A modified Poisson-Boltzmann theory and Monte Carlo simulation study of surface polarization effects in the planar diffuse double layer

The equilibrium structural properties of an electric double layer in the presence of a dielectric discontinuity at a plane charged interface are studied through a modified Poisson - Boltzmann equation and some Monte Carlo simulations. The diffuse double layer is modelled by a symmetric valency restricted primitive model electrolyte next to a uniformly charged planar wall whose relative permittivity (epsilon(w)) is, in general, different from that of the electrolyte (epsilon(r)) leading to polarization or image forces. The situations when the wall is a conductor (epsilon(w ->)infinity, epsilon(w)not equal epsilon(r)), or when the wall is an insulator (epsilon w = 1, epsilon w not equal epsilon r) are treated and contrasted with the case epsilon w not equal epsilon r, that is, no imaging. The results show that the influence of image forces on the diffuse double layer structure can be substantial, especially at or near zero surface charge density. The theoretical predictions are in nearly quantitative agreement for 1: 1 salts or in semi-quantitative or better agreement for 2: 2 salts with existing Monte Carlo simulations in the literature and some new simulations presented here.