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

Abstract

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 inorder to understand ion size effects on the partition of neutral and charged particles atplanar interfaces and the ionic selectivity of slit nanopores. We introduce a variationalscheme that can go beyond the mean-field (MF) regime and couple in a consistent waypore-modified core interactions, steric effects, electrostatic solvation and image-chargeforces, and surface charge induced electrostatic potential. Density profiles of neutralparticles in contact with a neutral hard wall, obtained from Monte Carlo (MC) simulationsare compared with the solutions of mean-field and variational equations. A recentlyproposed random-phase approximation (RPA) method is tested as well. We show that inthe dilute limit, the MF and the variational theories agree well with simulation results, incontrast to the RPA method. The partition of charged Yukawa particles at a neutraldielectric interface (e.g. an air–water or protein–water interface) is investigated. It isshown that as a result of the competition between core collisions that push theions toward the surface, and repulsive solvation and image forces that excludethem from the interface, a concentration peak of finite size ions sets in close tothe dielectric interface. This effect is amplified with increasing ion size and bulkconcentration. An integral expression for the surface tension that accounts forexcluded volume effects is computed and the decrease of the surface tension withincreasing ion size is illustrated. We also characterize the role played by the ionsize in the ionic selectivity of neutral slit nanopores. We show that the complexinterplay between electrostatic forces, excluded volume effects induced by corecollisions and steric effects leads to an unexpected reversal in the ionic selectivity ofthe pore with varying pore size: while large pores exhibit a higher conductivityfor large ions, narrow pores exclude large ions more efficiently than small ones.