Supramolecular organisation of tri-n-butyl phosphate in organic diluent on approaching third phase transition

The supramolecular organisation of organic phases of the extractant tri-n-butyl phosphate (TBP) in alkane diluents is studied. Effects of extractant and nitric acid concentration, temperature and acid nature on third phase formation are presented. The aim is to gain information on the structural aspect of the organic phase just before phase splitting in liquid - liquid extraction. Small-angle X-ray and neutron scattering combined to vapour pressure osmometry, tensiometry and conductivity are used to characterise these systems. It is shown that organic phases of TBP in equilibrium with acid solutions are organised in reverse interacting aggregates and that these interactions govern the formation of the third phase. These aggregates disappear at high temperatures, even in the presence of HNO3, to form regular molecular solutions. The shape and size of the aggregates are not modified by varying the acid nature, HNO3 or TBP concentrations, whereas low temperatures and polarisability of the aqueous micellar core affect the strength of the interactions. The sticky sphere model proposed by Baxter is used successfully to model the small reverse micelles of the organic TBP phases and quantify their interactions.