Interactions that occur at phase boundaries (in particular, liquid-liquid interfaces in solvent extraction, solid-liquid interfaces in used fuel dissolu­tion, in the environment, in electrometallurgy and in the cleanup of wastes at the former nuclear weapons complex) by definition control the rates of materials transfer which in turn often governs the efficiency of the separa­tion process. In solid-liquid separation systems, molecular motions on the solid side of the interface are generally limited while the fluid phase motions are quite dynamic. In liquid-liquid interfaces, both sides of the interface are

in a state of dynamic movement. At the interface, solute and solvent mol­ecules reorder their relative structures to facilitate phase transfer. In solvent extraction, surface active molecules arrange themselves to assist in the transfer of polar species into the less polar regime represented by the organic phase. The organization of these molecules at the interface controls the rate of mass transfer. Such systems have been investigated, but the necessity of probing a dynamic interfacial zone of with dimensions of only a few molecular diameters hinders a complete understanding. Computational modeling studies have attempted to create a rational framework for advanc­ing understanding of interfacial interactions, but progress is slow.