The effect of a diffuse layer in the solid phase at the silver iodide/aqueous electrolyte interface

Abstract

We examine models of the silver iodide/aqueous electrolyte interface, having diffuse layers of charge in both solid and liquid phases which are described by the Gouy-Chapman theory, and also a Stern inner region without adsorbed counterions on the aqueous side of the phase boundary. The diffuse layer in the solid phase is assumed to be due to lattice (Frenkel) defects. We consider two mechanisms suggested by Honig, whereby a surface layer of ions (or vacancies) is formed at the boundary of the solid phase. In one, the surface ions are “frozen-in” and in the other, the surface density of Ag + ions or vacancies is variable, being governed by a Langmuir-type entropic term in the electrochemical potential of a surface ion. The two models are tested by calculating the dependence of the interfacial differential capacity on the potential drop across the interface. It is shown that Honig's fixed charge condition without an inner Stern region does not reproduce the experimental variation of capacity with potential. In the case of the variable charge model general agreement with the experimental capacity-potential relation, on both positive and negative sides of the zero point of charge (z.p.c.) and for different concentrations of indifferent electrolyte, can be obtained provided we introduce an inner Stern region having a capacity which depends on the interfacial potential. We propose an alternative mechanism for establishing the surface charge. This assumes that surface imperfections are such that the surface charge consists of both Ag + and I − ions or vacancies. The net surface charge is then very small compared with the charge due to either Ag + or I −. It is shown that for such a model the presence of the diffuse layer in the solid phase merely shifts the z.p.c., justifying the usual approach by colloid chemists to the double layer in the silver iodide system, where any solid diffuse layer is ignored.