Dynamics of ion transfer across a liquid–liquid interface: A comparison between molecular dynamics and a diffusion model

Most theoretical approaches to ion transfer dynamics across a liquid–liquid interface describe the process as a stochastic crossing of a one-dimensional barrier whose shape is a priori unknown. We describe a molecular model of the ion transfer dynamics across an interface between two immiscible polar and nonpolar liquids. The results of extensive molecular dynamics trajectory calculations for the ion transfer are compared with the solution of a diffusion equation for an ion moving in an external field. The external field used in the equation is obtained from an independent free energy calculation using non-Boltzmann sampling. Near quantitative agreement is found, with discrepancies that may be attributed to solvent–shell exchange dynamics.