Microscopic roles of hydration on interfacial ion transfer after water finger break.

This article investigates the microscopic processes following the break of a transient water finger (WF) during ion transfer across a water–oil interface using molecular dynamics (MD) simulations. It reveals that nascent hydrated ion clusters formed in the oil phase exhibit excess hydration compared to equilibrium states due to the unstable WF structure, leading to subsequent relaxation via evaporation/condensation of hydrating water, diffusion, minor recapture by WF, and ion pair formation. The study quantifies these kinetics through free energy surface calculations and diffusion dynamics, estimating the nonequilibrium hydration region thickness to be on the order of nanometers. Additionally, it demonstrates that ion pairing and the concentration of hydrated ions in the oil phase are highly sensitive to trace water content, which influences the ion transport rate across the interface. These findings provide detailed molecular-level insights into ion transport mechanisms at liquid–liquid interfaces relevant to separation, extraction, and membrane processes.