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

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.