Eco‐Smart Ionic Liquids in the 21 st Century: A Comprehensive Review on Molecular Innovation for Sustainable Environmental Solutions

Ionic liquids (ILs) have matured from laboratory curiosities to eco‐smart, molecularly tunable media underpinning sustainable chemical technologies. This review integrates a bibliometric mapping of the 2000–2025 literature with a critical synthesis of IL design principles, structure–property relationships, and application landscapes. Additionally, an outline of how cation/anion engineering—including task‐specific and bio‐based ILs (BILs) and deep eutectic solvents (DESs)—governs viscosity, polarity, conductivity, electrochemical stability, and solvation, enabling “designer solvents” tailored for green catalysis, separations, and energy systems, is also addressed. Characterization (Fourier transform infrared [FTIR], nuclear magnetic resonance [NMR], X‐ray diffraction [XRD], thermogravimetric analysis [TGA]/differential scanning calorimetry [DSC], electrochemistry, and UV–Vis) is linked to performance metrics to establish predictive correlations. Applications span VOC‐free catalysis, CO 2 capture and selective extraction, advanced oxidation and adsorptive remediation (including IL–MOF, IL–biochar, and magnetic IL hybrids), and electrochemical platforms (lithium‐ion batteries [LIBs]/sodium‐ion batteries [SIBs], supercapacitors, fuel cells, redox‐flow, and solid‐state devices). A bibliometric perspective documents rapid growth and thematic diversification toward energy and environmental technologies, with increasing emphasis on BILs/DESs. Persistent barriers—costly synthesis/purification, high viscosity and mass‐transfer penalties, toxicity/biopersistence for certain ions, and energy‐intensive recycling are analyzed alongside mitigation strategies (halogen‐free BILs, structural tuning, membrane/electrochemical recovery, and standardized lifecycle assessment [LCA]). Looking forward, we highlight hybrid IL‐materials (IL–metal–organic frameworks [MOFs], poly(IL)s), circular‐economy integration, and AI/ML‐driven quantitative structure–property relationship (QSPR) screening as accelerants for scalable, low‐impact deployment. Collectively, ILs are positioned as molecular innovations for sustainable environmental solutions, translating tunable ion chemistry into practical gains in green manufacturing, clean energy, and wastewater remediation.