ELECTROACOUSTIC AND DIELECTRIC STUDY OF SURFACE CONDUCTION

The triple layer model (TLM) is here used to interpret measurements of electrophoretic mobilities at zero and high (1 MHz) frequencies, and the complex conductivities over the frequency range from 1 to 20 MHz for a monodisperse latex with carboxyl head groups. The TLM divides the double layer into a compact and diffuse region and assumes that the surface charge on the particle arises by dissociation of surface groups. The measurements were made over a wide pH range in KCl solutions at concentrations of 0.001 and 0.01 M. Most of the parameters for the triple layer model of the interface are estimated from the potentiometric titration data. The ζ potentials are calculated from electrophoresis data using the measured surface conductances obtained from the complex conductivity of the colloid. It is found that these ζ values are sm aller than values calculated directly from the conductivity data. In order to reconcile these two sets of data we consider two alternative electrokinetic mechanisms in the TLM: (i) the zeta potential is measured some distance (from 1 to 3 nm) from the Outer Helmholtz Plane (OHP) and the anomalous conductivity occurs in the diffuse layer or (ii) the ζ potential is measured in the OHP and the anomalous conductivity occurs in the Stern layer. With the latter interpretation, a consistent value of about 0.5 is obtained for the mobility coefficient of the Stern layer charge, independent of the electrolyte concentration. The first model does not lead to a consistent interpretation; the diffuse layer potential calculated from the electrokinetic data is larger than the maximum possible value estimated from the adsorption data. The second model also gives a reasonable description of the zeta potentials and surface conductances for the entire pH range over which measurements were possible. © 1993 Academic Press, Inc.