Ionic Liquids and Deep Eutectic Solvents as Functional Interfacial Phases in Membrane-Based Separations

In this Spotlight on Applications, we describe our progress in developing membrane-based separation processes in which ionic liquids (ILs) and deep eutectic solvents (DESs) act as functional interfacial phases rather than merely as alternative solvents or extractants. Starting from stability limitations identified in supported ionic liquid membranes, our work evolved toward architectures in which polymeric materials selectively contact, confine, distribute, or stabilize ionic and eutectic liquid phases. We organize these contributions according to the location and function of the active interface: external polymer-ionic liquid interfaces in dense membrane systems; localized ionic or eutectic phases confined within multilayer membrane architectures; distributed polymer-DES domains in dense hybrid membranes; membrane-stabilized contact with mobile immiscible liquid phases; and liquid-vapor-membrane interfaces in ionic-liquid-driven osmotic distillation. Each architecture builds on an established membrane-process category - perstraction, coated liquid membranes, polymer inclusion membranes, membrane contactors, and osmotic distillation - and is distinguished by the location, flow regime, and function of the ionic or eutectic phase. These configurations were applied to fermentation-product recovery, selective separation of target molecules, ethanol dehydration, food concentration, and water recovery from saline streams. Across these applications, performance emerged from the synergistic coupling of polymer sorption and diffusion, liquid-phase affinity, wetting, morphology, interfacial stability, and resistance distribution. COSMO-RS provided a complementary progression from solute-ionic-liquid screening to direct polymer screening and, subsequently, to thermodynamic interpretation of hybrid polymer-eutectic membrane environments. Collectively, these studies indicate that the polymer, ionic or eutectic phase, solute, interface, operating mode, and membrane architecture should be designed as an integrated interfacial system.