1507
Langmuir 1992,8, 1507-1508
The Electrophoretic Mobility of Hydrocarbon Particles in KCl Solutions Dave E . Dunstan Department of Chemical Engineering] Princeton University, Princeton] New Jersey 08544 Received January 7, 1992. In Final Form: April 16, 1992 The electrophoreticmobilities of docosane particles in aqueous KC1 solutions have been measured over the range 10” to 0.1 M. The measured mobilities are negative and vary with KC1 concentration. Interpretation of the mobilities using the theory of O’Brien and White’ indicates that the magnitude of the electrokinetic surface charge, u, increases while the effective { potential decreases with electrolyte concentration. A mechanism giving rise to the effective surface charge via preferential solubilization of the C1- ions in the interfacial region is postulated.
Introduction The mobility maxima with electrolyte concentration which have been observed for polystyrene latices have been an area of considerable i n t e r e ~ t . ~The - ~ reasons postulated for the observed maxima have centered around the concepts of surface hairiness and co-ion adsorption.2>6In an attempt to clarify the arguments pertaining to surface hairiness, the experiments reported herein were conducted. While polystyrene latices may have polymeric entities distending into solution, it would seem highly improbable that the uncharged short chain hydrocarbon (CZZ) would be “hairy”. That a mobility maximum is observed for particles which do not have polymeric hairs distending from the surface indicates that surface hairiness is not responsible for the mobility maxima observed for polystyrene latices.2-5 The apparent negative electrokinetic surface charge which changes with KCl electrolyte concentration is a remarkable observation. The idea of solvent perturbation in the interfacial region is posed as a possible reason for the observed behavior.
Experimental Section In all experimentsdoubly distilled water (2D)from a KMnOl/ KOH first stage was used. Aldrich LR docosane, 99% pure, was distilledunder vacuum. Fluka microselect KC1was used without further purification. The docosane particles were prepared by melting a small amount of the hydrocarbon on top of 50 mL of water in a sealed,clean conical flask. This procedure was repeated several times with fresh 2D water in order to remove any watersoluble/surface-active impurities which may have still been present after the distillation. The water was heated to approximately 50 “C (higher than the hydrocarbon melting temperature, 44 “C). The flask was then placed in a sonicator at ambient temperature and sonicated while cooling. This procedure formed stable,very low volume fraction suspensions of highly scattering particles. The average particle size as determined by light scattering in the Delsa was 0.4 pm radius with a polydispersity of 0.20. The suspension conductivity in water was 1.7 pS cm-l. All mobilities were measured using a Coulter Delsa 440 laser doppler apparatus. Measurements were made at both stationary (1) O’Brien, R. W.; White, L. R. J. Chem. SOC.,Faraday Trans. 2 1978, 74,1607. (2)Chow, R. S.;Takamura, K. J. J. Colloid Interface Sci. 1988, 125, 226. (3)van der Put, A. G.; Bijsterbosch, B. H. J. Colloid Interface Sci. 1983, 92, 499. (4)Midmore, B. R.;Hunter, R. J. J. Colloid Interface Sci. 1988,122, 521. (5)van der Put, A. G.; Bijsterbosch, B. H. J. Colloid Interface Sci. 1980, 75,512. (6)Zukoski, C. F.;Saville, D. A. J. Colloid Interface Sci. 1988, 114, 45. (7)Voegtli, L. P.; Zukoski, C. F. J. Colloid Interface Sci. 1991, 141, 92.
layers with the reported values being taken as the average of the two which were in agreement to within 10%. Consistencybetween the two values being used to indicate that no contaminationwas present in the cell. The mobility values were measured as a
series by taking a stock solution of the suspension in water and adding small aliquots of KC1 solutions to dilute to the final concentrationby mass. Solutionswere stirred after addition of the KC1 and left for approximately 5 min before the mobilities were measured. In this way the volume fraction of the suspension was kept approximately constant and the same particles were used in all measurements. Any subtleties in the preparation of the particles were eliminated using this procedure.
Results and Discussion Figure 1shows the measured electrophoretic mobility versus KC1 concentration for the docosane particles. Purging the suspension with NOincreased the mobility from-2.9 fim cm/sV in CO2-saturated water (pH = 5.8,1.5 X lo* M H2C03, conductivity 1.7 pS/cm) to -4.0 pm cm/ M H+/OH-). SV(pH = 7.0, conductivity0.5 pS/cm, 1X The electric field strength was varied from 80 to 400 V/cm with no observed variation in the electrophoretic mobility in water and at 1.07 X M MC1. This indicates that the observed mobilities are not due to a perturbation in the suspension due to the applied electric field. It is easy to envisage the presence of surface-active contamination as being responsible for the apparent charge on the particles. The distillation of the docosane under vacuum should have removed these and other impurities. The dependence of the mobility on KC1 concentration is not explained by the presence of impurities. Bubble persistence on the KCl solutions showed there to be no detectable surface-active agents present. Significant negative mobilities were observed before the KC1 was introduced into the suspension. The increase in mobility upon removal of the H2C03 by N2 purging suggests that the observed mobilities are due to the presence of ions in the suspension and not impurities. Other workers have also measured negative electrophoretic mobilities for paraffins in electrolyte solutions. The purity of their samples is not quoted h o w e ~ e r . The *~~ same general behavior is observed in the prior work as in this study. While the hydrocarbon-water interface may have some roughness on a molecular scale, the surface should not be “hairy” in the sense that the polystyrene surface is postulated to be.2 That mobility maxima are observed for both polystyrene latex and docosane suspensions would indicate that surface hairiness is not responsible for the observed behavior. Although the mobility maxima ob(8) Abramson, H. A. Electrokinetic Phenomena; ACS Monograph; American Chemical Society: Washington, DC, 1934. (9)Mooney, M. J. J.Phys. Chem. 1931,35, 331.
0743-7463/92/2408-1507$03.00/00 1992 American Chemical Society
Letters
1508 Langmuir, Vol. 8, No. 6, 1992 -2.01
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Figure 1. Electrophoretic mobility versus KC1 concentration for docosane particles in aqueous suspension. The point at 1.5 X 10-6 is for H&03 equilibrated water. The error for each point is f0.2 pm cm/s V.
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Figure 2. (a) Interpreted { potential and (b) electrokinetic surface charge, u, for the docosane particles versus KCl concentration using the theory of O'Brien and White.' served for polystyrene latices interpret as { potential maxima,4p5which is not the case for the docosane particles, both require the electrokinetic surface charge to increase with electrolyte concentration. In this sense both are nonclassical electrokinetic systems. The difference between the two is attributed to the finite surface charge on the latices superimposed on the behavior observed for the bare hydrocarbon surface. The t potentials and electrokinetic charge densities, u, interpreted using the theory of O'Brien and Whitel are shown in parts a and b of Figure 2, respectively. These show a decreasing { potential (negative) and increasing electrokinetic surface charge (also negative)with increasing electrolyte concentration. That the effective electrokinetic surface charge increases with electrolyte concen-
tration would suggest that C1- ions are adsorbed to the surface of the hydrocarbon particles. The physical basis for this is dubious. For the ions to adsorb to the hydrocarbon surface, they must be partially removed from the highly favorable aqueous environment. In fact ions should be excluded from a region near the hydrocarbon surface not adsorbed to it. Experimental data obtained using the surface forces apparatus show that no ion adsorption to the hydrocarbon surface o c ~ u r s . ~No ~J~ repulsion of an electrostatic nature is observed between hydrocarbon surfaces up to a concentration of 0.1 M KBr in the work of Kurihara et al.l0 In fact, a long range attraction is observed between hydrocarbon surfaces, the range and magnitude of which depend on the concentration of electrolyte. It has been postulated that the hydrophobic interaction arises from the destructuring of the water in the region adjacent to the surface. It is postulated here that the effect of the hydrophobic surface on the distribution of ions in the interfacial region gives rise to the observed negative mobilities. While the ions, both K+ and C1-, are overall "excluded" from the interfacial region, a slight preferential solubility of the C1ions in the interfacial region gives rise to an apparent negative charge on the particles. The destructuring of the water causes the entropy of the water near the surface to be increased. The entropies of solubilization of the K+ and C1- ions are different12 giving rise to a different solubility in the interfacial, destructured, region. The concentration dependence of u then arises from an increase in the number of C1- ions preferentially solubilized in the interfacial region with increasing concentration. The observed negative mobilities in H2COa-equilibratedwater and Nz-purged solutions suggests that the HCOS-and OHions are preferentially solubilized relative to the H+ ion. The range of the region of preferential solubility also changes with electrolyte concentration.lOJ1 At high concentrations the ions themselves perturb the water structure significantly and the range and magnitude of the perturbation caused by the surface should therefore change. The complete picture is complicated in that both the range and magnitude of the preferential adsorption change with electrolyte concentration. The preferential solubility of the C1- ions near the surface gives rise to not only an effective surface charge but also very strong retardation effects, as the effective electrokinetic surface charge is very mobile. The mobility maximum then arises from an interplay between these two effects. At higher electrolyte concentrations the range of the perturbation region is decreased such that the retardation effects dominate and the mobility goes through a maxima. The classical electrokinetic theory would therefore appear to be inappropriately applied to these and polystyrene latex systems. In a forthcoming paper, the above model will be discussed in view of the conductivity and dielectric response data. Obviously, a freely mobile effective electrokinetic surface charge will help to account for the larger than theoretically predictable conductance and dielectric response increments measured for latex dispersions. Further work on other hydrocarbons and electrolytes is being conducted.
Acknowledgment. I thank Dudley Saville for obtaining financial support: NSF Equipment Grant CTS8910184 and NASA's Microgravity Sciences and Applications Program Grant NAG 8-878. (10)Kurihara, K.; Kato, S.; Kunitake, T. Chem. Lett. 1990, 1666. (11) Christenson, H. K.; Fang,J.; Ninham, B. W.;Parker, J. L. J.Phye. Chem. 1990,94,8004. (12)Robinson,R. A.;Stokes, R. H. Electrolyte Solutions;Butterworth London, 1959.
