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

Nanoconfinement reduces the favorable hydration free energies of single ions, and 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 nonpolarizable carbon nanotubes filled with water as minimal, hydrogen-atom-like models to evaluate the single-ion intrinsic confinement hydration free energy penalty (ΔΔG_hyd). In tubes of radius R = 7.5 Å, we predict ΔΔG_hyd values 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 ΔΔG_hyd for the Na⁺/Cl^- pair by an amount exceeding the Debye-Hückel 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.