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

TL;DR

This study develops a variational field-theoretic model to analyze how ion size influences ion distribution and selectivity at dielectric interfaces and slit nanopores, revealing complex size-dependent behaviors.

Contribution

It introduces a variational approach that extends mean-field theory to include core collisions, solvation, and image effects, providing new insights into ion size effects.

Findings

Ion size causes a concentration peak near dielectric interfaces.

Pore size affects ionic selectivity, reversing trends for large and small ions.

The model agrees with Monte Carlo simulations in dilute conditions.

Abstract

A previously developed field-theoretic model [R.D. Coalson et al., J. Chem. Phys. 102, 4584 (1995)] 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. We show that in the dilute limit, the MF and the variational theories agree well with MC simulation results, in contrast to a recent RPA method. The partition of charged Yukawa particles at a neutral dielectric interface (e.g 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 towards 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. We also characterize the role played by the ion size on 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 exhibits a higher conductivity for large ions, narrow pores exclude large ions more efficiently than small ones.