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 a re confined by two charged walls. When the walls are thin, the confined ele ctrolyte 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 sys tem has been studied previously using a singlet integral equation. Our dens ity-functional (DF) study is more general because the fluids inside and out side the walls are constrained to have the same chemical potential and beca use solvent effects are considered albeit at a crude level. The singlet int egral equation does not consider the chemical potential constraint explicit ly. We find that for the low density PM, the DF and integral equation appro aches yield, except for a very narrow pore, very similar results. When solv ent molecules are considered, the profiles become oscillatory. The co-ion d ensity profiles are particularily interesting because the repulsive electro static potential and the effect of the increased pressure in "pushing" the co-ions against the wall compete.