Binding of Multiple DEHP^- Extractants to Er³⁺ in Bulk Water and at the Liquid-Liquid Interface

The binding of rare earth elements to extractants is a key step in their separation and purification by solvent extraction. Classical MD simulations are used to investigate the equilibration configuration and binding dynamics of one, two, and three DEHP^- to Er³⁺ in both bulk water and at the water-dodecane interface. The equilibrium radial distribution function g(r) shows that Er-O and Er-P radial separations within the first and second hydration shells are essentially identical for all bulk and interface simulations, though other aspects of the spatial configuration of heteroleptic Er³⁺(H₂O)_x(DEHP^-)_n complexes are different. The binding dynamics was fast for all interfacial and bulk binding events. Hydration water molecules rotate about the Er³⁺ ion and away from the binding DEHP^- upon its approach and water molecules are ejected in well-defined directions from the incoming DEHP^-. Potential of mean force calculations yield the height of the reaction barrier, which provides insight into the results of calculations of the binding dynamics. Interfacial binding of DEHP^- to rare earth ions is expected to dominate the solvent extraction process, and we find that the probability of interfacial binding of a third DEHP^- to Er³⁺ is an order of magnitude lower than the probabilities of binding the first two DEHP^- or of binding one, two, or three DEHP^- to Er³⁺ in bulk water.