 103, MIl FIG. 2. (a) Effecl of water addition on the suspension settling rate.

Increase in surfactant concentration expands the domain of water concentration in which adsorption takes place. 8: AOT = 8.5 X 10-3 mole/ liter; l:.; AOT = 26 X 10-3 mole/liter.

4 406 MALBREL AND SOMASUNDARAN In Fig. 2a, the stability of the suspension is reported in terms of settling rate as a function of residual water concentration at two different surfactant concentrations (8.5 and 26 X 10-3 mole/liter). Figure 2b shows the corresponding water adsorption isotherms. As the water concentration in the system is increased, the 10 IGI RESIDlML VIITER CDflCENTRRTIDH x 103, MIl FIG. 2. (a) Effecl of water addition on the suspension settling rate. Increase in surfactant concentration shifts the suspension destabiliation towards higher water concentrations. (b) Adsorption of water on alumina from the AOT /cyclohexane solution. Increase in surfactant concentration expands the domain of water concentration in which adsorption takes place. 8: AOT = 8.5 X 10-3 mole/ liter; l:.; AOT = 26 X 10-3 mole/liter. Imll suspensions exhibit a succession of flocculated and stable states. At low water concentration, the settling occurs rapidly. It can be seen that the onset of stabiliation corresponds to a sharp increase in the amount of water adsorbed on alumina, suggesting that the adsorption of water plays a critical role in the stabiliation phenomenon. It is generally accepted that the stabiliation by Aerosol OT of oxide particles is due to the development of electrostatic repulsive forces between particles when the dissociation of adsorbed surfactants and the subsequent desorption of the anions lead to the formation of charges at the solidi liquid interface ( 10). The results presented here show that, even though a monolayer of the surfactant is adsorbed at the interface in all cases, stabiliation takes place only when trace amounts of water are added to the suspension. This observation is in agreement with the conclusion of McGown, ParfItt, and Willis ( 4) on the role played by water in charge development at the solidi AOT adsorbed layer interface. Thus, it can be concluded that water plays a major role in the dissociation of the surfactant ions. At higher water concentrations, Fig. 2a shows a sharp increase in the suspension settling rate at both surfactant concentrations studied. But as the surfactant concentration increases, so does the water concentration at which the flocculation takes place. In these ranges of water concentrations, no relationship can be clearly established a priori between the suspension stability and the water adsorption. The shape of the water adsorption isotherm is similar to that of the one obtained for vapor adsorption on solid substrates. An analogy between the two adsorption phenomena can be used to rationalie the water adsorption data. Vapor adsorption on a solid substrate is a function of the vapor pressure, P, in contact with the solid. An increase in the vapor pressure increases the adsorption gradually until the vapor pressure reaches values at which the vapor starts to condense on the solid, leading to a sharp increase in the amount of vapor adsorbed. By analogy, water adsorption from the AOT Icyclohexane solution is controlled by the water concentration in the solution, [H2O]. The adsorption of a gas is ultimately limited by its saturating vapor pressure, Po. Similarly, it is possible to interpret the water adsorption as being controlled by a critical 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 /cyclohexane solution, a phase change is observed J 10.

This critical concentration is re- ferred to as the critical emulsion concentration 0.10 (C.E.C.) and is shown Fig. 3 as the limit ti:; between the two domains of solution behavior. '" ;.==.

5 SUSPENSIONS IN NONPOLAR MEDIA 407 water concentration at which a phase change I.J» in solution is observed. In the AOT /cyclohexane solution, a phase change is observed J 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 Fig. 3 as the limit ti:; between the two domains of solution behavior. '" ;.==.:t:':- Results presented in this figure were obtained at various surfactant concentrations by monitoring the sharp change in the turbidity of the solution as water is gradually added to it. Gas adsorption varies as a function of temperature: as the temperature is increased, so is the saturating pressure of the gas, Po. Classically, gas adsorption isotherms are plotted as a function of relative vapor pressure of the gas, P/ Po, to compensate the effect of temperature on the adsorption. Assuming that the surfactant concentration in solution is playing the role of temperature in the gas adsorption, an analo- : O.f/A.o 0.4 o.e (OJ/C.E.C normalied water concentration, [H2O]/ FIG. 4. (a) Normalied suspension settling rate data C.E.C. The good superimposition of the two showing the superimposition of the two data sets shown in Figure 2a. (b) Normalied water adsorption data. The D Z; "t:. x >-... a t- 8» _I..--- ==:'- J /. II //.,i --"'" gous isotherm normaliation is possible and of the is shown water in adsorption Fig. 4, in O. o.z 0.. D which the data are plotted as a function of IHZOJ/C.e.c. "- "b 0 )( ;::: a:... w c.j CJ c.j a: w...» lid! IUJ IU.... via I.I optical I" turbid.. V' y / y Micellar 8olutlMl optically claar ,0 I'" AEROSOL OT CONCENTRATION x 103,MIl FIG. 3. AOT IWater/Cyclohexane phase diagram showing the change of critical emulsion concentration (C.E.C.) as a function of surfactant concentration. The points reported on the diagram represent the concentrations at which the turbidity measurements were performed (.: optically clear; +: turbid). two isotherms are also wen superimposed by this normalization 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. water adsorption isotherms (Fig. 4b) indicates that the C.E.C. is indeed a controlling parameter of the adsorption of water on the solid. It justifies the use of the normaliation procedure for interpreting the settling rate data. Figure 4a shows the results of the normaliation of the two sets of settling rate data. Again the two curves coincide, suggesting that the flocculation phenomenon is also controlled by the C.E.C. of the solution. However, from the figure, it can be seen that the flocculation is not directly due to the water condensing on the mineral surface since water condensation I 4.C.wl Vol I)], No. 2, DecembeF 1989

408 MALBREL AND SOMASUNDARAN starts above (H2OJ/C.E.C. = 0.70, whereas the suspension flocculation occurs between 0.2 and 0.4. Two mechanisms have been put forward in the literature to explain the suspension flocculation at high water concentrations.

proposed a capillary bridging mechanism due to a layer of water "binding" the particles together (6). The results presented here do not help to choose between the two mechanisms proposed.

Addition of trace amounts of water has been shown to control stabiliation when an ionic surfactant such as Aerosol OT is used as stabilier.

An increase in surfactant concentration in solution expands the domain of stability of the suspension.

6 408 MALBREL AND SOMASUNDARAN starts above (H2OJ/C.E.C. = 0.70, whereas the suspension flocculation occurs between 0.2 and 0.4. Two mechanisms have been put forward in the literature to explain the suspension flocculation at high water concentrations. Mc- Gown et al. postulated a decrease in the magnitude of the electrostatic repulsive forces between particles as the amount of adsorbed water increases (4). On the other hand, Kandori et al. proposed a capillary bridging mechanism due to a layer of water "binding" the particles together (6). The results presented here do not help to choose between the two mechanisms proposed. However, the normaliation procedure proposed is thought to provide a means to compare results obtained under different conditions, which is necessary to understand the flocculation phenomenon at high water concentrations. CONCLUSIONS A systematic investigation of the effect of water on the colloidal stability in apolar media has demonstrated the critical role played by water in stabiliation phenomena. Addition of trace amounts of water has been shown to control stabiliation when an ionic surfactant such as Aerosol OT is used as stabilier. At high water concentrations, flocculation was observed and it was established that its onset varies with the surfactant concentration in solution. An increase in surfactant concentration in solution expands the domain of stability of the suspension. An analogy with gas adsorption was found to be useful for describing the adsorption of water on alumina in the presence of AOT. This analogy was also used to interpret the stability data and to develop a model for the suspension behavior. The effect of surfactant concentration on the stability suggests that the suspension behavior is controlled by the solvation power of the solution for water. When the surfactant concentration is increased, the amount of wa- ter that can be dissolved in the micellar s0- lution increases. The water concentration at which water "condensation" takes place on the surface of the colloid is shifted toward higher water concentration and so is the water concentration at which flocculation takes place. The observed destabiliation may be due to a decrease in the magnitude of the electrostatic repulsive forces between the particles or to a capillary bridging phenomenon. The normaliation procedure proposed in this note allows the comparison of data obtained under different experimental conditions and should therefore be useful for the determination of the mechanism controlling the suspension stability in the presence of the water. ACKNOWLEDGMENTS The authors acknowledge the financial support of the NSF (MSM andcbt »and the New Yorlc Mining and Mineral Resources Research Institute. REFERENCES I. "Interfacial Phenomena in the New and Emerging TecIlnologjes," (Proceedings of the NSF Workshop held at the Univty of Colorado, Boulder, Colorado, May 1986)(W. B. Krant and D. T. Wasan, Eds.), HITEX PubI., McKay, R. B., in "Interfacial Phenomena in Apolar Media" (M. F. Eicke and G. D. Parfitt, Eds.), Surfactant Science Series, Vol. 21, p. 361, Dekker, New York, Capes, C. E., and Gernlain, R. J., in "Physical Oeaning ofcoai" (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. Colloid 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, Novotny, V., Colloids Surfaces 2, 373 (1981). J-u.; -- Sd-.. VoL In. No

EFFECT OF SOLIDS CONCENTRATION ON POLYMER ADSORPTION AND CONFORMATION

2 EFFECT OF SOLIDS CONCENTRATION ON POLYMER ADSORPTION AND CONFORMATION Tsung-yuan Chen,. Chidambaram Maltesh,2 and Ponisseril Somasundaranl IHerny Krumb School of Mines Columbia University New York, New