CO2-Switchable Phase Separation of Nonaqueous Surfactant-Free Ionic Liquid-Based Microemulsions

Surfactant-free microemulsions (SFMEs) have shown great potential in many aspects such as chemical reactions and material preparation. Ionic liquid (IL)-based SFME is a promising member in this family. However, switchable phase separation of nonaqueous IL SFMEs has not been reported up to now although it is of great importance for the separation of products and recycling of the systems. Herein, a new kind of CO2-responsive SFMEs composed of quaternary ammonium IL, dimethyl sulfoxide, and ethyl acetate has been developed. It is worth noting that the microemulsion can be reversibly switched between emulsification and complete phase separation upon alternative CO2 and N-2 bubbling at atmospheric pressure. The microstructures and phase behavior of the systems before and after CO2 bubbling have been systematically studied by phase diagrams, electrical conductivity, dynamic light scattering, optical microscopy, and atomic force microscopy, respectively. Mechanism studies verify that the reversible switching is attributed to the generation and restoration of more hydrophilic carbamate salts from the reaction of CO2 with IL anions, which results in poor solubility of ILs in the nonaqueous microemulsions and thus reversible breaking and rebuilding of the microemulsions. Based on this unique phase behavior, the CO2-switchable SFMEs are used as a microreactor for room temperature synthesis of Zn-based metal-organic frameworks, and effective reaction and microemulsion recycling have been achieved. This study may provide a new strategy for the integration of reaction, separation, and recycling procedures to obtain sustainable chemical processes.