Determination of the structure in organic solution combining experimental characterization and molecular dynamic simulation
Résumé
In the frame of the nuclear fuel recycling, various solvent extraction processes have been developed using mono-or di-amide extractants in an aliphatic diluent. In order to better understand the extraction mechanisms involved in such extraction processes, a detailed description of the organic phases is essential. The properties of the solvent extraction system have traditionally been understood from concepts rooted in coordination chemistry. However, since the researches done by Osseo-Assare in the beginning of the 90s, it is now well established that, due to the extractants amphiphilic properties, the organic phases involved in such processes (especially at high solute concentrations) are not molecular solutions of extractants but, rather, structured solutions with an organization at the supramolecular scale. The speciation in organic phase after solvent extraction remains therefore challenging since both level of description have to be assessed concomitantly. To overcome this issue, a new approach, combining experimental studies with molecular dynamic (MD) simulations has been developed. The solutions were first prepared at the laboratory and characterized in order to obtain: (i) the phases composition (quantitative analysis of all the organic phase constituents) that allowed us to build virtual solution "boxes" for the MD simulations with precisely the same composition than the experimental ones, and (ii) the structural data related to this experimental phases (manly: densities and Small and Wide Angle X-Ray Scattering-SWAXS). Molecular dynamic simulations were then performed and the structural data were computed from the simulation trajectories. As soon as these computed data match the experimental ones (Figure 1), the simulated solution is assumed to be representative of the experimental one. The MD simulation can then be accurately analyzed to describe the structure of the solution at both the molecular and the supramolecular levels.
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