2066
Langmuir 1991, 7 , 2066-2071
Microelectrophoresis of Silica in Mixed Solvents of Low Dielectric Constantt Marek Kosmulski* and Egon Matijevie' Department of Chemistry, Clarkson University, Potsdam, New York 13699 Received March 4, 1991
Electrophoretic mobilities of monodisperse, spherical silica particles (300 and 700 nm in diameter) in dioxane/water mixtures have been studied at 1:1 electrolyte concentrationsover the range 10-6-10-'mol dm4. At dioxane contents of 80-9596 (w/w), the potentials are negative in KOH solutions and positive at sufficiently high HC1 concentrations. The addition of KC1 in increasing amounts leads to a decrease in the mobility of silica in 80% dioxane/water, while at 90 and 95% dioxane, the charge reversal from negative to positive takes place. An analogous effect is observed with RbCl and CsCl, but not with LiCl and NaC1. At still higher dioxane concentrations (>97.5%), the mobilities approach zero within the experimental error at the studied range of electrolyte concentrations.
Introduction Traditionally, the stability of lyophobic colloids in aqueous and nonaqueous systems is considered as a separate problem. (In this paper nonaqueous means that water is not the main component of the dispersing phase, although the liquid medium may contain water.) In both cases the behavior of dispersions is dependent on the properties of the electrical double layer, with the (-POtential as a measurable parameter; thus, regardless of the medium, the systems should show some similarities. However, the experimental conditions of {-potential determinations in aqueous and nonaqueous dispersions are different. In the former case, such measurements are carried out at a well-defined ionic strength (usually with a "neutral" 1:l electrolyte) and a known concentration of the potential determining ions (pdi). For many sols (e.g., simple and composite oxides) hydrogen and hydroxyl ions play the role of pdi, and consequently, the surface potential is defined by the pH. The studies in nonaqueous media are usually done in one of two ways. In the first case both the solid and the liquid are subjected to more or less sophisticated purification, especiallyto drying, so that the effective {-potential is considered to be due to the solid surface/solvent interaction only. Typical examples of such a treatment are early electroosmosis investigations by Coehn, who formulated the rule that solids of a higher dielectric constant assume positive charge in contact with a medium of a lower dielectric constant, t. The importance of the latter on colloid stability and (-potential has been affirmed in many studies.lt2 More recently, Labib and Williams3s4argued that the (-potential of solid particles in organic solvents may be predicted by taking into account the acidity of the solid (for oxides and related materials defined by the position of the isoelectric point (iep)) and the donicity and the acceptor number of the liquid.6 High proton affinity of Supported by a contract with the XMX Corp., Burlington, MA. t On leave of absence from the Laboratory of Adsorption and Physical Chemistry of Surfaces, Polish Academy of Sciences, Lublin, Poland. (1)Dukhin, S. S. In Surface and Colloid Science.; MatijeviC, E., Ed.; Wiley-Interscience: New York, 1974;Vol. 7,p 1. (2)Lyklema, J. Adu. Colloid Interface Sci. 1968, 2, 65. (3)Labib, M. E.;Williams, R. J. Colloid Interface Sci. 1984,97,356; 1987,115, 330. (4)Labib, M.E. Colloids Surf. 1988,29, 293. (5) Gutmann, V. Coord. Chem. Rev. 1976,18, 225.
the liquid and high acidity of the solid are favorable conditions for negative {-potentials. The second group of studies deals with organic solvents containing controlled amounts of so1utes,2p6 most frequently ~ a t e r , 3 in - ~the - ~ presence of e1ectrolytes"ll and surfactants.7J2J3 Early reports have already shown that low concentrations of electrolytes strongly affect the {-potential in organic solvents. Increasing concentrations of salts decrease the mobility, while acids and bases may cause charge reversals1 The latter phenomenon is also observed in aqueous systems, indicating a similarity in the charging mechanism in different solvents. An elementary calculation shows that a very low concentration of an electrolyte in an organic medium is sufficient to produce a high {-potential. The electric charge per particle, q, at a very low salinity (Ka 0, where K is the Debye parameter) may be estimated from Coulomb's law with J, = fi
-
q = 4m(a (1) Assuming a particle radius a = 0.15 km (as used in the present study), t = 5.6 (90% dioxane), and a liquid to solid ratio of 25 000 (typical for electrophoretic measurements), eq 1 shows that {-potential of 50 mV corresponds to the adsorption of 2 X lo-" mol of potential determining ions from 1kg of the solution. This amount is much below the sensitivity of analytical methods, and the content of ions in even the best purified organic liquids is expected to be higher. Obviously, exceedingly low concentrations of pdi may cause the formation of an electrical double layer, which affects the stability and the electrokinetic mobility of a colloid dispersion. For this reason, in the present study water content and salinity were adjusted instead of attempting to purify and dewater the system. Experimental and theoretical difficulties related to the stability and electrical properties of nonaqueous disper(6) Parfitt, G. D.; Peacock, J. In Surface and Colloids Science; Matijevib, E., Ed.; Wiley-Interscience: New York, 1978;Vol. 10, p 163. (7) Kandori, K.; Kon-no, K.; Kitahara, A. Bull. Chem. SOC.Jpn. 1984, 57, 3419. (8)Hidalgo-Alverez,R.; Delgado, A.; Callejas, J.; Gonzalez Caballero, F. Colloid Polym. Sci. 1985, 263, 941. (9)Lotenberg, E. H. P.; Stein, H. N. Colloids Surf. 1986, 17, 305. (IO) Hudson, G. F.; Raghavan, S. Colloid Polym. Sci. 1988, 266, 77. (11)Goodwin, J. W.; McDonald, F.; Reynolds, P. A. Colloids Surf. 1988, 33, 1. (12)Chowdiah, P.; Watson, D. W.; Gidaspow, D. Colloids Surf. 1983, 7, 291. (13) Kosacz, J. Mater. Sci. Forum 1988,25-26, 413.
0743-7463/91/2407-2066$02.50/0 0 1991 American Chemical Society
Langmuir, Vol. 7, No. 10, 1991 2067
Microelectrophoresis of Silica in Mixed Solvents
sions have been amply emphasized. Essential progress with respect to earlier work may be achieved (a) by the use of monodispersed, spherical colloidal particles,14instead of polydispersed systems or solids of irregular shape, and (b) by carrying out experiments in the new generation of electrophoresis equipments, which give a histogram of mobilities instead of a mean value and avoid subjective visual observation of particle movement. These features are especially important for measurements carried out in partially coagulated dispersions, in which the mobility of aggregates is different from that of single particles.
Experimental Section The following chemicals were used as delivered: l,4-dioxane HPLC (Aldrich),KCl (AR, Mallinckrodt), NaCl, LiCl, and Dilutin KOH and HCl (Baker), RbCl and CsCl (ultrapure, Johnson Matthey), bromophenol blue (Fisher), bromocresol green, methyl red, and phenol red (Spectrum). Dioxane was proved to be peroxide-free by the iodide method.16 Doubly distilled water had a conductance of 1pS cm-l. Two dispersions of silica consisting of uniform spherical particles were prepared by hydrolysis of tetraethyl orthosilicate (TEOS) in the presence of ammonia.lB The respective diameters, as determined from electron micrographs, were d = 0.3 and 0.7 pm, and the corresponding surface areas were 11.3and 6.6 m2g1, as determined by nitrogen adsorption (Quantasorb apparatus). The particle diameter of the first sample, determined from the diffusion broadening of the mobility histogram peaks in aqueous systems, was 0.3 pm, in agreement with the above value. The analysis of the diffusion broadening in nonaqueous systems led to a particle size of 0.4 0.1 pm, suggesting that the mobility peaks were indeed due to single particles. Portions of concentrated silica dispersions (40 w t %) were diluted with a 10-fold amount of water and washed to remove excess ammonia until the conductance of the supernatant solution fell below 20 p S cm-l. Portions of such a diluted and washed dispersion were combined with the mixed solvent and electrolyte solutions in proportions necessary to achieve a desired solvent composition, electrolyte concentration, and solid/liquid (w/w) ratio of 1:25000 for d = 0.3 pm and of 1:7000 for d = 0.7 pm silica particles. Additional experiments were carried out with the latter sample of a ratio 1:70000, and no difference in mobilities was observed. Since the degree of dissociationof electrolytesin mixed solvents rich in dioxane is low, albeit unknown, one cannot carry out measurements at a constant ionic strength. For this reason the results are presented in terms of mobilities, but the corresponding t-potentials are only best estimates. Silica dispersions in mixed solvents were treated in an ultrasonic bath (Branson ModelB1200R-4) for 3 min, then stored for at least 4 h a t 25 OC, and again ultrasonicated for 3 min before the beginning of the mobility measurements, Portions of the dispersions (30 cm3) were used to wash and fill the cell in the DELSA 440 system (Caulter Electronics) at 25 "C, using the frequency range of 500 Hz and the time of 50 s. The particle velocity profiles across the electrophoretic cell for various dioxane and electrolyte concentrations showed parabolic shape, symmetric relative to the cell axis, and most measurements were carried out in the upper stationary layer. The mobilities assumed more negative values at higher voltages, until a plateau was reached at 60-80 V. Above 100 V, the difference in mobilities was observed when the polarity of the electrodes was altered. Therefore, 100V was chosen as astandard electric field for measurements carried out in mixed solvents, while 20 V was applied for aqueous systems. The values of the refractive indexes of dioxane/water mixtures were interpolated from literature data."
*
(14) MatijeviE, E. Annu. Rev. Mater. Sci. 1985, 15, 483; Acc. Chem. Res. 1981, 14, 22. (15) Encyclopedia of Industrial Chemical Analysis; Snell, F. D., Ettre, L. S., Eds.; Interscience: New York, 1971; Vol. 11, p 522. (16) Staber, W.; Fink, A,; Bohn, E. J. Colloid Interface Sci. 1968,26, 62.
-
A
10"mol dm"
Hc':
-0.2-0.1
0
0.1
fi /
- 0.1
0
0.1
0.2
m2 V ' s "
Figure 1. Mobility histograms of silica (d = 0.3 pm) in a 95% (w/w) dioxane/water mixture in the presence of 1 X 1od (a) and 1 X lo-' mol dm+ HCl (b). The frame denotes the area that is integrated. The mobility histograms observed in nonaqueous systems for smaller silica particles and low solid contents consisted of two peaks as shown in Figure 1 (95% dioxane), type a being typical for mobilities of lpl 1O-O m2 V-l s-l and type b for Ipl 5X lO-"J m2 V-l s-1. In both cases, a very sharp peak occurs at zero or nearly zero mobility while the second one is broad, and in type b, it overlaps in part with the first peak. When the mobility is sufficiently high, the sharp peak completely disappears. The mobilities presented in figures to follow were calculated by integration of the part of the broad peak that is not superimposed onto the sharp peak, as illustrated in Figure 1. When two peaks were no longer distinguishable (which occurs at lpl
