On water reorientation dynamics in cation hydration shells

The effects of ions on liquid water's structural, dynamical, and thermodynamical properties have key implications for a wide range of biological and technological processes. Based on simulations and analytic modeling, we have recently developed a framework that rationalizes the effects of solutes and interfaces on water reorientation dynamics. However, this picture still misses some contributions of the cations to the experimentally measured slowdown or acceleration of water dynamics. All-atom classical simula-tions also face some limitations in quantitatively reproducing water structural and dynamical features in ionic aqueous solutions. Here, we show that a scaled-charge approach can successfully reproduce experimental trends and that ab initio descriptions are not required. We show that a picture where the cation would lock a water molecule dipole and lead to partial OH reorientation is both incorrect for some ions, and largely exaggerated for others. We demonstrate that a combination of two effects on the hydrogen-bond (H-bond) exchange dynamics explains the ambient temperature acceleration of water reorientation next to a cesium cation, and the retardation next to lithium and magnesium cations. First, ions create a local excluded volume, which hinders the approach of possible new H-bond partners, leading to a retarding contribution. However, they also perturb the local water structure, reducing the energetic cost of elongating the initial H-bond. For magnesium and lithium cations, the excluded volume effect dominates, which leads to an overall retardation of the H-bond exchange. For the cesium cation, at room temperature, this latter contribution overcomes the excluded volume effect, leading to an acceler-ation; moreover, the strong temperature dependence observed in the experiments, going from a large acceleration close to freezing to a retardation close to boiling, is understood by the key enthalpic effect of the elongation contribution. Overall, our framework now provides a comprehensive understanding of cations' and anions' effects on water reorientation dynamics.(c) 2022 Elsevier B.V. All rights reserved.