Temperature-Responsive Ionic Liquids: Polymer Structure, Phase Transition Behaviors and Applications

Temperature-responsive ionic liquids (TRILs) represent a significant advance of smart functional materials with thermally tailorable phase transition behaviors. TRILs exhibit typical lower critical solution temperature (LCST) and upper critical solution temperature (UCST) liquid–liquid phase transitions, as well as versatile liquid–solid transition modes involving crystallization, precipitation, and liquid crystal formation, which endow TRILs with superior tunability inaccessible to conventional solvents and ordinary ILs, enabling reversible regulation of solubility, miscibility, ion transport, and interfacial properties under temperature stimuli. This review systematically elaborates the structural characteristics of TRIL cations and anions, clarifies the intrinsic mechanisms of thermally triggered phase transitions, and comprehensively summarizes state-of-the-art advances in multidisciplinary applications covering water treatment, gas separation, controlled drug delivery, switchable catalysis, and electrochemical energy storage. We fully discuss the structural advantages and working principles of TRIL-based smart systems, and analyze the current bottlenecks including high intrinsic viscosity, limited regulation range, ambiguous biocompatibility, and inadequate long-term cycling stability. Future research directions are further proposed to guide the rational molecular design, composite modification, and multi-stimuli integration of high-performance TRIL materials. This work aims to provide a comprehensive and systematic reference for the development and industrial translation of next-generation thermoresponsive functional materials with case-specific environmental advantages.