The structure of electrolytes at charged surfaces: Ion–dipole mixtures

Abstract

The detailed structure of the double layer is investigated using a model fluid consisting of hard spheres with embedded point charges in a solvent of hard spheres with embedded point dipoles against a hard wall with smeared-out surface charge. Such a model treats solute and solvent particles on an equal basis, unlike the primitive model of electrolytes. The statistical mechanics is solved using the mean spherical approximation for all interactions. This limits the validity of any results to the regime of low ionic concentrations, where, in this approximation, the model fluid has the correct limiting behavior for bulk thermodynamic quantities. In this regime, simple analytic results for the surface properties are given, which are correct to order (κR). In particular, the surface potential has the classical Stern layer form, with solvent structuring responsible for the inner layer capacitance. This result is the first derivation, as opposed to postulation, of Stern layer behavior. In addition, the polarization density oscillates about the continuum theory result for 3–4 molecular diameters away from the surface. Such behavior shows the difficulty in defining a local dielectric constant close to the surface.