PRObing the aqueous interface of a Ionic Liquid during Extraction – PROfILE
What happens at the interface between an aqueous phase and an immiscible "oil" phase is stressed to be of vital importance in processes like phase transfer catalysis, separation by liquid-liquid extraction or assisted ion extraction. The interface is not a mere boarder, but a peculiar region of the solution where reactive species (in the case of phase transfer catalysis) or ions and extractants (in the case of assisted ion transfer) presumably meet and interact with each other, prior being transferred into the preferred bulk liquid phase. Some species (e.g. hard ions) can be "repelled" by the interface, while others are "attracted", thereby forming organized structures, ranging from locally concentrated "random" solutions, to highly organized systems (e.g. more or less concentrated layers, multilayers, films, etc…). The interfacial landscape depends on the precise content of the two "bulk" phases and can be fairly complex, in terms of composition and structure.
In this proposal we focus on the interfacial processes involved in cations extraction by specific ligands ("extractants"), and aim at describing, at the nanoscopic and molecular levels, the characteristics of water-"oil" interfaces at different stages of the process. Instead of classical organic "oil" phases, we select hydrophobic Room-Temperature Ionic liquids ("ILs") that form biphasic systems with water. Ion extraction to such ILs is generally much more efficient than to classical molecular liquids, sometimes involving different mechanisms. Furthermore, the molecular structure of their aqueous interfaces involved in extraction processes remains quite elusive. As a result, the detailed sequence of complex formation (ion + counterions + extractant molecules) and transfer, as well as the specificities of aqueous interfaces with ILs, compared to classical interfaces, are not well understood. Surface analytical tools to directly monitor molecular adsorption and organization at interfaces are thus of outmost importance. The innovation and breakthrough of the PROfILE project are to propose a new methodology to probe in situ the structure of water/IL interfaces, and to depict the interfacial extraction mechanism. For these purposes, model interfaces will be probed, coupling non-linear optical spectroscopy (Second Harmonic Generation "SHG"), extraction and surface tension experiments, and simulations (Molecular Dynamics "MD"). The content of the bulk phases will be analyzed by spectroscopic studies (EXAFS, metal luminescence, NMR, Hyper Rayleigh Scattering "HRS"), chemical analysis and MD. This fundamental work is supported by four complementary teams: ILM (Team-1), CMC (Team-2), IPNL (Team-3) and IPHC (Team-4). The combination of SHG and MD, namely SHG-MD, constitutes a novel approach to probe the water/IL interface, at equilibrium and during the extraction process. The ILs used in this project, noted CnmimTf2N, are based on different Cnmim+ imidazolium cations and on the Tf2N- anion. The biphasic system composed of the acidic aqueous phase with Eu(III) as target metal ion and of IL phase with heterocyclic organic ligand of BTP (bis-1,2,4-triazinyl pyridine) type will be our reference extraction system. Besides, we will combine the inherently surface specific optical technique of SHG with bulk phase analyses to give a detailed balance of the transferred species as well as the structure and the dynamics of the interface during the transfer reaction. Finally, MD simulations will bring the required complementary tool to provide a molecular picture of interfaces at different stages of the process. The results will allow us to understand at the microscopic level the modifications of the interface composition and structure along the transfer process, and to depict the underlying mechanisms. They will impact the burgeoning fields of ILs for the metal ions separation or recovery, especially for rare earth elements, and of IL interfaces.
Monsieur Emmanuel Benichou (Institut Lumière Matière)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
IPNL - CNRS Institut de physique nucléaire de Lyon
IPHC Institut pluridisciplinaire Hubert Curien
ILM - CNRS Institut Lumière Matière
CMC Chimie de la Matière Complexe
Help of the ANR 447,032 euros
Beginning and duration of the scientific project: January 2018 - 48 Months