How Temperature Rise Can Induce Phase Separation in AqueousBiphasic Solutions

Ionic-liquid-based acidic aqueous biphasic solutions(AcABSs) recently offered a breakthrough in thefield of metalrecycling. The particular mixture of tributyltetradecylphosphoniumchloride ([P4,4,4,14]Cl), acid, and water presents the unusual character-istic of a lower solution critical temperature (LCST), leading to phaseseparation upon a temperature rise of typically a few tens of degrees.We address here the microscopic mechanisms driving the phaseseparation. Using small-angle neutron scattering, we characterized thespherical micelle formation in a binary ionic liquid/water solution andthe micelle aggregation upon the addition of acid due to the screeningof electrostatic repulsion. The increase in both the acid concentrationand the temperature eventually leads to micelleflocculation and phaseseparation. This last step is achieved through chloride ion adsorptionat the surface of the micelle. This exothermic adsorption compensates for the entropic cost, leading to a counterintuitive behavior,and may be generalized to a number of molecular systems with an LCST.

Automated summary

Ionic-liquid-based acidic aqueous biphasic solutions have made a breakthrough in metal recycling. The separation of phases can be achieved by controlling the temperature and acid concentration, which is facilitated by the adsorption of chloride ions on the surface of micelles.