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

Abstract

A previously developed field-theoretic model [R.D. Coalson et al., J. Chem.\nPhys. 102, 4584 (1995)] that treats core collisions and Coulomb interactions on\nthe same footing is investigated in order to understand ion size effects on the\npartition of neutral and charged particles at planar interfaces and the ionic\nselectivity of slit nanopores. We introduce a variational scheme that can go\nbeyond the mean-field (MF) regime and couple in a consistent way pore modified\ncore interactions, steric effects, electrostatic solvation and image-charge\nforces, and surface charge induced electrostatic potential. We show that in the\ndilute limit, the MF and the variational theories agree well with MC simulation\nresults, in contrast to a recent RPA method. The partition of charged Yukawa\nparticles at a neutral dielectric interface (e.g air-water or protein-water\ninterface) is investigated. It is shown that as a result of the competition\nbetween core collisions that push the ions towards the surface, and repulsive\nsolvation and image forces that exclude them from the interface, a\nconcentration peak of finite size ions sets in close to the dielectric\ninterface. We also characterize the role played by the ion size on the ionic\nselectivity of neutral slit nanopores. We show that the complex interplay\nbetween electrostatic forces, excluded volume effects induced by core\ncollisions and steric effects leads to an unexpected reversal in the ionic\nselectivity of the pore with varying pore size: while large pores exhibits a\nhigher conductivity for large ions, narrow pores exclude large ions more\nefficiently than small ones.\n