A NONLOCAL DENSITY-FUNCTIONAL THEORY OF ELECTRIC DOUBLE-LAYER - CHARGE-ASYMMETRIC ELECTROLYTES

A nonlocal density-functional theory of inhomogeneous ionic fluids pro posed by us recently [J. Chem. Phys. 100, 5219 (1994)] for symmetric e lectrolytes is extended to study the structure of electric double laye r for a charge-asymmetric (2:1) situation involving hard sphere ions o f equal diameter with a continuum or neutral hard sphere model for the solvent. The hard sphere contributions to the excess free energy dens ity and its derivatives for the inhomogeneous system are evaluated non perturbatively through a position-dependent effective weighted density , which is also used to obtain the corresponding ionic contributions t hrough a second-order functional Taylor expansion. The calculated resu lts for the continuum solvent model show reasonably good agreement wit h the available simulation results, while the layering effect due to h ard sphere exclusion and the charge inversion phenomena are some of th e interesting consequences arising from the molecular nature of the so lvent.