Structure of electric double layers: A simple weighted density functional approach

A simple weighted density functional approach is developed for inhomogeneou s ionic fluids and applied to the structure of the electric double layer us ing the restricted primitive model where the ions are considered to be char ged hard spheres of equal diameter. The formalism is nonperturbative with b oth hard-sphere and electrical contributions to the one-particle correlatio n function evaluated through a suitably averaged weighted density, the only input being the second-order direct correlation functions of the correspon ding uniform system. The approach is designed in such a way, that the calcu lation of the weight function is decoupled from the weighted density. Numer ical results on the ionic density profile and the mean electrostatic potent ial near a hard wall at several surface charge densities are shown to compa re well with available simulation results. The corresponding results for th e nonprimitive molecular solvent model provide insight into the layering ef fect and the charge inversion phenomena. (C) 1999 American Institute of Phy sics. [S0021-9606(99)51145-8].