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

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.