Double-Stimuli-Responsive (Temperature and pH) Aqueous Biphasic Systems Comprising Ionic Liquids

Abstract

Aqueous biphasic systems (ABS) can integrate multiple unit operations and operate under continuous mode, contributing to the development of sustainable separation processes. Encouraged by the designer solvent features of ionic liquids (ILs), we herein propose their use as components of double-stimuli-responsive (temperature- and pH-driven) ABS. Resorting to choline-alkanoate-based ILs as the pH-responsive components and poly(propylene glycol) (PPG 400) as the thermo-responsive component, the ABS ternary phase diagrams are determined at various temperature (25–45 °C) and pH (3–7) conditions. While the liquid–liquid phase diagram response to temperature obeys a lower critical solution temperature-like behavior, the response to pH correlates with the pKa of the IL anion parent acid. The simultaneous responsiveness to temperature and pH is then shown, whose results inspire the development of customizable separation techniques as proved with the simultaneous (one-step) separation of two dyes. By a proper customization of the IL chemical structure and stimuli applied, ABS may be designed to improve the performance and sustainability of separation processes.