Molecular simulation of the interface between two immiscible electrolyte solutions

In this paper we analyze the properties of an interface between two immisci ble electrolyte solutions. The study is conducted using high concentrations of an ionic salt in the aqueous phase (1 and 3 mol.dm(-3) MgCl2). The prof ile of the electric potential drop across the interface is calculated, and it is concluded that the surface potential is originated by the orientation al anisotropy of the solvents near the interface. The ionic density profile shows a monotonic decrease as the ions approach the interface. The resulta nt ionic net charge density was found to be null across all the simulation box, within statistical uncertainty, confirming that the potential drop is only caused by the orientation of the solvent molecules. The orientational structure of the solvents is not disturbed by the presence of the ions. The radial distribution functions for the hydration shell of the ions show tha t the hydration shell remains almost unaffected at the interfacial region. The hydration number is reduced with the increase of the ionic concentratio n, a fact explained by the decrease of the H2O/ion ratio. Ionic diffusion i s anisotropic in all extension of the simulation box, being slower toward t he interface than parallel to it. This anisotropy is due to the existence o f an impenetrable barrier to displacements normal to the interfacial plane. It was also observed that the diffusion near the interface was faster than in the bulk solution. This effect is mainly caused by the smaller density of the interfacial region.