Résumé
Ion separation by liquid-liquid extraction is a process based on the difference in affinity of a solute between two immiscible phases. In hydrometallurgy, to promote the selective transfer of metal ions to the phase to be enriched, an amphiphilic phase transfer agent called ligand is usually added in the organic phase. However, when the energy of "ligand/metal ion" complexation is small and the solubilization of the complex in the oil phase depends on the aggregation of the ligand in this phase, then the structural organization of the ligands at the water/oil interface can be decisive for the transfer kinetics. Thus, detailed knowledge of the supramolecular structuring of these ligand-containing interfaces is essential to understand and to simulate ion transfer. Unfortunately, obtaining experimental data on the structure of these buried interfaces at the nanoscale is very complex which makes them almost non-existent. Recent experiments combining X-ray and neutron reflectivity on extractant molecules used for actinide/lanthanide separation have made it possible to access this information highlighting some differences between interface and interphase. The analysis of these measurements shows that trivalent cations can be repelled or attracted by extractant-enriched interface depending on the nature of the ligand molecule.The observed experimental structure being the result of a complex treatment of numerous reflectivity data, it was necessary to verify if the experimental results could be corroborated by the use of molecular modelling without any prior assumption on the ligand molecule. Molecular Dynamics (MD) simulations of real solutions were performed, explicitly accounting for polarization effects of all atoms using AMBER software to simulate a water/oil interface containing malonamide extractant molecules (typically DMDOHEMA) distributed at equilibrium between the interface and the organic phase. Comparison of measurement and MD results confirms not only the analysis of large instrument data, but also efficiency of MD simulations in interpreting the experiments.