Liquid-liquid extraction of uranyl by an amide ligand: interfacial features studied by MD and PMF simulations

We report a molecular dynamics study of biphasic systems involved in the liquid-liquid extraction of uranyl nitrate by a monoamide ligand (L = N,N-di(2-ethylhexyl)isobutyramide, DEHiBA) to hexane, from pH neutral or acidic (3 M nitric acid) aqueous solutions. We first describe the neat interfaces simulated with three electrostatic models, one of which including atomic polarizabilities. The free energy profiles for crossing the water/hexane interface by L or its UO2(NO3)2L2 complex are then investigated by PMF (potential of mean force) calculations. They indicate that the free ligand and its complex are surface active. With the polarizable force field, however, the complexes have a lower affinity for the interface than without polarization. When DEHiBA gets more concentrated and in acidic conditions, their surface activity diminishes. Surface activity of UO2(NO3)2L2 complexes is further demonstrated by demixing simulations of randomly mixed DEHiBA, hexane, and neutral or acidic water. Furthermore, demixing of randomly mixed solvents, L molecules, UO2(NO3)2 salts, and nitric acid shows in some cases complexation of L to form UO2(NO3)2L2 and UO2(NO3)2L complexes that adsorb at the aqueous interfaces. These features suggest that uranyl complexation by amide ligands occurs "right at the interface", displaying marked analogies with the liquid-liquid extraction of uranyl by TBP (tri-n-butyl phosphate). Regarding the positive effect of nitric acid on extraction, the simulations point to several facets involving enhanced ion pairing of uranyl nitrate, decreased affinity of the complex for the interface, and finally, stabilization of the complex in the organic phase.