THE DISCRETENESS-OF-CHARGE EFFECT IN ELECTRIC DOUBLE LAYER THEORY

Abstract

The Stern–Grahame–Devanathan theory of the electrical double layer in aqueous systems is modified to include the so-called discreteness-of-charge effect of Esin and Shikov and Ershler. This provides an explanation of a number of phenomena which are at variance with the Stern theory. A simple method of incorporating the above effect into the Stern theory is suggested by the work of Grahame and is equivalent in principle to the discrete-ion approximation employed by the Russian authors. It is shown that the effect can be interpreted in terms of a 'self-atmosphere' potential at the counterions adsorbed in the Stern layer. This provides a new term in the energy of an adsorbed ion, which is very nearly proportional to the surface density of these ions and which had hitherto been included in the specific adsorption potential in the Stern adsorption isotherm. This energy is not small and accounts for the conclusion reached by Grahame that the adsorption potential varies strongly with the surface charge. Grahame found that the potential at the plane separating the compact and diffuse parts of the double layer in the solution phase (i.e. the outer Helmholtz plane) at the mercury –aqueous electrolyte interface displays a maximum as the potential across the interface is varied, and this property is reproduced by the theory. The effect of ion size on the adsorption isotherm is also considered.