Thymol-based deep eutectic solvents: a comprehensive overview of classifications, preparation methods, stability, and...

Thymol, a naturally occurring phenolic monoterpenoid predominantly found in Thymus kotschyanus and Achillea ageratum, has emerged as a key building block in the design of hydrophobic deep eutectic solvents (DESs). Owing to its strong ability to form stable eutectic systems with a wide variety of hydrogen bond donors and acceptors, including carboxylic acids, amides, amines and nitrogen-containing heterocycles, alcohols and diols, terpenoids (oxygenated derivatives of terpenes), quaternary ammonium salts, phenols and their derivatives, as well as other functional compounds, thymol has enabled the development of both binary and ternary DES formulations. These thymol-based DESs are increasingly recognized as a versatile class of green solvents, offering key advantages such as low toxicity, biodegradability, tunable physicochemical properties, and simple, energy-efficient preparation. This review provides a comprehensive and systematic overview of thymol-based DESs, with particular emphasis on their classification, preparation strategies, and phase behavior. Key thermophysical properties, including melting point, viscosity, density, and thermal stability, are critically analyzed, alongside the stability of these systems in aqueous environments to better understand their practical applicability. In addition, the broad application landscape of thymol-based DESs is briefly investigated, spanning extraction technologies for analytical, separation, and decontamination purposes; CO₂ capture systems with tunable absorption performance; and bioactive roles, including antibacterial and antifungal activity. Emerging applications, such as hydrophobic eutectogels, biomass processing, transdermal drug delivery, green synthesis of functional nanomaterials, and bioactive plasticizers for sustainable food packaging, are also discussed to underscore their growing interdisciplinary relevance. By consolidating current knowledge and identifying existing research gaps, this review aims to provide valuable insights into the design, properties, and future potential of thymol-based DESs as multifunctional and sustainable solvents for advanced scientific and industrial applications.