Résumé
Liquid-liquid extraction of metal ions is a process based on the difference in affinity of a solute between two immiscible phases. To promote the selective transfer to the phase to be enriched, an amphiphilic phase transfer agent called ligand is usually added in the organic phase. However, as 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, 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, experimental data on the structure of these interfaces at the nanoscale are scarce due to the complexity of measuring these buried interfaces.Our experiments combining neutron and X-ray reflectivity on two ligand-enriched interface (DMDOHEMA and DMDBTDMA) used for separation of lanthanides and actinides form fission products in the DIAMEX process have made it possible to access this interfacial structure. The observed experimental structure being the result of a complex treatment of numerous reflectivity data, it was necessary to corroborate the experimental results by the use of Molecular Dynamics (MD) simulations without any prior assumption on the ligand molecule. 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 (DMDOHEMA) distributed at equilibrium between the interface and the organic phase. ResultsWe have observed by reflectivity the formation of either interface or interphase depending on the nature of the ligand molecule. As consequence, the trivalent cations can be repelled or attracted by extractant-enriched interface with some macroscopic effects on the extraction kinetics. The MD simulations confirmed the distribution at the interface obtained by reflectivity data treatment. There is a 20 Å layer of adsorbed ligand at the interface followed by depletion between the interface and the bulk solvent. The surface tension calculated from the excess of molecule at the interface is in good agreement with the experimental data as well as the aggregation in the organic phase. The well structure interface with a low mobility of the DMDOHEMA is confirmed.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.