Beyond Poisson-Boltzmann: Images and correlations in the electric double layer. II. Symmetric electrolyte
An extension of the Poisson-Boltzmann theory of the electric double layer is applied to a symmetric electrolyte between two charged planar surfaces. This analytic treatment includes the effects of images and of ion correlations via the Debye-Hückel closure for the direct correlation function. It is demonstrated that at large separations the corrections to Poisson-Boltzmann theory appear as an effective surface charge. A means of correcting the apparent ion binding inferred from fitting experimental force data is also given. The extended theory shows good agreement with accurate numerical calculations for systems with low coupling. The generalization of the Onsager-Samaras limiting result to higher concentrations and to the case of the surface free energy of a charged dielectric interface is also presented.