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

During ion transfer through a water-oil interface, a water finger (WF) is transiently formed and breaks to leave hydrated ions in the oil phase. The present work investigated subsequent microscopic processes following WF by molecular dynamics (MD) simulation. The nascent ions tend to have excessive hydrating water as a consequence of unstable WF formation, implying subsequent relaxation. Subsequent kinetics of the ions including evaporation/condensation of hydrating water, diffusion, recapture of ions by WF, and ion pair formation were comprehensively examined with calculations of free energy surfaces and diffusion dynamics. The thickness of the interface associated with non-equilibrium hydration is estimated to be the order of ∼nm, and the WF recapture is a rather minor process in the range. The observed ion current through the liquid-liquid interface was shown to be sensitive to the water content in the oil because the concentration of ions including their hydrated clusters in the oil phase is sensitive to the trace amount of water concentration.