The statistical mechanics of the electric double layer

Recent theoretical studies of the electrical double layer, useful at small electrode charge and based upon the assumption that an electrolyte can be regarded as a mixture of charged hard spheres and dipolar hard spheres, are outlined. It is found that, at small electrode charge at least, the conventional idea of an inner layer and a diffuse layer described by the Gouy-Chapman theory yields the same potential difference and charge adsorption isotherm as do our recent studies. However, the microscopic picture underlying the conventional ideas is incorrect and can be misleading. The interfacial region of the solvent is not compact but is about the same thickness as the double layer. The conventional result for the effective potential of an ion at the electrode is too small. In addition, it is found that, even assuming that the solvent can be represented by a uniform dielectric medium, the Gouy-Chapman theory is significantly in error at high electrode charge. This error becomes more pronounced as the dielectric constant is decreased or the valence of the ions increased. © 1983 Elsevier Sequoia S.A.