Atomistic understanding of hydration shell mechanics modulating freezing dynamics of alkali chloride aqueous solution

Understanding the ion-specific effects on the freezing dynamics of supercooled ionic solutions is vital for advancements in atmospheric sciences and water resource technologies. In this study, the hydration mechanics of five alkali metal ions (M = Li+, Na+, K+, Rb+, and Cs+) and their impact on the modulation of freezing dynamics in ionic solutions were elucidated. Alkali ions with thick subshells form soft, loosely bound hydration shells (Rb+ and Cs+) that facilitate frequent ligand exchange owing to weakened Coulombic interactions between the ion core and the surrounding water molecules. These soft hydration shells cause frequent ligand rearrangements that disrupt the crystallization of ice by degrading its lattice structure. In contrast, the thin subshells (Li+ and Na+) of the alkali ions construct rigid and ordered hydration shells, which impose kinetic constraints on nucleation and hinders ice formation. These findings offer atomistic-level insights into the modulation of the freezing behavior of supercooled ionic solutions by the hydration structure of ions.