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Ionic-liquid based acidic aqueous biphasic solutions (AcABS) recently offereda breakthrough in the field of metal recycling. Indeed, the mixture oftributyltetradecylphosphonium chloride (P$_{44414}$Cl) and acid with watercontent larger than 60 \% presents a phase separation with very good extractionefficiency for metallic ions. Moreover, this ternary solution presents a LowerSolution Critical Temperature (LCST), meaning that the biphasic area of thephase diagram increases upon increase of temperature, in other terms the phaseseparation from a homogeneous liquid can be induced by an elevation oftemperature, typically a few tens of degrees. We address here the microscopicmechanisms driving the phase separation. Small Angle Neutron Scatteringprovides us with structural information for various acid content andtemperature. We characterized the spherical micelle formation in the binaryionic liquid/water solution and the micelle aggregation upon addition of acid,due of the screening of electrostatic repulsion. If addition of salt leads toidentical transitions in the solution, the ionic strength is not a relevantparameter and more subtle effects such as ion size or polarizability have to betaken into account to rationalize the phase diagram. The increase of both acidconcentration and/or temperature eventually leads to the micelle flocculationand phase separation. This last step is achieved through chloride ionadsorption at the surface of the micelle with an enthalpy of adsorption of$\sim$ 12 kJ/mol. The attraction between micelles can be well understood interms of DLVO potential. This exothermic adsorption compensates the entropiccost, leading to the counter-intuitive behavior of the system.

How temperature rise induces phase separation in acidic aqueous biphasic solutions