Spherical Macroions in Strong Fields

Abstract

The steady-state response of a dilute salt solution of spherical macroions to strong electrical fields has been calculated on the basis of the nonlinear diffusion equations, the Navier-Stokes equation, and Poisson's equation. The electrophoretic mobility shows essentially no nonlinear response at the field strengths considered, but the effective charge and dipole moment, the latter proportional to the conductivity change, show a strongly nonlinear response. At moderate field strengths part of the ion atmosphere is stripped away, but at still larger fields the macroion traps counterions. A positively charged macroion together with its ion atmosphere then has a net negative charge. If the Debye length k-1 and radius a are such that ka > 1, as holds for the numerical illustrations, nonlinearity in the polarization or nonquadratic response of the effective charge sets in at field strengths that scale inversely with particle radius. The field strength can therefore be quite low compared to what is required for Wien effects in strong electrolyte solutions, where the required field scales as k. © 1983, American Chemical Society. All rights reserved.