Revisiting the Definition, Solid–Liquid Equilibria, and Thermodynamic Nonideality of Deep Eutectic Solvents

Since their introduction, deep eutectic solvents (DESs) have been widely used, often without a rigorous or consistent definition. This has led to misuse of the term, where DESs are applied without considering their underlying thermodynamics, resulting in misclassification and studies that overlook essential physicochemical behaviors. This review aims to provide a comprehensive understanding of DESs by examining their definitions, thermodynamic properties, phase behavior, and modeling approaches through representative examples. First, common misconceptions regarding DES definitions are addressed, followed by a clear, thermodynamically grounded definition. Next, the types of components capable of providing the nonideal thermodynamic interactions necessary for DES formation are outlined. The review then emphasizes the importance of constructing solid–liquid equilibrium (SLE) phase diagrams for accurately defining DESs, discussing both experimental and thermodynamic modeling methods for determining these diagrams. Recent studies on SLE determination of DESs using various approaches are also summarized. Finally, the effects of water on DES stability and other challenges are discussed, along with perspectives for future research. Overall, DES research holds great promise, and this review supports the rational design, selection, and modeling of DESs, which are essential for efficient and practical process development.