Hydronium ion at the water/1,2-dichloroethane interface: Structure, thermodynamics, and dynamics of ion transfer.

Molecular dynamics simulations including umbrella sampling free energy calculations are used to examine the structure, thermodynamics, and dynamics that accompany the transfer of the classical hydronium ion (HO) across the water/1,2-dichloroethane interface. The calculated free energy of transfer (17 ± 1 kcal/mol) is somewhat larger than the experimental value (14 kcal/mol). A detailed examination of the hydration structure is provided, and several dynamical properties as a function of the distance along the interface normal are calculated. In particular, it is shown that the hydronium ion is transferred as an Eigen species, and while the three hydration shell water molecules' average structure is conserved during the transfer, they may be exchanged with nearby water molecules with a rate that decreases as the ion enters the organic phase.