Ion size effects upon ionic exclusion from dielectric interfaces and slit nanopores

A previously developed field-theoretic model (Coalson et al 1995 J. Chem. Phys. 102 4584) that treats core collisions and Coulomb interactions on the same footing is investigated in order to understand ion size effects on the partition of neutral and charged particles at planar interfaces and the ionic selectivity of slit nanopores. We introduce a variational scheme that can go beyond the mean-field (MF) regime and couple in a consistent way pore-modified core interactions, steric effects, electrostatic solvation and image-charge forces, and surface charge induced electrostatic potential. Density profiles of neutral particles in contact with a neutral hard wall, obtained from Monte Carlo (MC) simulations are compared with the solutions of mean-field and variational equations. A recently proposed random-phase approximation (RPA) method is tested as well. We show that in the dilute limit, the MF and the variational theories agree well with simulation results, in contrast to the RPA method. The partition of charged Yukawa particles at a neutral dielectric interface (e. g. an air-water or protein-water interface) is investigated. It is shown that as a result of the competition between core collisions that push the ions toward the surface, and repulsive solvation and image forces that exclude them from the interface, a concentration peak of finite size ions sets in close to the dielectric interface. This effect is amplified with increasing ion size and bulk concentration. An integral expression for the surface tension that accounts for excluded volume effects is computed and the decrease of the surface tension with increasing ion size is illustrated. We also characterize the role played by the ion size in the ionic selectivity of neutral slit nanopores. We show that the complex interplay between electrostatic forces, excluded volume effects induced by core collisions and steric effects leads to an unexpected reversal in the ionic selectivity of the pore with varying pore size: while large pores exhibit a higher conductivity for large ions, narrow pores exclude large ions more efficiently than small ones.