Solubilisation–microstructure relationship of trivalent salts in hydrotropic ternary systems

We show here that there are four different regimes of pronounced mesoscopic structuring in the monophasic region of the water/sodium salicylate/ethyl acetate ternary phase diagram. Surprisingly, the solubility of various lanthanides is highest in these regimes containing pre-nucleation clusters and not in the complexation regime. All four local maxima of solubility are characterised by their scattering and explained in terms of the driving forces behind the structuring and as a function of the added ions. The position of the critical point was determined by regions where critical opalescence is maximal. After centrifugation, the points located at the boundary of the diphasic zone separate into two immiscible fluids of equal volumes. While the highest solubility of metal ions is found in the region of critical fluctuations, a still high solubility combined with a significant selectivity was found in the region of the pre-nucleation clusters. This new and previously unknown feature can also be explained and may pave the way for a new, green procedure of selective rare-earth metal recovery.