Finding Order in the Disordered Hydration Shell of Rapidly Exchanging Water Molecules around the Heaviest Alkali Cs and Fr.

We report the structural and dynamical characterization of the intrinsically disordered hydration shells of the heaviest alkali ions, Cs and Fr, obtained in ab initio molecular dynamics simulations. The knowledge of solvation and complexation properties of short-lived Fr is very limited and mostly based on extrapolations from the smaller alkali metal ions. To this end, we provide a critical insight into Fr solvation, demonstrating an extreme example of disordered solvation with no distinction between the ion-bound and solvent-bound states of water based on the ion-water distance. However, these two states are distinguished through distance-solvent rearrangement correlation, where either coordination number or electric field is employed to treat solvent rearrangement. Utilizing reaction rate theory, we find that the water exchange time scale for Fr (2.1-2.3 ps) is unexpectedly slower than for Cs (0.5-1.2 ps), because Fr experiences stronger nonequilibrium solvent effects. This study provides a new perspective on weak and hydrophobic solvation.