as4
Ind. Eng. Chem. Res. 1993,32, 854-858
Mechanisms Influencing the Stability of a Nonaqueous Phosphor Dispersion David D. Mysko a n d John C. Berg' Department of Chemical Engineering BF-IO, University of Washington, Seattle, Washington 98195
The factors which influence the stability against flocculation of a dispersion of yttrium tantalate particles in a nonaqueous solvent system are investigated. The binder resins used in the formulation of the dispersion are found to adsorb to an appreciable extent, and should therefore confer steric stability on the dispersion. The nonaqueous solvent (butyl acetate/propanol) is found to influence the surface charge of the particle through an acid-base interaction with the surface hydroxyl groups present on all hydrate oxide surfaces. The donor number (as defined by the Gutmann donicity scale) of these surface hydroxides is found to be approximately 5.5 kcal/mol. The anionic surfactant Aerosol-OT, used as a dispersant in the formulation investigated, is found to charge the phosphor particles positively in the nonaqueous solvent. At low surfactant concentrations the charging behavior is controlled by the electrostatic interaction of the ionized surfactant with the charged surface and is characterized by a rapid change of the zeta potential with surfactant concentration. At higher surfactant concentrations, the surface charge is controlled by the relative solubilities of the surfactant ions in solution. Small amounts of water are found to influence the sign and magnitude of the particle surface charge in the presence of surfactant, as evidenced by the fact that the zeta potential passes through a maximum as the water concentration of the bulk solution is increased.
Introduction The phenomena determining the stability against flocculation of particles in suspension have been, and still are, some of the most frequentlystudied areas of colloid science. The bulk of the investigations reported in the literature have been concernedwith aqueous systems, and as a result, the behavior of dispersed particles in nonaqueous media is less well understood. The London-van der Waals attractive forces present between all molecules and particles is independent of the nature of the dispersing solvent. The only place the medium enters the calculation of the attractive forces is through the Hamaker constant, and the calculation of Hamaker constants from theoretical principles in nonaqueous solvents may actually be easier than in water, since most nonaqueous solvents lack water's nonideal, strong self-associatingproperties. The treatment of steric interactionsin nonaqueoussystems is also straightforward. Since the solvent enters the governing equations for steric stabilization through its relative solubility interactions with the adsorbed polymer molecules,nonaqueous systems (as opposed to aqueous systems) offer a wider range of suitable solvent-polymer combinations. It is the relatively low dielectric constant of organic solvents, and thus the electrostatic behavior of electrically charged particles dispersed in these solvents, which is the main difference between aqueous dispersions and their nonaqueous counterparts. Frequently an ionic surface-active dispersant is added to nonaqueous dispersions in an attempt to charge the surface of the dispersed particles and thereby electrostatically stabilize the dispersion against flocculation. In the system under study this dispersant is the anionic surfactant Aerosol-OT (sodium bis(Bethylhexy1) sulfosuccinate). Severalstudies of the zeta potential of particles dispersed in solutions of this surfactant have been published: McGown et al. (19651, Kitahara et al. (1967, 1977, 19801, and Cooper and Wright (1974). In each of
* To whom correspondence should be addressed.
these studies, trace amounts of water in the system had a significant impact on the measured particle zeta potentials. Another mechanism of surface charging in nonaqueous media is through surface group dissociation. The ionization of oxide surface groups in nonaqueous solvents has been the focus of several publications, notably Labib and Williams (1984,1986),Fowkes et al. (1982),and Huang et al. (1991). This research was motivated by the need to characterize the stabilizing mechanisms of a particular dispersion of fluorescent particles in a nonaqueous solution. The particles were to be incorporated in a polymer film. This fluorescent film is used to develop X-ray pictures (in medicine, dentistry, etc.) using photographic film, which is normally not affected by X-rays. The action of the fluorescentscreen is seen in Figure 1. When the fluorescent particles in the film are struck by X-rays, they absorb them and emit energy in the form of visible light, which exposes the photographic film directly beneath the screen. The process used to manufacture the fluorescent screen involves incorporating the phosphor particles into some type of support film. The particles used are yttrium tantalate (other phosphor materials are also in current use, but the YTa04 system is the one under current investigation), and the support film is an acrylic resin. The particles are ball milled with a solution of the acrylic binder resins (Carboset resins, B. F. Goodrich), and the dispersant Aerosol-OT in a butyl acetateln-propyl alcohol solvent system. The ball-milled dispersion is then spread very thinly onto a polyester support film with a doctor knife, and air-dried. The even distribution of particles on the film is the critical factor in manufacturing a good product. Sometimes during the drying process particle aggregation can occur, leaving a maldistribution of particles on the film, making the film unusable. This failure of dispersion stability, is not well understood; if the cause of this failure were known, steps could be taken to avoid it, thus saving time and resources.
0888-5885/93/2632-0854$04.00/0 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 855
lllllllllllll n
““nUEI”l
s m
X-Rayr
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Phompphic Film
Figure 1. X-ray screen schematic.
Materials and Methods The yttrium tantalate (YTa04) particles supplied by the Du Pont Co. had a mean particle size of approximately 6 pm, and their density was reported as 7.55 g/cm3. Due to the large density difference between the particles and the dispersingsolvents,the mean particle size was reduced by grinding to approximately 1 Fm for electrophoresis measurements. Particle sizes were determined by centrifugal sedimentation using a Horiba CAPA-500 automatic particle analyzer. The Carboset binder resins used in this study were acrylic terpolymers of butyl acrylate, ethyl methacrylate, and methacrylic acid. The Carboset 525 polymer has a molecular weight of 200 OOO and an acid number of 76-85 while the 526 polymer has a molecular weight of 175 OOO and an acid number of 104. The acid number is ameasure of how much methacrylic acid is contained in the polymer, and is defined as the number of milligrams of potassium hydroxide (KOH)needed to neutralize 1 g of the polymer in solution. The n-butyl acetate and n-propyl alcohol used in this research were both reagent grade liquids from J. T. Baker. The composition of the “butyl acetatelpropanol solvent” mixture used throughout this research was determined from the binder solution recipe, as 58.2% (by volume) butyl acetate and 41.8% (by volume) propanol. In the determination of the acid-base nature of the surface of the YTa04 particles, several solvents were used. These solvents were chosen to span a range of acidicbasic strength as measured through the Gutmann donor number (Gutmann, 1975). Measurementsof the electrophoreticmobilityweremade using the Malvern Zetasizer IIc which measures particle velocities using laser Doppler interferometry. The electrophoretic mobility of the particles was converted to a zeta potential through the use of the OBrien and White (1978) solutions to the general electrophoresis problem with the computer program Mac Mobility (availablefrom Department of Mathematics, University of Melbourne, Parkville VIC 3052, Australia). The ionic concentration in the nonaqueous solutions (in the absence of any added electrolyte)was approximatedby modeling all ionic species as K+ or C1-. The limiting ionic molar conductivity (Ao) of KC1 in water is 149.65 S cm2/mol. By adjusting this value for the viscosity of a solvent (v,Jb = T ~ A Oand ~) measuring the conductivity of a dispersion in that solvent, one can obtain the equivalent concentration of KC1 in that dispersion and thus determine the relationship between the mobility and the zeta potential. Solvent conductivities were measured using a Radiometer CDM83 conductivitymeter. Thesame approach wastakenwith the mobility experiments using Aerosol-OT. Due to the lowdielectricnatureofthesolventsusedwith the AerosolOT, complete ionization of the surfactant could not he assumed, and the conductivity approach to ionic concentration determination was again employed. The Ai, value for Aerosol-OT in cyclohexane was reported by Kitshara et al. (1967) to be 120 S cm2/mol. Again adjusting this value for the viscosity change, the molar conductivity of the surfactant in the butyl acetateipropanol solvent is calculated to be 128 S cm2/mol. It was necessary to use
the O’Brien and White solutions since the value of XU, the inverse Debye length times the particle radius, for these dispersions was always in the intermediate region between the Hackel and the Helmholtz-Smoluchowski regions (1 < K a < 100). In the determination of the affect of water on the charging behavior of the oxide particles in solution, the Karl-Fischer titration waa used to determine the concentration of water in the solutions accurately. In this study the water determination titrations were carried out in a back-titration mode due to the slowness of the KarlFischer reaction at ambient temperatures. The titrations were performed using a Radiometer RTS-822 automated titration system. The resolution of a titration performed by this method was typically within lC-20 ppm of water. The drying of the dispersions was carried out by removing as much contaminating water as possible from both theoxide particlesurfaceand thedispersingsolutions. The solventswere dried using an activated molecular sieve (type 4A) from J. T. Baker Chemical Co. The surfactant Aerosol-OT was dried by storage in a nitrogen-filled vacuum desiccator over PzO5 at a pressure of 1-2 mmHg, for at least 1 week. Aerosol-OT in its pure form is a very strong hygroscopic agent, and even after 2 months in the desiccator, solutions made from the surfactant still contained measurable, albeit small, amounts of water. The YTaOa particles were dried by heating the particles to a temperature of 250 “C at a pressure of approximately 2 X 1o-L Torr. The particles were held at this temperature and pressure for at least 2 h, and upon removal of the vacuum heating, they were immediately removed to a drybox containing P2O5, which was replaced daily. AU subsequent water-sensitive dispersions were prepared in this drybox. Results and Discussion Adsorption isotherms of the Carboset resins in the butyl acetate/propanol solvent onto the yttrium tantalate particles were obtained gravimetrically, allowing adsorption equilibriation times of 48 h. The adsorption isotherms seem to follow Langmuir type adsorption kinetics according to the equation
Using linear regression the constants which characterize the adsorption, namely rm and K , were determined. For the Carboset 525 we obtain rm = 1.85 mg/mz and K = 1.139 L/g. For the 526 resin the constants are calculated to be r, = 1.427 mg/m2 and K = 0.9908 L/g. The acidity or basicity of the surface hydroxyl groups present on all metal oxide surfaces, due to chemisorption ofwater, aredetermined through the electrophilicstrength of the metal atom in the oxide. With some information as to the charge and radius of the metal ion in the lattice responsible for the surface charging, Parka (1965) postulated that one should be able to roughly predict the isoelectricpoint (IEPS) of the oxide in an aqueous system. In the case of yttrium tantalate, however, there are two possible metal ions which may determine the charging behavior of the oxide surface, namely Y3+and Ta5+. The yttrium(II1) ion is a fairly basic ion and should produce a surface similar to that of alumina (A1203, IEPS = 9). The tantalate(V) ion on the other hand is a very acidic ion, and if it were responsible for the charge of the oxide surface,it would produce a surfacesimilar to or more acidic than that of silica (SO2,IEPS = 2). Experiments were
856 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993
shear plane from a radius close to the particle surface to one near the outaide of the polymer adlayer, one would be able to calculate the approximate polymer adlayer thicknesa from the Gouy-Chapman approximationof the diffuse double layer according to tanh( 4-68.0 '
T,,
:!,AcclicMhydnds , , '
5f0
10.0
15.0
20.0
25.0
30.0
Solvent DN ( kcal I mol )
Figure 2. YTa04 acid-base charging in organic media. 2
4 0.4
t/ -32.0 0
2
4 6 Carboset 525 Cow. (Pn)
8
10
Figure 3. Effect of polymer adsorption on YTa04 zeta potential.
therefore performed in order to determine which of these two ions seemsto be responsible for the acid-base charging behavior of the surface. Dispersions of YTa04 were prepared in solvents with a wide range of donor numbers as in the work of Labib and Williams (1984). The zeta potential of the phosphor particles in each of these solvents was measured, and the results can be seen in Figure 2. The surface donor number of the phosphor particles is approximately 5.5 kcal/mol. Thisvalue for the surface donor number (DN) agrees much more closely with that of alumina (DN = 7 kcal/mol) as opposed to silica,which remains negatively charged in this series of liquids and is therefore more acidicthan the lowest member of this series according to Labib and Williams (1986). One may then conclude that it is the Y3+ion which is mainly responsible for determining the acid-base charging behavior of the YTa04 particle. The acid-base interactions studied through the effect of solvent donor number on acquired particle charge may take place not only between the particle surface and the solvent, but also between the solvent and Carboset polymer and/or between the Carboset polymer and the particle surface (Fowkes et al., 1982). Investigations were conducted to determine if the acidic of basic nature of the Carboset resins (from either the basic ester groups or the acidic carboxylic acid groups) is such that they may be involved in proton transfer with either the surface of the particles (and through desorption charge the surface) or the solvent (and through adsorption charge the surface). If either of these scenarioswere to occur, one would observe a correlation between the amount of adsorbed polymer present on the particle surface (the adsorption isotherm) and the measured zeta potential of the polymer adsorbed particles. Figure 3 demonstrates the profound effect that adsorption of even a small amount of polymer (the adsorption isotherm is represented by the solid line) has on the zeta potential. If one were to attribute the decrease in observed zeta potential solelyto a shift of the particle hydrodynamic
$)
= tanh(
f)
eXp(-KA)
(2)
e
Here and T b are the dimensionless zeta potentials = {e/kT) at radii a and b from the particle surface (in this case, the hydrodynamic shear plane with and without adsorbed polymer, respectively),z is the counterionvalency (assumed to be l), K is the reciprocal of the Debye length (calculatedby modeling the conductivityof the dispersion as attributable to KCl), and A is the polymer adlayer thickness. Using the above equation, one can calculate the polymer adlayer thickness to be approximately 150 nm, 6 times the hydrodynamic size of a free polymer chain in solution (25 nm) as measured by photon correlation spectroscopy using a Brookhaven BI-2030AT light scattering system operating at 90'. It seems improbable that this magnitude of polymer chain expansion can be attributed to the interaction of neighboring polymer molecules at the particle surface (deGennes, 1980), and this fact seems to suggest that a portion of the decrease in the zeta potential may indeed be caused by proton transfer from the polymer to the particle surface and/or from the solvent to the polymer (and then to the particle surface through polymer adsorption). While the chain expansion calculated from the decrease in the zeta potential may be extreme, the polymer adlayer is most definitely thicker than the free polymer size in solution. It is well-knownthat even a few chains extending beyond the average adlayer thickness (these longer chains could be accounted for by the large polydispersity (0.4) of the resin's molecular weight) will appreciably affect the effective hydrodynamic particle size. Therefore, any method which uses hydrodynamics to measure effective particle sizo or adlayer thickness (such as electrophoresis) will inevitably measure an increase in the radial position of the shear plane due to polymeric adsorption which is larger than the actual adlayer thickness. A more rigorous treatment of the hydrodynamic thickness of an adsorbed polymer layer in the presence of shear is treated by Anderson et al. (1991). The investigation into the charging behavior of the phosphor particles through ion adsorption from solution began with a study of the effect of the concentration of the surfactant Aerosol-OT on the zeta potential of the particles. The first series of electrophoretic mobility measurements performed were those to determine the effect of Aerosol-OT on the zeta potential of the phosphor particles, with the particles, surfactant, and solvent in an "asreceived" condition. In this condition the oxide particles will have a layer of water adsorbed to the hydrophilic particle surface. The surfactant solutions in which these particles were dispersed also contained anywhere from 400 to lo00 ppm of contaminating water. The results (Figure 4) show a maximum in particle mobility (at approximately 10-12 mM Aerosol-OT) as the concentration of the surfactant is increased. This result has been observed before for an Aerosol-OT/oxide combination in a nonaqueous solution (McGown et al., 1966). The steep initial rise is caused by cation (sodium) penetration into the adsorbed water layer on the oxide particle. The decrease in the mobility after the maximum is theorized to be caused by two factors: the increased
Ind. Eng. Chem. Res., Vol. 32, No. 5 , 1993 857 35
30
30
20
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0 75
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Figure 4. YTa04zeta potential for the "wet" and 'Dry" cases as a function of Aerosol-OT concentration.
Figure 5. Variation of YTa04 zeta potential with bulk water and Aerosol-OT concentration.
adsorption of anionic surfactant molecules at the oxide surface which is enhanced by the anion's electrostatic attraction to the cation-rich water layer and/or the loss of some of the cation-rich water layer to the interior of reversed Aerosol-OT micelles (in solution) which are more abundant at the higher Aerosol-OT concentrations. As one might infer from the theorized charging mechanisms, interfacial water plays an important role in this system. The experiments were repeated with particles which had been dried under vacuum and solutions from which most moisture had been removed. The results (Figure 4) show that in the "dry" state the magnitude of acquired charge is less. This can be explained by the fact that the positive charge on the particle surface is brought about by preferential penetration of the positive sodium ions to the hydrophilic surface layer of the particle. In the dry state this layer is much thinner, and therefore the positive charge is diminished. As expected, water concentration in solution and adsorbed water at the particle surface are important parameters in determining the magnitude and sign of the acquired particle charge. Therefore the next step in the investigation was to quantify the effect of water by determining the charging behavior of the phosphor particles as a function of solution water concentration, while holding the surfactant concentration fixed. The bulk water concentration was increased until water-in-oil emulsion formation was apparent. By varying the water concentration in solution, and dispersing dried particles from which the bulk of adsorbed surface water had been removed, there were varying amounts of physisorption of bulk water at the the particle surface (this adsorbed water layer will eventually condense to form a film around the particle at higher bulk water concentrations). The dispersions produced in this method were of such low weight percent solids, and the solids were of such a low specific surface area, that the adsorption of this water at the particle surface did not appreciably affect the bulk water concentration. The results of this investigation are presented in Figure 5, which demonstrates the behavior that has been observed in many other nonaqueous Aerosol-OT dispersions (McGown et al., 1965; Kitahara et al., 1967, 1977, 1980; Cooper and Wright, 1974, a maximum (or minimum) in particle zeta potential with increasing water concentration. Figure 5 shows that this maximum can occur at a much higher water concentrations than has previously been observed. In the past, these types of experiments have been performed in solvents such as cyclohexane, heptane, and xylene: very nonpolar, low permittivity solvents. Even with the solubilizing power of the surfactant, the water solubility in these solvents is much lower than in the solvent system studied here. In
the past when such water-semisoluble systems (such as the butyl acetate/propanol system used in this investigation) were studied, no appreciable change was seen in the water concentration range studied for hydrocarbon solvents, and it was postulated that water played little if any role in particle charging (Cooper and Wright, 1974). This, as can be seen in the graph, is not the case; rather it seems that the curve is simply spread out over a wider range of water concentrations. The final step performed in this investigation was to determine whether the particle's initial charge in the absence of surfactant (from acid-base interaction of the oxide surface groups with the solvent) plays an important role in the sign and magnitude of the particle's charge in the presence of surfactant, or if the charging mechanism is mainly driven by the relatively lipophobic character of the sodium cation (and its affinity for the hydrophilic particle surface). This distinction was unresolvable in the previous experiments due to the fact that the ion which carried the opposite charge of the particle surface in the absence of surfactant and the less soluble ion in solution were the same (Na+). The question was answered by measuring the electrophoretic mobility of the phosphor particles in an acidic solvent (one which charged the particle surface positively in the absence of surfactant) at various Aerosol-OT concentrations. The solvent chosen for this investigation was the most acidic solvent (DN = 0.0) used in the acid-base investigation, 1,2-dichloroethane. If the only driving force for particle charging in the presence of the surfactant Aerosol-OT is the hydrophilic nature of the sodium ion, one would expect the trend for particle charging in an acidic solvent such as dichloroethane to be the same as the trend in the butyl acetate/ propanol solvent, except that the whole curve would be shifted in the positive zeta potential direction an amount equal to the difference between the zeta potential in each solvent at zero Aerosol-OT concentration (Figure 6a). If electrostatics played a dominant role in particle charging (i.e., if the initial electrostatic charge on the particle surface caused increased adsorption of the counterion from solution), one would expect the trend in the acidic solvent to be a mirror image of the charging trend in the basic butyl actate/propanol system (Figure 6b), since the roles of the ceion and counterion would be reversed. The results from this investigation are presented in Figure 7. As can be seen in this figure, the hydrophilic nature of the sodium ion does play a role in the charging of the phosphor particles in 1,2-dichloroethane. This is evidenced by the approach to a finite positive zeta potential at high surfactant concentrations. As the difference in solubility between the surface-active anion and the hydrophilic cation increases, one would expect this finite positive zeta potential to increase as well. The most
858 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993
1
Aaosol-OT concarwfion (mu)
I
Amsol-OT Conccnmtion (mu)
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Figure 6. Theoretical charging scenarios: (a) solubility controlled; (b) charge controlled. I
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surfactant. Through electrostatic interaction of the surface with the ionized surfactant, there is increased adsorption of the oppositely charged ionic species. The asymptotic value of the zeta potential with increasing surfactant conentration is, however, determined by the permittivity of the solvent (i.e., solubility of surfactant ions and counterions in the solvent). As expected, contaminating water was found to play a major role in determining the sign and magnitude of the acquired charge of the particle surface in the presence of surfactant. The magnitude of the zeta potential passed through a maximum as the water concentration of the bulk solution was increased. This maximum in zeta potential was on the order of 8-10 mV greater than the zeta potential of the dry dispersion of the same surfactant concentration. With a further increase in bulk solution water concentration past the maximum, the zeta potential decreased to 0.
"
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A-OT Conc. (mM)
Figure 7. YTa04 zeta potential vs Aerosol-OT concentration in 1,2-dichloroethane and butyl acetate/propanol solvents.
interesting finding from Figure 7 is the degree to which electrostatics plays a role in the charging of this system. The drop in zeta potential with the addition of even minute amounts of surfactant is extremely rapid and demonstrates the preferential adsorption of the surface-active anion due to electrostatic attraction to the positively charged particle surface. Conclusions The Carboset resins were found to adsorb to the particle surface to an appreciable extent. Equilibrium adsorption was attained after approximately 40 h of adsorption time, with plateau adsorption of the order of 1.5 mg/m2. The conformation of the adsorbed chains is quite extended from the surface, due to a combination of good solvency and adsorbed-chain interactions at the particle surface, which should produce significant steric stabilization. The charging of the phosphor particles by the anionic surfactant Aerosol-OTin a butyl acetate/propanol solvent is characterized by charge reversal from negative to positive zeta potentials at low surfactant concentrations. Upon further increase of surfactant concentration, past approximately 10 mM, the zeta potential approaches a limiting value of approximately 25 mV. The role of the solvent in determining the charging behavior of the particles in solution is 2-fold. The solvent influencesthe surface charge through acid-base interaction with surface hydroxyl groups. The surface donor number of YTa04 was found to be approximately 5.5 kcal/mol. This surface charge due to acid-base interaction of the particle with the solvent will then influence the charging behavior of the phosphor particles in the presence of
Acknowledgment This work was supported by a grant from the Du Pont Company. Literature Cited Anderson, J. L.; McKenzie, P. F.; Webber, R. M. Model for Hydrodynamic Thickness of Thin Polymer Layers at Solid/Liquid Interfaces. Langmuir 1991,7, 162-166. Cooper, W. D.; Wright, P. Electrophoresis of Colloidal Copper Pthalocyanines in Low Permittivity Media. J. Chem. SOC., Faraday Trans. 1 1974,70,858-867. de Gennes, P. G. Conformations of Polymers Attatched to an Interface. Macromolecules 1980,13,1069-1075. Fowkes, F. M.; Jinnai, H.; Hostafa, M. A.; Anderson, F. W.; Moore, F. W. Mechanism of Electric Charging of Particles in Nonaqueous Liquids. In Colloids and Surfaces in Reprographic Technology; Hair, M., Croucher, M. D., Eds.; American Chemical Society: Washington DC, 1982. Gutmann, V. Solvent Effects on the Reacitvites of Organometallic Compounds. Coord. Chem. Rev. 1975,18,225-255. Huang, Y. C.; Fowkes, F. M.; Lloyd, T. B. Acidic and Basic Nature of Ferric Oxide Surfaces. J. Adhes. Sci. Technol. 1991,5,39-56. Kitahara, A.; Karasawa, S.; Yamada, H. The Effect of Water on Electrokinetic Potential and Stability of Suspensions in Nonpolar Media. J. Colloid Interface Sci. 1967,25,490-495. Kitahara, A.; Ammo, M.; Kawasaki, S.;Kon-no,K. The Concentration Effect of Surfactants on Zeta Potential in Non-aqueous Dispersions. Colloid Polym. Sci. 1977,255, 1118-1121. Kitahara, A.; Tamura, T.; Kon-no, K. Effect of Water on Flocculation of Carbon Black in Nonaqueous Surfactant Solutions. Sep. Sci. Technol. 1980,15,249-261. Labib, M. E.; Williams, R. The Use of Zeta-Potential measurements in Organic Solvents to Determine the Donor-AcceptorProperties of Solid Surfaces. J. Colloid Interface Sci. 1984,97,356-366. Labib, M. E.; Williams, R. An Experimental Comparison between the Aqueous pH Scale and the Electron Donicity Scale. Colloid Polym. Sci. 1986,264, 533-541. McGown,D. N. L.; Parfitt, G. D.; Willis, E. Stability of Non-Aqueous Dispersions: I. The Relationship Between Surface Potential and Stability in Hydrocarbon Media. J . Colloid Sci. 1965,20,650664. O'Brien, R. W.; White, L. R. Electrophoretic Mobility of a Spherical Colloidal Particle. J . Chem. SOC.,Faraday Trans. 2 1978, 74, 1607-1626. Parks, G. A. The Isoelectric Points of Solid Oxides,Solid Hydroxides, and Aqueous Hydroxides, and Aqueous Hydroxo Complex Systems. Chem. Rev. 1965,65, 177. Received for review September 28, 1992 Revised manuscript received January 25, 1993 Accepted February 9,1993
