Quadrupole-mediated dielectric response and the charge-asymmetric solvation of ions in water

TL;DR

This paper introduces a modified dielectric continuum model incorporating quadrupole effects to better distinguish cation and anion solvation thermodynamics in water, aligning well with simulation data.

Contribution

It proposes a physically motivated modification to dielectric continuum models to account for quadrupole moments, improving the description of ion solvation asymmetry.

Findings

The model captures charge asymmetry in ion solvation thermodynamics.

Solvation free energies from the model closely match simulation results.

The approach extends dielectric models to include higher-order multipole effects.

Abstract

Treating water as a linearly responding dielectric continuum on molecular length scales allows very simple estimates of solvation structure and thermodynamics for charged and polar solutes. While this approach can successfully account for basic length and energy scales of ion solvation, computer simulations indicate not only its quantitative inaccuracies but also its inability to capture some basic and important aspects of microscopic polarization response. Here we consider one such shortcoming, a failure to distinguish the solvation thermodynamics of cations from that of otherwise-identical anions, and we pursue a simple, physically inspired modification of the dielectric continuum model to address it. The adaptation is motivated by analyzing the orientational response of an isolated water molecule whose dipole is rigidly constrained. Its free energy suggests a Hamiltonian for dipole fluctuations that accounts implicitly for the influence of higher-order multipole moments, while respecting constraints of molecular geometry. We propose a field theory with the suggested form, whose nonlinear response breaks the charge symmetry of ion solvation. An approximate variational solution of this theory, with a single adjustable parameter, yields solvation free energies that agree closely with simulation results over a considerable range of solute size and charge.