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

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Abstract
By means of molecular dynamics (MD) simulations of the biphasic behaviors of CyMe4-BTPhen (2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline) and its complexes with a heavy f-block metal ion, americium(III) (Am3+) in “oil”/water binary solvents, two of the key factors that determine the efficiency of a liquid-liquid extraction protocol, i.e. 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, were delineated and discussed. In the simulations, 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. The simulations showed that the charge states of both ligand and Am-L 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 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 fundamental chemical concept, i.e. minimization of hydrophilicity and maximization of lipophilicity to enhance phase transfer from aqueous phase to 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.
Funding Information
  • National Natural Science Foundation of China (11564015, 21473206, 91026000, 91226105)
  • Chinese Academy of Sciences (Y1515540U1, Y2291810S3)

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