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Key Factors Determining Efficiency of Liquid–Liquid Extraction: Implications from Molecular Dynamics Simulations of Biphasic Behaviors of CyMe4‑BTPhen and Its Am(III) Complexes
journal contribution
posted on 2020-02-24, 18:08 authored by Ziyi Liu, Xu Ren, Rongri Tan, Zhifang Chai, Dongqi WangCyMe4-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.