Room temperature ionic liquids and deep eutectic solvents: distinct classes or a continuum of liquid matter?

The evolution of low-melting ionic and molecular solids has followed a continuous path from classical molten salts and eutectic mixtures to modern room-temperature ionic liquids (RTILs) and deep eutectic solvents (DESs). Although these materials have traditionally been classified according to composition, melting point, phase behavior, or specific intermolecular interactions, growing experimental and theoretical evidence indicates that they are best understood as dynamic condensed phases in which Coulombic interactions, ion pairing, coordination chemistry, hydrogen bonding, dispersion forces, and supramolecular organization act cooperatively across multiple length and time scales. Consequently, molten salts, RTILs, and DESs should not be regarded as fundamentally distinct classes of liquids, but rather as members of a continuous landscape of ionic, molecular, and ionic-molecular fluids in which the balance between ionicity, molecular association, and supramolecular organization governs structure and properties. By revisiting the historical development of the field, from eutectic fused salts and Friedel-Crafts melts to modern nanostructured ILs and DESs, this Review proposes a unified framework for understanding and classifying liquid matter beyond the traditional boundaries separating ionic and molecular systems.