Synergism by coassembly at the origin of ion selectivity in liquid-liquid extraction

In liquid-liquid extraction, synergism emerges when for a defined formulation of the solvent phase, there is an increase of distribution coefficients for some cations in a mixture. To characterize the synergistic mechanisms, we determine the free energy of mixed coassembly in aggregates. Aggregation in any point of a phase diagram can be followed not only structurally by SANS, SAXS, and SLS, but also thermodynamically by determining the concentration of monomers coexisting with reverse aggregates. Using the industrially used couple HDEHP/TOPO forming mixed reverse aggregates, and the representative couple U/Fe, we show that there is no peculiarity in the aggregates microstructure at the maximum of synergism. Nevertheless, the free energy of aggregation necessary to form mixed aggregates containing extracted ions in their polar core is comparable to the transfer free energy difference between target and nontarget ions, as deduced from the synergistic selectivity peak.