Water-mediated interactions between hydrophobic and ionic species in cylindrical nanopores

TL;DR

This study investigates how confinement in cylindrical nanopores affects the interactions of hydrophobic and ionic molecules in water, revealing that pore size and charge influence solvation and ion-pair formation.

Contribution

The paper provides a detailed analysis of how cylindrical confinement alters molecular interactions and ion pairing, combining Monte Carlo simulations with analytic entropy estimates.

Findings

Confinement eliminates the solvent separated minimum in free energy profiles.

Adding charges restores the solvent separated minimum and promotes ion pairing.

Pore size and charge density critically influence solvation and ion-pair stability.

Abstract

We use Metropolis Monte Carlo and umbrella sampling to calculate the free energies of interaction of two methane molecules and their charged derivatives in cylindrical water-filled pores. Confinement strongly alters the interactions between the nonpolar solutes, and completely eliminates the solvent separated minimum (SSM) that is seen in bulk water. The free energy profiles show that the methane molecules are either in contact or at separations corresponding to the diameter and the length of the cylindrical pore. Analytic calculations that estimate the entropy of the solutes, which are solvated at the pore surface, qualitatively explain the shape of the free energy profiles. Adding charges of opposite sign and magnitude $0.4e$ or $e$ (where $e$ is the electronic charge) to the methane molecules decreases their tendency for surface solvation and restores the SSM. We show that confinement induced ion-pair formation occurs whenever $l_B/D \sim O(1)$, where $l_B$ is the Bjerrum length, and $D$ is the pore diameter. The extent of stabilization of the SSM increases with ion charge density as long as $l_B/D < 1$. In pores with $D \le 1.2$ nm, in which the water is strongly layered, increasing the charge magnitude from $0.4e$ to $e$ reduces the stability of the SSM. As a result, ion-pair formation which occurs with negligible probability in the bulk, is promoted. In larger diameter pores that can accomodate a complete hydration layer around the solutes, the stability of the SSM is enhanced.