Tetrahedral entropy captures non-monotonicity of electrical conductivity in aqueous monatomic ions

TL;DR

This paper introduces a tetrahedral entropy measure that explains the non-monotonic relationship between ionic size and electrical conductivity in aqueous monatomic ions, addressing limitations of previous entropy theories.

Contribution

It proposes a new entropy metric based on tetrahedral water ordering that captures non-monotonic conductivity behavior, advancing understanding of ion hydration effects.

Findings

Tetrahedral entropy correlates with ionic conductivity variations.

Water's tetrahedral structure changes non-monotonically with ion size.

The new entropy measure explains experimental conductivity trends.

Abstract

The intriguing relationship between entropy and diffusion is a subject of much current interest. However, the experimentally observed unusual non-monotonic dependence of limiting ionic conductivity on inverse ion size is neither described by the Adam-Gibbs entropy crisis theory nor by the Rosenfeld entropy scaling. This failure is obvious because throughout the size variation the bulk entropy of the solvent remains the same, or undergoes infinitesimal change. We show that it is the entropy experienced by the tagged ion that needs to be calculated. This entropy can be quantified, at least partly, by the change in the tetrahedral ordering of water molecules in the hydration layer of the ions which exhibits a nonmonotonic size dependence.