Physicochemical properties and thermal-responsive phase separation of poly(ethylene glycol)-based ionic liquids

Abstract

• A series of PEGylated ionic liquids were synthesized. • Physico-chemical properties of PEGylated ionic liquids were measured. • AOT-based PEGylated ILs exhibit good surface activity in aqueous solutions. • PEGylated ionic liquids exhibit typical low critical solution temperature phase behavior. • Thermal-regulated phase separation system has great promise to separate metal ions. How to design thermal-responsive functional ionic liquids that can respond in a reversible manner still remains a challenge for the construction of ionic liquid-based smart systems, and they easily find applications in various scientific fields, including phase transfer catalysis, smart soft materials, controlled drug delivery and green separation. In this work, a series of PEGylated ionic liquids were prepared, and their structures were confirmed by NMR and FTIR spectra. The thermal properties of these ionic liquids were characterized by DSC and TGA. In addition, their physicochemical properties were measured in detail, including density and viscosity. Different models were used to fit the density and viscosity data. The critical micelle concentrations of the Aerosol OT-based PEGylated ionic liquids were also determined by the surface tension method, and they exhibited good surface activity. Furthermore, these PEGylated ionic liquids exhibit typical low critical solution temperature phase behavior. To illuminate the underlying mechanism of thermal-induced phase separation, temperature-dependent FTIR spectra were used to analyze the interaction between ionic liquids and water molecules. Finally, the thermally regulated PEGylated IL phase separation system was proposed as an efficient strategy to separate heavy metal ions and rare earth metal ions from water media. The results presented here offer key information to design new thermoresponsive ionic liquids and their applications.