Solvation Dynamics of HEHEHP Ligand at the Liquid-Liquid Interface

Actinide-lanthanide separation (ALSEP) has been a topic of interest in recent years as it has been shown to selectively extract problematic metals from spent nuclear fuel. However, the process suffers from slow kinetics, prohibiting it from being applied to nuclear facilities. In an effort to improve the process, many fundamental studies have been performed, but the majority have only focused on the thermodynamics of separation. Therefore, to understand the mechanism behind the ALSEP process, molecular dynamics (MD) simulations were utilized to obtain the dynamics and solvation characteristics for an organic extractant, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEHEHP). Simulations were conducted with both pure and biphasic solvent systems to evaluate the complex solvent interactions within the ALSEP extraction method. The MD simulations revealed solvation and dynamical behaviors that are consistent with the experimentally observed chemical properties of HEHEHP for the pure solvent systems (e.g., hydrophobic/hydrophilic behaviors of the polar head group and alkyl chains and dimer formation between the ligands within an organic solvent). When present in a biphasic solvent system, interfacial behaviors of the ligand revealed that, at low concentrations, the alkyl side chains of HEHEHP were parallel to the interfacial plane. Upon increasing the concentration to 0.75 M, tendency for the parallel orientation decreased and a more perpendicular-like orientation was observed. Analysis of ligand solvation energies in different solvents through the thermodynamic integration method demonstrated favorability toward n-dodecane and biphasic solvents, which is in agreement with the previous experimental findings.