Department of Chemistry, University of California, Santa Cruz, California 95064
The thermodynamics and dynamics of NaCl ion-pair dissociation at the water/1,2-dichloroethane liquid/liquid interface are examined using a continuum electrostatic model, molecular dynamics free energy calculations and non-equilibrium dynamic trajectory calculations. The continuum model shows increased stability of the ion-pair relative to that in bulk water and strong dependence of the potential of mean force on the orientation and location of the ion-pair relative to the interface. These are in qualitative agreement with the molecular dynamics results. In particular, the equilibrium free energy calculations show that the ion-pair is locally stable at the interface and that the dissociation must involve ion transfer and considerable change in the interface structure. These are also confirmed by the non-equilibrium dynamics calculations: Dissociation of the ion pair at the interface involves a simultaneous transfer of both ions into the aqueous side of the interface. The faster transfer of the sodium than the chloride ion influences the lifetime of the ion-pair at the interface. In particular, a strong dependence of the ion-pair's stability on its orientation is found.