The vapor-liquid interface potential of (multi)polar fluids and its influence on ion solvation

TL;DR

This study uses molecular dynamics simulations to explore how multipolar contributions at vapor-liquid interfaces affect ion solvation, revealing a complex interplay that influences electrostatic contributions beyond polarization effects.

Contribution

It provides new microscopic insights into the role of quadrupolar and dipolar contributions in vapor-liquid interface potentials and their impact on ion solvation, especially in water.

Findings

Quadrupolar and dipolar contributions significantly influence the vapor-liquid interface potential.

Competing interface potentials lead to partial cancellation, reducing the net effect on ion solvation.

Ion-specific electrostatic contributions are dominated by multipolar effects beyond polarization.

Abstract

The interface between the vapor and liquid phase of quadrupolar-dipolar fluids is the seat of an electric interfacial potential whose influence on ion solvation and distribution is not yet fully understood. To obtain further microscopic insight into water specificity we first present extensive classical molecular dynamics simulations of a series of model liquids with variable molecular quadrupole moments that interpolates between SPC/E water and a purely dipolar liquid. We then pinpoint the essential role played by the competing multipolar contributions to the vapor-liquid and the solute-liquid interface potentials in determining an important ion-specific direct electrostatic contribution to the ionic solvation free energy for SPC/E water---dominated by the quadrupolar and dipolar contributions---beyond the dominant polarization one. Our results show that the influence of the vapor-liquid interfacial potential on ion solvation is strongly reduced due to the strong partial cancellation brought about by the competing solute-liquid interface potential.