Zeta Potential of Highly Charged Thin Double-Layer Systems

Electrophoretic measurements under d.c. conditions involve a fundamental ambiguity for particles with a high zeta potential; two very different values of zeta potential can be ascribed to the same mobility. In the case of a polystyrene sulfate latex studied previously, selecting the higher (absolute) value gave a more consistent picture of the interface and also removed an anomaly in the electrokinetic behavior as a function of indifferent electrolyte concentration. Confirmation that the higher value is the correct one, however, was still needed. The ambiguity is resolved unequivocally here by the use of high-frequency mobility measurements using electroacoustics, since the mobility spectra for the high and low zeta potentials are very different. The interpretation is supported by the dielectric relaxation spectra of this material in the megahertz region.