Non-local free-energy density-functional theory applied to the electrical double layer

Publisher Website
Google Scholar
Abstract
This paper reports the predictions of a non-local free energy density functional theory of the ion density and the mean electrostatic potential profiles of the restricted primitive model of the electrical double layer. Results for unsymmetrical 2:1 electrolytes predicted by three different non-local free energy density functional approximations are compared with those of Monte Carlo simulations and of the modified Gouy-Chapman theory. The diffuse layer potentials are also compared with those of some recent theories of the electrical double layer. In the free energy density functional theory the hard sphere repulsive contribution to the Helmholtz free energy is represented by non-local density functional models that have appeared in previous work. The mean spherical approximation for the bulk electrolyte is used to model the electrostatic part of the non-uniform ion-ion correlations present in the interface. In the case of singly charged counterions the agreement between the free energy density functional theory and the Monte Carlo simulations is within a few per cent at the densities and surface charges studied. In the case of doubly charged counterions, however, the agreement is poor at low electrolyte concentrations, although good at high concentrations. The density functional theory correctly predicts the existence of an extremum in the diffuse layer potential as a function of the surface charge density.