The interfacial structure of a mixture of charged hard spheres and dipolar hard spheres near a charged wall

In the past few years, there has been a renewed interest in the structure of an electrolyte at an electrode interface (the so-called electric double layer). If only the ions are taken onto account explicitly, with the solvent represented by a uniform dielectric medium of constant dielectric constant epsilon, the potential difference across the double layer is much smaller than is observed experimentally. This remains true even if the Poisson-Boltzmann theory of Gouy and Chapman (GC) is replaced by more refined theories, such as the hypernetted chain approximation. Presumably, this error is due to the simplified treatment of the solvent. If it is assumed that the dielectric constant epsilon in the so-called inner region between the hard, flat, charged wall (the electrode) and a plane passing through the centers of the charged hard spheres in contact with the wall is different from the bulk value epsilon, the potential drop across the interface is where sigma is the diameter of the charged hard spheres sigma/2 is then the distance of closest approach to the wall), -E/4pi, is the charge density on the wall, and phi is the potential difference between the plane of sigma/2 and infinity.