Marie PLAZANET1, Gautier MEYER1, Isabelle BILLARD2, Ralf SCHWEINS3, Jean-François DUFRÊCHE4
1LIPhy- CNRS, Grenoble, France
2LEPMI, Grenoble, France
3Institut Laue Langevin, Grenoble, France
4ICSM, Marcoule, France
Ionic-liquid-based acidic aqueous biphasic solutions (AcABSs) recently o?ered a breakthrough in the ?eld of metal recycling. The particular mixture of tributyltetradecylphosphonium chloride ([P4,4,4,14]Cl), acid, and water is a biphasic solution. It presents the unusual characteristic of a lower solution critical temperature (LCST), meaning that phase separation occurs upon a temperature rise, typically here a few tens of degrees. In this solution, metallic ions dissolved in the monophasic state migrate upon heating toward their preferred phase, enabling the system to be used for efficient extraction. In the present work, we dig into the mechanisms driving the counter intuitive phase separation upon temperature rise observed in this system. The phase separation is moreover similarly observed with salt (NaCl) instead of acid (HCl), both mechanisms for separation being expected to be identical. Using small angle neutron scattering, we could identify the micellar structure under various conditions of acid and temperature, observing the micelle aggregation eventually leading to the phase separation. Temperature rise and addition of acid both induce the phase separation in an apparently similar mechanism.
In salt containing solutions, the fine titration of the chloride ions present in the solution revealed a thermally activated, exothermic adsorption of these ions at the micelle surface. Such ionic adsorption, similarly to acid addition, induces a screening of electrostatic interactions between the micelles, then their aggregation. These regions of lower density eventually collapse and cause the phase separation with a ionic-liquid rich phase on top of an acid-rich phase.
Such a mechanism of ionic or molecular adsorption at the micelle surface could be generalised for numbers of molecular systems presenting a Lower Critical Solution Temperature.
 G. Meyer, R. Schweins, T. Youngs, J.-F. Dufrêche, I. Billard and M. Plazanet, How Temperature Rise Can Induce Phase Separation in Aqueous Biphasic Solutions, J. Phys. Chem. Let. (2022), 13, 2731−2736.