Solvation of positive ions in water: The dominant role of water-water interaction

TL;DR

This study uses first-principles quantum simulations to reveal that water-water interactions predominantly influence the solvation structure of positive ions in water, overshadowing the direct ion effects.

Contribution

It demonstrates that water-water interactions are the key factor in ion solvation, regardless of ion size or charge, through multiscale first-principles molecular dynamics.

Findings

Water-water interactions dominate the first solvation shell.

Ion influence is limited to the first shell.

Electronic polarization effects are crucial in solvation dynamics.

Abstract

Local polarization effects, induced by mono and divalent positive ions in water, influence (and in turn are influenced by) the large scale structural properties of the solvent. Experiments can only distinguish this process of interplay in a generic qualitative way. Instead, first principles quantum calculations can address the question at both electronic and atomistic scale, accounting for electronic polarization as well as geometrical conformations. For this reason we study the extension of the scales' interconnection by means of first principle Car-Parrinello molecular dynamics applied to systems of different size. In this way we identify the general aspects dominating the physics of the first solvation shell and their connection to the effects related to the formation of the outer shells and eventually the bulk. We show that while the influence of the ions is extended to the first shell only, the water-water interaction is instead playing a dominant role even within the first shell independently from the size or the charge of the ion.