Ionic Liquid Mixtures in Task-Specific Applications: Linking Composition, Physicochemical Properties, and Performance

Ionic liquids (ILs) are often described as designer solvents. However, a single cation–anion pair rarely optimizes phase stability, transport properties, and task-specific functions simultaneously. Mixing introduces composition as a continuous design variable, which can suppress crystallization, reorganize coordination and hydrogen-bond networks, redistribute ions on interfaces, and balance viscosity, conductivity, selectivity, and reactivity. This critical review analyses binary and higher-order IL mixtures, including common-ion, reciprocal, double-salt, eutectic, and glass-forming systems, with a focus on studies published from 2018 to 2026, and selected foundational reports. Applications include CO2 capture and gas separation, liquid separations, biomass processing, electrochemical energy technologies, thermal management, catalysis, electrospray propulsion, and biological systems. The review connects composition-dependent performance to phase behavior, thermal stability, density and excess volume, transport properties, interfacial organization, polarity, hydrogen bonding, and acid–base descriptors. Special attention is given to counterexamples and to distinguishing eutecticity, thermodynamic non-ideality, and functional synergy. The evidence indicates that the most useful formulation does not necessarily maximize a single property or coincide with the eutectic composition. Its benefit frequently lies in expanding the operating window or balancing multiple requirements. Rational formulation therefore requires composition-resolved measurements, explicit reference states, and multiobjective optimization from molecular structure to process or device performance.