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.