Combined effect of confinement and dielectric exclusion on ion adsorption in slits, pores, and cavities

We present simulation results for the Donnan equilibrium between a homogeneous bulk reservoir and inhomogeneous confining geometries with varying number of restricted dimensions, dc. Planar slits (dc = 1), cylindrical pores (dc = 2), and spherical cavities (dc = 3) are considered. The walls have a negative surface charge density. Because different dielectric constants are used in the reservoir and confined system, we used the Donnan grand canonical Monte Carlo method [Boda and Gillespie, J. Mol. Liq. 391, 123372 (2023)] to simulate the equilibrium. The systems with larger confining dimensionality produce greater adsorption of counterions (cations) into the confinements, so cation selectivity increases with increasing dimensionality. The systems with smaller dielectric constants produce more effective coion (anion) exclusion, so cation selectivity increases with decreasing dielectric constant. The combined effect of a more confining space and solvation penalty produces even more efficient anion exclusion and cation selectivity than each separately.