Solvation Dynamics of HEHEHP Ligand at the Liquid–Liquid Interface

Abstract

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.

Key Factors Determining Efficiency of Liquid–Liquid Extraction: Implications from Molecular Dynamics Simulations of Biphasic Behaviors of CyMe4-BTPhen and Its Am(III) Complexes

“…For example, these studies covered the ligands of BTPhen, 20 BTBP, 22 18-Crown-6, 53−56 tri-n-butyl phosphate (TBP), 57−59 calix [4]arene, 60 N,N-di(2-ethylhexyl)isobutyramide (DEHi-BA), 61 and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEHEHP). 62 It is now established that the complexation of ligands with metal ions happened at the interface. The key influencing factors relevant to the composition of the extraction systems, for example, the effect of synergistic reagent and counter-ions, the nature of the diluents, the acidity of the aqueous phase, and their effect on the dynamics of extracted species and their migration behavior to cross the interface, have been also analyzed for specific model systems in earlier works, and it remains to enrich and generalize our understanding on these issues to assist the advancement of experimental studies.…”

“…To assist the development of LLExt protocols, in an earlier work, MD simulations have been carried out to study the biphasic behavior of extractants and the extracted complexes. For example, these studies covered the ligands of BTPhen, BTBP, 18-Crown-6, − tri- n -butyl phosphate (TBP), − calix[4]­arene, N , N -di­(2-ethylhexyl)­isobutyramide (DEHiBA), and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEHEHP) . It is now established that the complexation of ligands with metal ions happened at the interface.…”

Dynamic and Solvation Behaviors of ALSEP Organic Ligands

“…Ab initio calculations were carried out using the Gaussian09 software package. 23 Geometry optimizations of HEHEHP and n-dodecane were taken from earlier work 13 while the structures of T2EHDGA and nitrate were optimized using the same level of theory (M06-2X//6-311G(2d,d,p)). Partial atomic charges (Table S1, Supporting Information) were obtained using the restrained electrostatic potential (RESP) method at MP2/cc-PVTZ// M06-2X/6-311G(2d,d,p) level of theory (Tables S1 and S2, Supporting Information).…”

“…Several solvent extraction processes have been developed to target this difficult separation including the SANEX, GANEX, and advanced TALSPEAK processes (acronyms are defined within the Supporting Information), as well as the recently developed Actinide-Lanthanide Separation Process (ALSEP). 5,6,9,13 However, each of these processes have drawbacks at the industrial scale.…”

Interfacial Behavior of Acidic Organophosphorus Extractant Monolayer at Air-Water Interface: Subphase pH and Spreading Solvent Effect

“…而二氯甲烷溶剂铺 展的 P507 单分子膜中聚集体较少, 分子单体不能依靠 "晶核"快速聚并成聚集体 [16] . P507 分子在界面以单体 形式或二聚体形式存在, 其极性端深入水相的角度和深 度必定会存在差异, 这必然会影响到磷氧基团水化及其 萃取水相离子的能力 [37] . 因此, 对聚集体或二聚体分子 在界面如何排布和取向仍需深入研究.…”