Zooming into the polarity of deep eutectic solvents

Deep eutectic solvents (DESs) continue to gain momentum across analytical chemistry, catalysis, extractions, and separations, yet polarity, one of their most influential properties, remains inconsistently defined, measured, and interpreted. Despite its central role in governing DES behaviour, polarity accounts for nearly 1% of all reported DES physicochemical characterisation studies. This review offers a unique unified synthesis of how polarity is described in DESs and what fundamentally governs it. Molecular probe-based and interfacial methods, together with computational approaches are evaluated in terms of what aspect of the DES environment they actually measure. By analysing reported datasets across methods and DES families, the review shows that polarity is strongly context-dependent; it is primarily shaped by hydrogen bond acceptor and donor identities, and impacted by water, additives, and temperature. Method-dependent variability and probe sensitivity are identified as major sources of inconsistency in the literature. The review highlights the gap in current practices and argues that polarity descriptors must be selected and interpreted based on analytical intent rather than convention. This review positions polarity as a mechanistic and design-relevant property, rather than a standalone characterisation metric. Ultimately, a fundamental understanding of DES polarity is indispensable for the rational design, optimisation, and deployment of DESs in real-world applications; otherwise, the capabilities of these multifunctional agents will remain only partially realised.