ELECTRICAL PROPERTIES OF AQUEOUS COLLOIDAL DISPERSIONS (DIELECTRIC RELAXATION, ELECTROKINETIC PHENOMENA)

KYUNG-HEE LIM, Purdue University

Abstract

The dielectric relaxation method has been used in studies of aqueous dispersions of monodisperse polymer microspheres, oil- in-water (o/w) emulsions, and liquid microcrystallites and vesicles of surfactants sodium 4-(1'-heptylnonyl)benzenesulfonate (SHBS) and sodium di-(2-ethylhexyl)sulfosuccinate (Aerosol OT or AOT). Dielectric relaxation phenomena, i.e., large low-frequency dielectric constants and a strong dependence of the dielectric constant on the A.C. frequency, were observed. Dielectric constants and conductivities of aqueous dispersions of monodisperse polymer microspheres of sizes of 2.0, 1.1, 0.45, and 0.09 (mu)m were measured at various particle weight fractions (phi)(,w) and double layer thickness (kappa)('-1). The specific dielectric increments (sdi's) ((epsilon) - (epsilon))/(phi)(,w) were independent of (phi)(,w) and depended strongly on the particle radius a and on (kappa)a. Although the (zeta) potential should be an important variable, the dependence of sdi on the (zeta) potential was not clearly established, because (zeta) varied with (kappa). A distribution of charac- teristic relaxation times or frequencies was found for the first time to correspond to a given size. This distribution becomes narrower and closer to monodisperse the larger the value of (kappa)a. The central char- acteristic frequency (nu)(,o) is proportional to (a + (kappa)('-1))('-n) where n = 1.50 (+OR-) 0.12. This value differs from the value of 2 which is predicted by available models. These results are important for developing the di- electric relaxation method for estimating size distribution and pos- sibly surface charges in concentrated aqueous colloidal dispersions. Experimental results for these dispersions suggest that particle sizes, even if they are widely distributed, and electrical surface properties can be probed by the dielectric relaxation method. For developing EMTA (Electrophoretic Mass Transport Analyzer) method as an independent technique measuring electrophoretic mobilities or (zeta) potentials of particles, migration and diffusion of charged species in binary electrolyte solutions at a fixed current have been studied. Our results demonstrate that for calculating electrophoretic mobilities from measurements of concentration changes of charged species at different positions in an electrolytic cell, the electrode reactions must be known and unsteady state transport models should be used. (Abstract shortened with permission of author.)

Degree

Ph.D.

Subject Area

Chemical engineering

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