Density-functional theory for an electrolyte confined by thin charged walls

Results are reported for the primitive model of an electrolyte and for the solvent primitive model of an electrolyte for the case where these fluids are confined by two charged walls. When the walls are thin, the confined electrolyte inside the walls is affected by the charge on both the inside and the outside of the walls. In the case of the primitive model (PM), this system has been studied previously using a singlet integral equation. Our density-functional (DF) study is more general because the fluids inside and outside the walls are constrained to have the same chemical potential and because solvent effects are considered albeit at a crude level. The singlet integral equation does not consider the chemical potential constraint explicitly. We find that for the low density PM, the DF and integral equation approaches yield, except for a very narrow pore, very similar results. When solvent molecules are considered, the profiles become oscillatory. The co-ion density profiles are particularily interesting because the repulsive electrostatic potential and the effect of the increased pressure in "pushing" the co-ions against the wall compete.