Anomalous Ion Confinement Penalties and Giant Ion-Screening Effects in One-Dimensional Nanopores

TL;DR

This study investigates how nanoconfinement in nanopores significantly alters ion hydration energies, especially for chloride ions, revealing giant ion-screening effects and deviations from classical models, with implications for ion rejection and reactivity.

Contribution

The paper introduces a minimal classical model to quantify ion confinement hydration penalties and uncovers large, ion-specific effects and novel screening phenomena in nanopores.

Findings

Ion confinement increases hydration free energy penalties up to 7.8 kcal/mol.

Chloride ions experience larger penalties than sodium ions under confinement.

Electrolyte presence drastically reduces confinement penalties, exceeding classical estimates.

Abstract

Nanoconfinement reduces the favorable hydration free energies of single ions, which is correlated with ion rejection and modified chemical reactivity in water-filled nanopores. Many factors contribute to the magnitude of the observed confinement effect. Here we use simple classical force fields and non-polarizable carbon nanotubes filled with water as minimal, "hydrogen atom"-like models to evaluate the single-ion intrinsic confinement hydration free energy penalty (Delta Delta G(hyd)). In tubes of radius R=7.5 Angstrom, we predict Delta Delta G(hyd)'s that are up to 7.8 kcal/mol, are much larger for Cl$^-$ than the smaller Na+ ion, and contradict the canonical Born Equation for ion solvation. Adding a 1.0~M background electrolyte reduces Delta Delta G(hyd) for the Na$^+$/Cl$^-$ pair by an amount exceeding the Debye-Huckel estimate in unconfined media by almost an order of magnitude. We identify concentration-dependent ion-screening of confinement effects as a major, unheralded consequence of electrolytes in cylindrical nanopores.