2611
Langmuir 1992,8, 2611-2615
Effect of Salt on Polymer Solvency: Implications for Dispersion Stability Maryann B. Einarson and John C. Berg' Department of Chemical Engineering, BF-10, University of Washington, Seattle, Washington 98195 Received March 16,1992. In Final Form: July 8, 1992 The adsorption of a macromolecule to the surface of a colloidal particle provides a steric component to the stabilization of the dispersion. Dispersion stabilities of systems with varying chain length of the stabilizing moiety are compared as a function of electrolyte concentration and counterion valence. Block copolymers of poly(ethy1ene oxide)/poly(propyleneoxide), PEO/PPO, are used 88 the steric stabilizers. Experimental stability domains are determined in terms of the early-stage kinetics of flocculation by measuring the change in effective diameter with time. Systems having the shortest chain length stabilizer are marginally stable, requiring only sufficient electrolyte to collapse the electric double layer to induce flocculation,as reported earlier." Systems with long chain length polymers, however, are robustly stable, requiring the reduction of polymer solvency,which occurs at high salt concentrations,to induce flocculation. The copolymer having the higher molecular weight PEO increases stability 100-foldcompared to the lower molecular weight PEO stabilizer in the presence of NaCl and BaClz. For relatively high salt concentrations, changes in stability, which may be due to the reduction of polymer solvency, are discussed by considering the behavior of the second virial coefficient of the polymer as a function of electrolyte concentration.
Introduction Polymer adsorption to a colloid surface has a wide range of practical applications. Whether the presence of the adsorbed polymer acts to promote dispersion stability or to flocculate the dispersion depends on the molecular weight, the amount adsorbed, the charge on the polymer, if any, the polymer structure, and the solvency of the polymer in the solvent. An overviewof the factors affecting the stability of steric systems is given by Napper.2 At plateau adsorption of neutral polymers, the chain length of the polymer affecta the magnitude of the steric repulsion. For a charged particle the difference between the stability provided by adsorbed long-chain molecules are adsorbed short-chain surfactants often distinguishes steric and electrosteric stabilization. Systems with adsorbed polymer can be flocculated by reducing the polymer solvency to 8 conditions. On the other hand, if the system is stabilized electrosterically with a combination of an electric double layer and a rather thin adlayer, it may be flocculated by adding sufficient electrolyte to collapse the electric double layer. In this case, the steric repulsions acting alone are not enough to maintain stability. For conditions of good solvency in an aqueous medium, long-chain macromolecule adlayers have been shown to be strong stabilizers, producing a dispersion which is insensitive to salt concentrations which correspond to a collapsed electric double l a ~ e r . ~The * ~incipient flocculation of a dispersion with high MW adlayers was found to correspond to the attainment of 6 conditionsas observed by critical flocculation temperature (cft) measurements of poly(ethy1ene oxide), PEO, stabilized dispersions.6 For a large number of model systems which were sterically stabilized by high molecular weight polymer adlayers, Nappefl hae shown a correlation between the cft and the 8 temperature. However, the correlation between the cft
* To whom correspondence should be addressed.
(1) Einanon, M.B.; Berg, J. C. J. Colloid Interface Sci., in prees. (2) Napper, D. H.Polymeric Stabilization of Colloidal Dispersiom; Academic Press: London, 1983. (3)Ash, S. G.; Clayfield, E. J. J. Colloid Interface Sci. 1976,55,645. (4) Cowell, C.; Vincent, B. J. Colloid Interface Sei. 1983,95,573. (5)Napper, D. H. J. Col&+ Interface Sci. 1970,33,384. (6) Napper, D. H.J. Colloid Interface Sci. 1970,32, 106.
and 8 temperature breaks down with low MW stabilizers and weaker, physically adsorbed p ~ l y m e r . ~Systems flocculate under better than 8 conditions as observed with PEO (MW < 6000)in the presence of 0.26 MgS048 and PEO/PPO block copolymers in the presence of varying amounts of KC1 and N a ~ S 0 4 . ~ Polymer solvency may be modified through a temperature change or the addition of nonsolvent. Cosgrove and co-workers1° recently showed the adsorbed polymer layer thickness decreases on approachto flocculationusing both small-angle neutron scattering and photon correlation spectroscopy. The temperature was varied in the presence of electrolyte to progressively worsen polymer solvency. The reduction in the solvency of PEO solutions upon addition of electrolyte is often characterized by reporting the decrease in the cft (theminimum temperature at which flocculation occurs) and 8 temperature, with increasing salt concentration. Boucher and Hines" did an extensive study of free PEO in solution and reported the 6 temperature as a function of electrolyte concentration. The dt was observed to decreasealmost linearly with increasing salt concentration.lZ Napper6 studied the flocculation effectiveness of inorganic salts for systems which were sterically stabilized by adsorbed PEO. The observed cation order of decreasing flocculation effectiveness in these studies was mono-, di-, and then trivalent cations, which is the reverse of that for electrostatic stabilization.6W1 In this work the manner in which dispersion stability is affected by changes in polymer solvency with increasing salt concentration is examined. In the regime of low molecular weight stabilizers, it is of interest to investigate the difference in dispersion stability behavior as a function (7) Cowell,C.;Li-Lin-On,F. K. R.;Vincent,B. J. Chem. Soc., Faraday Tram. 1 1978, 74,337. (8) Cowell, C.; Vincent, B. In The Effect of Polymers on Dispersion Properties; Tadroe, T. F., Ed.; Academic Prees: London, 1982. (9) Tadros, T. F.;Vincent, B. J. Phys. Chem. 1980,84,1575. (10) Cosmove. T.: Crowlev. - . T.: Rvan. - . K.:. Webster, J. Colloids Surf. 1991,51, 2g5. . (11) Boucher. E. A.: Hines. P. M. J. Polvm. Sci... Polvm. Phvs. Ed. 1976,14, 2241. ' (12) Bailey, F.E.;Koleeke, J. V. In Nonionic Surfactants; Schick, M. J.; Ed.;Marcel Dekker: New York, 1987; Chapter 16. '
0743-7463/92/2408-2611$03.00/00 1992 American Chemical Society
2612 Langmuir, Vol. 8, No. 11,1992
Einarson and Berg
Table I. Properties of PEO/PPO Block Copolymers Pluronic L-44 L-35
L-43
MW~M
MWPEO
2200
lo00
1900 1850
950 650
of electrolyte concentration between systems which flocculate upon removal of the electric double layer and systems which flocculate at high salt concentrations due to a reduction in polymer solvency of the adlayer. The effect of the molecular weight of the polymer adlayer on dispersion stability as a function of electrolyte concentration and counterionvalenceis examined. Threeneutral, low-to-moderateMW block copolymers of PEO and PPO were investigated. Stability ratios were determined from initial aggregation kinetics obtained from photon correlation spectroscopy. Polymer solvency is characterized as a function of electrolyte concentration and counterion valence through intrinsic viscosity measurements of free PEO solutions.
Experimental Section Aggregation Experiments. The model colloidal dispersion of monodisperse, well-characterized polystyrene latexes was purchased from Interfacial Dynamics Corp. (Portland,OR). The latex dispersion, received suspended in distilled water at a concentration of 10.6 wt %, was stabilized by surface sulfate groups with a parking area of 2571 &/charged group. As determined from electron microscopy, the mean diameter was 0.086 pm. The NaCl, BaClZ, and AlCl, used were reagent grade. All water used in dilutions was deionized and doubly distilled with a pH of 5.5-6.0. The water was fiitered through a 0.20-pm Nylon filter just prior to use. For experiments using AlCL, all solutions were adjusted to a pH of 4.0 to minimize the presence of aluminum species other than Ais+, yet high enough to maintain stability of the latexes in the absence of added electrolyte. The pH of the solutions was adjusted through the addition of either HNOs or NaOH. A series of commercialABA block copolymersof poly(ethy1ene oxide), PEO, and poly(propy1eneoxide), PPO, manufactured by BASF Wyandotte Corp. (Wyandotte, MI) were selected as the steric stabilizers and are characterized in Table I. The triblock Pluronic polymers are expected to adsorb such that the PPO section of the block copolymer anchors the polymer to the latex surface and the PEO chains extend out into the solvent under conditions of good solvency to provide the stabilizing moiety. For the Pluronic series, the plateau adsorption concentrations are well established when adsorbed to polystyrene 1atexes.l3J4 The polymer was added to the latex dispersion at plateau adsorption concentrations. This polymer/latex dispersion was stirred for 24 h to achieve equilibrium and refrigerated when not in use. All dispersions were used within 4 days of preparation. For a given aggregation experiment, latex (with or without adsorbed polymer) and electrolyte solutions were mixed in a test tube to provide a final particle number concentration of (7-10) X 1Og particles/mL and a polymer concentration of 2.5 X lo-' g/mL. This particle number concentration range was chosen such that the rate constant of aggregation is independent of the particle number concentration for polymer-coated dispersions. Latex number concentrations were determined by measuring the weight percent of solids in the latex stock solution supplied by the manufacturer and noting all successive dilutions made from the stock solution. Aggregation was induced by mixing equal volumes of latex dispersion and electrolyte at time zero. The subsequent change in diameter with time was monitored. The mean hydrodynamic diameter of the particles in the dispersion with and without polymer was determined by photon correlation spectroscopy (PCS) using a Brookhaven Instruments (Holtsville, NY)Model ~~
BI-2030laser light scattering setup. A Spectra-Physics Stabilite 15mWH e N e laser was used in conjunctionwith the Brookhaven correlator. The method of cumulants was used for data analysis. The reference PCS measuring conditions were a scattering angle of 90°, a wavelength of 632.6 nm,and a temperature of 24 "C. Data were typically collected for approximately 600 s for fast aggregation and up to 5OOO s for very slow aggregation. The rate constant of doublet formation can be calculated from the initial slope of the measured change in mean diameter, or radius, with time as measured by PCS using the method of cUmUlants:'6
where Rbl is the initial hydrodynamic radius at time zero, Nois the initial particle number concentration, and a is an optical factor that is a function of the particle radius, solvent refractive index, scattering angle, and wavelength of light in the medium. The initial slopes were evaluated at approximately the same number of normalized half-times. The stability ratio is expressed as the ratio of Smoluchowski's diffusion-limitad rate constant of doublet formation to the rate constant of doublet formation measured experimentally. As discussed earlier,' a normalized stability ratio, W*, is reported which represents the ratio of the fast aggregation rate constant for the bare latex dispersion measured experimentally to the measured slow aggregation rate constant of a latex dispersion with or without polymer. Thus, the absolute stability ratio is effectively normalized by the stability ratio describing fast aggregation of bare latex dispersions. Viscosity Experiments. The polymer used in the intrinsic viscosity experiments was a PEO homopolymer with a MW of 148 OOO, termed WSR N-10 Polyox from Union Carbide. Solutions were prepared with deionized water. The salts were all reagent grade. Experiments were performed in a thermostated water bath at 25 "C. The solvency of the polymer in solution, as described by the second virial coefficient, was measured viscometrically wing a Cannon-Fenske capillary viscometer. For a given electrolyte concentration, extrapolation of the viscosity to zero polymer concentration yielded the intrinsic viscosity, [VI. The ratio of the intrinsic viscosity of the polymer in solution to the intrinsic Viscosity of the polymer at B conditions determines the cube of the intramolecular expansion factor a:
The intramolecular expansion factor provides a measure of how the solvent perturbs the polymer,where a > 1indicates a polymer with an extended conformation from the B conformation. The second virial coefficient, Bz, may be determined from the Flory equation:16 (3) where YZ is the partial specific volume of the polymer, VI is the molar volume of the solvent, and MW is the molecular weight. The molecular constant, Cm, in eq 3 is
where (P)01/2 is the unperturbed rms end-bend length of the poly(ethy1ene oxide).
Results Normalized stability ratios calculated from the change in mean diameter with time are shown in Figures 1-3 for electrolyte-induced aggregation of the latex dispersions. The stability ratios are shown as a function of salt
~~
(13)Kayea, J. B.; Rawlins, D. A. Colloid Polym. Sci. 1979,257, 622. (14)Baker,J.ElectrostericStabiion.Ph.D. Diasertation,University of Washington, 1986.
(15)Virden, J. W.; Berg, J. C. J. Colloid Interface Sei. 1992,149,628. (16)Flory,P.J. Principles of Polymer Chemistry; Comell University Prese: Ithaca, NY,1953.
Langmuir, Vol. 8, NO.11,1992 2613
Effect of Salt on Polymer Solvency 12.0
10.0
8.0
i
6.0
4.0
o.im
i 0.01
0.1
2.0
1
\
10
1
NPoo.soO
0.0
Figure 1. Normalized stability ratios for bare and polymercoated latex dispersions as a function of NaCl concentration.
1 0.0
0.50
1.0
f
I
1.5
1 2.0
Elecuolyre conc (M
Figure 4. Effect of electrolyteon polymer solvencyas measured by the second virial coefficient.
f o.mi
0.01
0.I
1
--eo
Figure 2. Normalized stability ratios for bare and polymer-
coated latex dispersions as a function of BaClz concentration.
B
2
0.100 l'O0Io.'
10'
10.3
lo.'
lo.'
Id
ruq-m
Figure 3. Normalized stability ratios for bare and polymer-
coated latex dispersions as a function of AlCh concentration.
concentration for NaCl, BaC12, and AlC4. The stability ratio is close to unity for fast aggregation and approaches infinity for a completely stable dispersion. The dispersion stability of bare latex and Pluronic L-43 coated latex as a function of electrolyte concentration and counterion valence have been reported and discussed earlier' and are included in this study for purposes of comparison. As shown in Figures 1-3, increasing the PEO chain length e l i m i i t e s fast aggregationregardless of salt concentration and counterion valence. This effect is clearly shown by comparing the stability behavior of Pluronic L-43/latex and Pluronic L-35/latex systems in which the total molar masses of the adlayers are virtually identical, but the PEO molar mass is quite different. Similar stability behavior is observed between the Pluronic L-35/latex and Pluronic L-44/latex systems, which is not surprising since the two copolymershave almost identical PEO molecular weights. The reproducibility of the stability ratios is within 5% so
that the difference between the Pluronics L-35 and L-44 results is statistically significant. The dispersion stability of Pluronic L-35/latex or Pluronic L-44/latex systems is observed to be at least 2 orders of magnitude greater than the Pluronic L-43/latex system in the presence of NaCl or BaCl2. Such a large increase in the stability ratio with an increase in the chain length of PEO was not observed in the presence of AlC4, but an increase was still observed as evident in Figure 3. A decrease in stability is observed at high electrolyte concentrations for which electrostatic effects are removed. Interestingly, at even higher salt concentrations an increase in stability is observed for all electrolytes investigated. The change in the second virial coefficient, Bz, with increasing salt concentration determined from intrinsic viscosity measurements using eq 3 is shown in Figure 4. The second virial coefficient decreases with increasing concentrations of KC1, BaClz, or AlCls. The monovalent cation is the most effective in reducing B2, followed by the divalent and then the trivalent cations. A second set of viscosity experiments was performed with nitrate as the anion. The results have the same trends as observed with chloride as the anion.
Discussion The effect observed in increasing the MW of the stabilizing moiety is to change the system from marginally stable to robustly stable with respect to electrolyte concentration. For the low MW adlayer, the steric repulsion is comparable to electrostatic repulsion and thus stability is reduced upon the collapse of the electric double layer. Electrostatic repulsion is small compared to the steric repulsion for the higher MW PEO adlayers, giving a dispersion stability that is more robust upon the saltingout of the electric double layer. The steric repulsion of the higher MW PEO stabilizer is longer range than the lower MW stabilizer since the adlayer thickness is greater. The adlayer thickness of Pluronic L-43 was estimated to be 20 A.l The adlayer thickness of Pluronic L-44 was measured using PCS to be between 40 and 50 A by taking the differencebetween the hydrodynamic radii of the bare latex and the Pluronic L-44/latex system. There is some uncertainty in the measured value of the adlayer thickness; since the adlayer is only about 10% of the bare particle size, it almost lies within the scatter of the data. Although Pluronics L-44 and L-35 have approximately the same MW of PEO, the anchor polymer, PPO, ia of differing molecular weight. However, the anchorpolymer appears to have no significant effect on dispersion stability.
2614 Langmuir, Vol. 8, No. 11,1992 This is in agreement with findings by Napper17J8in which the cft was found to be relatively insensitiveto the presence and nature of the anchor polymer. Before the minimum in stability is reached, the dispersion stability of Pluronic L-35 is generally lower than Pluronic L-44 for all counterions examined. This is most likely due to Pluronic L-35 having a slightly smaller average MW than Pluronic L-44. Figures 1-3 show a decrease in stability for Pluronics L35 and L44 coated systems at relatively high salt concentrations. At the salt concentrations for which this decrease is observed, the sterically stabilized dispersion is undergoing weak flocculationfrom the reduced solvency of the polymer adlayer in the dispersion medium. As is evident in Figure 4, the solvency decreases with increasingly concentrated salt solutions. The polymer solvency, which may be described by either B2 or the Flory-Huggins x parameter, has been predicted by theory to be independent of polymer MW and concentration as well as electrolyte concentration. However, x has been shown experimentally to depend on all of these factors to some degree.2 Current expressions describing steric repulsion do not account for any of these effects. In Figure 4, the decrease in polymer solvency with increasingelectrolytewas tracked by measuring the second virial coefficient,B2. The second virial coefficient of PEO in pure water was determined to be 10.7 X lo4 cm3mol/g2. Poly(ethy1ene oxide) is strongly hydrated in solution through hydrogen bond formation with the ether oxygen.19*20 This salting-out effect is reflected in a collapsing of the polymer coil in solution. The potency of an electrolyte in reducing the second virial coefficient or the cft is directly related to its ability to convert water into a poorer solventfor the polymer.52l However,the complete mechanism behind the influence of electrolytes on the polymer solvency in water is still not understood. For electrolyte concentrations above 0.5 M the results are in accord with the literature6J1 on flocculation effectiveness, in which the monovalent cation is the most effectivein reducing polymer solvency. The order of cation flocculationeffectivenessdoes not appear to be so evident below this concentration. Napper generally examined flocculation effectiveness at salt concentrations of 2 M. Boucher and Hines studied the decreasein the cft of many inorganic salts including NaC1, KC1, CaC12, and MgCl2 over a concentration range of 0.5-3 M. Examination of the effect of counterion valence for latexes coated with the high MW PEO in Figures 1-3 showed that the concentration at the onset of weak flocculationwas of the order of 0.1, 0.01, and 0.001 M for NaC1, BaC12, and Ac13, respectively. This is not necessarily a contradiction to the flocculation effectiveness mentioned above since the di- and trivalent systems weakly flocculate at a concentration less than 0.5 M where the order of effectiveness is not as easily discernable, on the basis of Figure 4. For the Pluronic L-35/latex and Pluronic L-44/latex systems the stability is observed to pass through a minimum and then to increase with salt concentration for all counterions examined. Cation binding to the PEO is a possible explanation of this observation. An apparent association of cations has been observed with the PEO chain in solution. Both Na+and Ba2+have been observed (17) Napper, D. H.; Netschey, A. J . Colloid Interface Sci. 1971, 37, 528. (18) Napper, D. H. J. Colloid Interface Sci. 1969,29, 168. (19) Molyneux, P. In Water: A Comprehensiue Treatiee; Franks, F., Ed.;Plenum Press: New York, 1975; Chapter 7. (20) Antonsen, K. P.; Hoffman, A. S. In Biomedical Applications of
Polyethylene Glycol Chemistry; Harris,J. M., Ed.: Plenum: New York, to be published. (21) Erlander, S. R. J. Colloid Interface Sci. 1970, 34,53.
Eihcrrson and Berg
to complex with poly(ethy1ene oxide) in a manner resembling a crown ether in which the cation is held in a coiled polyether helix.12 An estimated six to seven ethylene oxide units are required to form the cation/polymerc0mplex.2~.23 The crown ether formation has been supported by nuclear magnetic relaxation studies24in an aqueous medium and by NMR22in acetonitrilefor poly(ethy1eneoxide)solutions containing salt. Yanagida22found the ability of PEO and Pluronics to extract an ion from solution to increase with the number of ethylene oxide units in the polymer chain. As the cation binds to the PEO to form a cation/polymer complex, the polymer chain may adopt a more elongated conformation as well as the possible occurrence of chain stiffening. These effects may be the cause of the observed increase in stability at high electrolyte Concentrations.At such high electrolyteconcentrations,for which this increase is observed, all electrostatic effects of the charged latex or the cation/polymer complexesshould be fully shielded. A similar decrease and subsequent increase in dispersion stability with increasing electrolyte concentration was recently observed by Virden26for PEO-stabilized vesicles upon addition of CaC12. An attempt was made to model the stability behavior of the lo00 MW adlayer systems. Both the bare and the 650 MW adlayedlatex systems were modeled earlier.' Current expressions for steric repulsion consider polymer solvency to be independent of electrolyte concentration. An exponential expression describing the decrease in Bz with increasingsalt concentrationwas obtained from curve fitting the change in second virial coefficientdatain Figure 4 for NaCl. The solid line drawn for the monovalent case in Figure 4 disregards three points at the lowest concentrations to present a monotonic relationship. Polymer solvency is a function of polymer MW in the oligomeric range. The exact cutoff beyond which polymer solvency is independent of MW has been reported2 to be in the range between lo00 and 2000,so at a PEO MW of lo00 the dependency is small if not absent. The simplistic exponential expression was incorporated into equations describingsteric repulsion. The stericrepulsionexpression was combined with the appropriate electrostatic repulsion and van der Waals attractive potential energy expressions as described earlier' to obtain the total interaction potential energy. The Marmur was used to relate the stability ratio to the pair interaction potential energy. A value of 4 X J, the overall best-fit value obtained from modelingthe bare latex case,was used as the Hamaker constant. An adlayer thickness of 45 A and a MW of 1o00, as specified by the manufacturer for the PEO in the block copolymer, were used for describing steric interactions. For the effectivevolume fraction of polymer in the adlayer, the best-fit value from modeling the dispersion stability of the lowest MW polymer adlayer system of 0.031 was used. In this previous modeling, polymer solvency was treated as independent of salt concentration. Figure 5 compares the experimental stability ratios of the higher MW adlayers, Pluronics L-35 and L-44, to the stability ratios determined from the model as a function of NaCl concentration. At low salt concentrations, the stability decreases as the electric double layer is progressively shielded, such that the electrostatic repulsion is negligible by a NaCl concentration of 0.07 M. Above this concentration the dispersion stability, which levels off (22) Yanagida, S.;Takahashi,K.; Okahara, M.Bull. Chem. SOC.Jpn. 1977,50,1386. (23) Crabb, N. T.;Persinger,H. E.J. Am. Oil Chem. SOC.1964,41,752. (24) Florin, E. Macromolecules 1985,18, 361. (25) Virden, J. W.; Berg, J. C. J . Colloid Interface Sci., in preee. (26) Mannur, A. J . Colloid Interface Sci. 1979, 72, 41.
Effect of Salt on Polymer Solvency : '
Langmuir, Vol. 8, No. 11, 1992 2615
-.-
L-35
- 0- .L44
ElecaosIatic
I \
Mannu model
0.
m
i
10.0
'
'
'
I
"""
0. I
1
10
NaCl m c (M)
Figure 6. Comparison of normalid experimentalstabilityratios to model calculations into which the effect of electrolyte on polymer solvency hae been incorporated.
around a stability ratio of 850, is provided by steric mechanisms. The reduction in polymer solvency becomes significant at 1 M NaCl, as evidenced by the predicted decrease in stability. Had the effect of electrolyte on polymer solvency not been accounted for, the stability ratio would remain constant at the plateau value for concentrations above 0.1 M. The plateau value predicted by the model is lower than that obtained from extrapolating the experimental data to lower salt concentrations assuming the experimental data plateaus at low salt concentrations. One possible reason may be that the adlayer thickness was underestimated. An adlayer thickness of 50 A rather than 45 A increases the stability ratio plateau value from around 850 to 1370. The decrease in the
stability ratio appears to occur at higher salt concentrations for the model as compared to the experimental reeulta. Stability ratios determined from the Wang modeP7were found to lie below those determined from the Marmur model. Although the model is rough, it does serve, if only in a preliminary manner,to describe the observed decrease in stability at high salt concentrations.
Conclusions Increasing the MW of PEO,the stabilizingmoiety, from 650 to lo00 eliminates fast aggregation for all salt concentrations and counterion valences investigated in this study. Stability ratios for systems with the higher molar mass stabilizers are 2 orders of magnitude greater than that of the lower molar mass in the presence of either NaCl or BaC12. At high salt concentrations the stability of the high M W adlayedlatex dispersions decreases due to a reduction in polymer solvency. Incorporating an equation describing the effect of salt on polymer solvency into the steric repulsion expression has been shown to be a reasonable, if only approximate, first-order attempt in describing this effect. With increasingly concentrated electrolyte solutions, however, dispersion stability passes through a minimum and then increases. This behavior is observed for all three counterions investigated. Acknowledgment. We thank Jorge Sunkel for performing the viscosity experimentsand James A. Baker for useful discussions. This work was supported in part by a grant from the IBM Corp. %&@tryNO. PS, 9003-63-6;PPO,106392-12-5;PEO,2532268-3; NaC1,7647-14-5;BaC12, 10361-37-2; AlCla, 7446-70-0. ~
~
~~~
~
(27) Wang, Q.J. Colloid Interface Sci. 1991, 146,W.
