Ionic and dipolar adsorption from an ion–dipole mixture. A model for the stern layer

Abstract

The details of solvent structure in the electrical double layer are investigated using a molecular model electrolyte which consists of a fluid mixture of hard spheres with embedded point charges in a solvent of hard spheres with embedded point dipoles. The charged surface is modelled as a hard wall with a uniform surface charge density. Effects due to the specific adsorption of ions and the specific adsorption and preferential orientation of solvents at the surface are also considered. The statistical mechanical treatment is the Mean Spherical Approximation. Simple analytic expressions, valid at low electrolyte concentrations and near the point of zero charge, are obtained for the effective Stern-layer capacitance for various types of charged surfaces. In this model the Stern layer is a derived rather than a postulated entity. Structural quantities such as potential and polarization profiles at the charged surface indicate that while the Stern model can correctly parameterize the thermodynamic properties of the double layer, it may not be an appropriate visualization of the microstructure of the interfacial region.