The Hydration Number of Li+ in Liquid Water

TL;DR

This study combines theoretical, ab initio, and classical simulations to determine that Li+ ions in liquid water predominantly have four water molecules in their inner hydration shell, challenging previous interpretations.

Contribution

It provides a comprehensive multi-method analysis showing that Li+ prefers a four-coordinated hydration structure in liquid water, contradicting earlier experimental interpretations.

Findings

Li+ most probable hydration number is four.

Six-coordinated structures are unstable compared to four-coordinated.

Simulations show rapid relaxation from six to four water molecules.

Abstract

A theoretical treatment based upon the quasi-chemical theory of solutions predicts the most probable number of water neighbors in the inner shell of a Li+ ion in liquid water to be four. The instability of a six water molecule inner sphere complex relative to four-coordinated structures is confirmed by an `ab initio' molecular dynamics calculation. A classical Monte Carlo simulation equilibrated 26 water molecules with a rigid six-coordinated Li(H2O)6+ complex with periodic boundary conditions in aqueous solution. With that initial configuration for the molecular dynamics, the six-coordinated structure relaxed into four-coordinated arrangements within 112 fs and stabilized. This conclusion differs from prior interpretations of neutron and X-ray scattering results on aqueous solutions.