Low-Frequency Dielectric Response of Polystyrene Latex Dispersions

Abstract

The low-frequency permittivity and the conductivity increments of a well-characterized polystyrene sulfate latex with constant surface charge density were measured in KCl electrolyte solutions. The ionic strength was varied in the range 0.4–6 mM. The data could be explained with one absolute surface-conductivity parameter over the complete concentration range. The titratable surface charge density correlates well with the surface conductivity, the ions in the double layer having the bulk mobility. The results are in good agreement with static conductivities of plugs composed of the same particles obtained in a previous study (1). The electrokinetic charge obtained from streaming potentials (1) in that study is significantly lower, confirming the presence of conduction behind the shear plane. In order to interpret the conductivity data, simple analytical expressions have been derived for the frequency-dependent complex conductivity of a dilute dispersion of spherical particles with relatively thin double layers (κa⪢1, whereais the particle radius and κ−1the Debye screening length). The expressions are restricted to binary suspending electrolytes. However, no restrictions on the ion mobilities and ion valencies are made. Our results agree well with the numerical results of the dielectric model of Mangelsdorf and White (2,3) and reduce to O'Brien's static conductivity results (4) in the limit of low frequencies.