A Review on Hydrophobicity, Toxicity, and Biodegradability of Deep Eutectic Solvents: Fundamentals and Characterization Techniques

Abstract

The continuous search for green, sustainable, and environmentally friendly solvents has led to the discovery of deep eutectic solvents (DESs). The unique properties of DESs allow them to be tailored to meet application requirements. However, the environmental sustainability of DESs cannot be established without understanding their hydrophobicity, toxicity, and biodegradability. This review critically evaluates the methods used to characterize these properties. The Karl–Fischer titration remains the most widely accepted technique for assessing hydrophobicity. The combination with NMR, GC-MS, LC-MS, TOC analysis, and solvatochromic studies provides better insight into water uptake, component leaching, and structural stability. The review demonstrates that hydrophobicity depends on the hydrophobic nature of the individual components and hydrogen-bonding interactions. The toxicity assessment using microbial assays, plants, mammalian cell lines, and animal models reveals significant inconsistencies across different biological models and experimental protocols. Similarly, biodegradability evaluated using OECD methods, BOD, UV-Vis spectroscopy, and GC-MS is influenced by microbial species and testing conditions. Nonetheless, the review highlights the lack of standardized protocols and emphasizes the need for harmonized methodologies to enable reliable environmental assessment for sustainable DESs.