Ionic exclusion phase transition in neutral and weakly charged cylindrical nanopores

TL;DR

This paper introduces a field theoretic variational approach to study ion behavior in cylindrical nanopores, revealing a phase transition between ionic penetration and exclusion influenced by pore size, concentration, and dielectric properties.

Contribution

It presents a novel theoretical framework for analyzing ion exclusion and penetration in nanopores, highlighting a first-order phase transition driven by dielectric effects and ionic interactions.

Findings

Ion in neutral pore experiences competing effects: cloud deformation and image forces.

A first-order phase transition occurs at small pore radii and low concentrations.

Counterion behavior is non-monotonic, with two distinct regimes depending on concentration.

Abstract

A field theoretic variational approach is introduced to study ion penetration into water-filled cylindrical nanopores in equilibrium with a bulk reservoir. It is shown that (i) an ion located in a neutral pore undergoes two opposing mechanisms: the deformation of its surrounding ionic cloud of opposite charge, with respect to the reservoir, which increases the surface tension and tends to exclude ions form the pore, and (ii) the attractive contribution to the ion self-energy of the repulsive image forces associated with the dielectric jump between the solvent and the pore wall, which becomes more and more screened when ions enter the pore. For pore radii around 1 nm and bulk concentrations lower than 0.2 mol/L, this mechanism leads to a first-order phase transition, similar to capillary "evaporation", from an ionic-penetration state to an ionic-exclusion state. The discontinuous phase transition survives within the biological concentration range (~ 0.15 mol/L) for small enough membrane dielectric permittivities (< 5). In the case of a weakly charged pore, counterion penetration exhibits a non-monotonic behaviour and is characterized by two regimes: at low reservoir concentration or small pore radii, coions are excluded and the energy barrier for counterions is reduced but image forces remain strong enough, so that the counterion partition coefficient in the pore decreases with increasing reservoir concentration up to a characteristic value; for larger reservoir concentrations, image forces are screened and the partition coefficient of counterions increases with the reservoir electrolyte concentration.