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How temperature rise induces phase separation in acidic aqueous biphasic solutions

Ionic-liquid based acidic aqueous biphasic solutions (AcABS) recently offered a breakthrough in the field of metal recycling. Indeed, the mixture of tributyltetradecylphosphonium chloride (P$_{44414}$Cl) and acid with water content larger than 60 \% presents a phase separation with very good extraction efficiency for metallic ions. Moreover, this ternary solution presents a Lower Solution Critical Temperature (LCST), meaning that the biphasic area of the phase diagram increases upon increase of temperature, in other terms the phase separation from a homogeneous liquid can be induced by an elevation of temperature, typically a few tens of degrees. We address here the microscopic mechanisms driving the phase separation. Small Angle Neutron Scattering provides us with structural information for various acid content and temperature. We characterized the spherical micelle formation in the binary ionic liquid/water solution and the micelle aggregation upon addition of acid, due of the screening of electrostatic repulsion. If addition of salt leads to identical transitions in the solution, the ionic strength is not a relevant parameter and more subtle effects such as ion size or polarizability have to be taken into account to rationalize the phase diagram. The increase of both acid concentration and/or temperature eventually leads to the micelle flocculation and phase separation. This last step is achieved through chloride ion adsorption at the surface of the micelle with an enthalpy of adsorption of $\sim$ 12 kJ/mol. The attraction between micelles can be well understood in terms of DLVO potential. This exothermic adsorption compensates the entropic cost, leading to the counter-intuitive behavior of the system.