Extraction of cadmium by TODGA–dodecane and TBP–dodecane: A comparative study by MD simulation

Molecular-scale understanding of diluent effects on ligand assembly for metal ion separations

Use of metal-selective ligands in solvent extraction is instrumental in extraction of critical materials and recycling, yet, diluent effects on extraction performance are not well understood. Experimental and empirical solvent parameters have been proposed to correlate with extraction performance, but are often inadequate predictors. We follow the hypothesis that the diluents' primary influence on extraction efficiency is whether or not it hinders assembly of the bulky extracting ligands into a geometry necessary for metal complexation. This behavior is readily accessible with molecular dynamics (MD), where the atomistic description of molecules can be applied to arbitrary extractant-solvent molecules and their mixtures. Several simulated quantities are considered, from both pairwise and graph theoretical analyses, and compared to experimental distribution ratio data for americium extraction by TODGA in a series of inert, non-interacting diluents. These simple properties, especially the formation of closed triplets corresponding to the 3 : 1 ligand : metal stoichiometric solvate, suggest a potential predictive power of this approach. This methodology provides a path forward to comprehensively understand and predict diluent effects in more complex systems involving different extracting ligands and multi-component diluent mixtures.

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

CyMe₄-BTPhen (2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline, denoted as L) has been considered as a promising extractant in lanthanide(III)/actinide(III) separation. Vast endeavors in its application put forward a compelling need on the understanding of the underlying mechanism in the liquid-liquid extraction. To address the issue of its dynamics in biphasic systems, we carried out molecular dynamics (MD) simulations of L and its complexes with a heavy f-block metal ion, americium(III) (Am³⁺) in "oil"/water binary solvents. Two types of organic phases have been considered, differing in the presence of octanol in the bulk n-dodecane or not, and the distribution of the solutes and their interfacial behaviors have been investigated. Two of the key factors that determine the efficiency of a liquid-liquid extraction protocol were delineated and discussed, that is, the appropriate ligand to enhance the lipophilicity of AmL complexes and appropriate way to form ion pairs to minimize the attraction between the complexes and aqueous phase. The simulations showed that the charge states of both ligand and AmL complexes were strongly correlated with their phase behavior, and the migration of neutral species was driven by van der Waals interactions while that of charged species by electrostatic interactions, indicating stronger lipophilicity of the former than the latter. The presence of octanol facilitated the migration of the ligand from the interface to the organic phase via hydrogen bond between its polar head and the ligand or the AmL complexes and constituted a polar core in the organic phase. This work bridged the widely used liquid-liquid extraction technique in chemistry to a fundamental chemical concept, that is, minimization of hydrophilicity and maximization of lipophilicity to facilitate phase transfer from the aqueous phase to the organic phase, and is expected to improve the understanding of dynamics of ligands and their complexes with metal ions and to contribute to the development of efficient protocols for phase transfer of target species.