Binding of Multiple DEHP – Extractants to Er 3+ 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 Er 3+ 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 Er 3+ (H 2 O) x (DEHP – ) n complexes are different. The binding dynamics was fast for all interfacial and bulk binding events. Hydration water molecules rotate about the Er 3+ 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 Er 3+ is an order of magnitude lower than the probabilities of binding the first two DEHP – or of binding one, two, or three DEHP – to Er 3+ in bulk water.