Deep Eutectic Solvents: Physicochemical Fundamentals, Extraction Mechanisms, and Industrial Perspectives

Deep eutectic solvents (DESs) are composition-sensitive liquid mixtures investigated for metal recovery, electrodeposition, biomass processing, catalysis, and carbon dioxide capture. Nevertheless, a low melting temperature, apparently facile preparation, or a renewable starting component does not, by itself, establish eutectic nonideality, separation selectivity, or environmental superiority. This critical narrative review evaluates the relationships among mixture phase behavior, chemical speciation, water content, transport, feed chemistry, and solvent regeneration. Evidence is assessed using an explicit hierarchy that distinguishes direct controlled experiments, realistic-feed demonstrations, analogous studies, and unvalidated process hypotheses. The main weakness identified across applications is the frequent substitution of first-cycle extraction or uptake values for whole-process performance. Accordingly, we propose a structure–speciation–transport–regeneration framework emphasizing specified product recovery, competing-element rejection, solvent makeup, impurity accumulation, cycle stability, and consistent process boundaries. The review also sets minimum experimental reporting requirements and a stage-gated evaluation to compare DES-based routes with aqueous hydrometallurgy, conventional separation processes, and ionic-liquid systems. Future demonstrations should prioritize independently characterized formulations, actual industrial feeds, repeated or continuous solvent operation, and integrated techno-economic and life-cycle assessment. DES-based technologies may be advantageous in particular chemical niches, but their technical and environmental merit must be demonstrated for each formulation and application.