Physicochemical and Biological Properties of Menthol and Thymol-Based Natural Deep Eutectic Solvents

Seven hydrophobic deep eutectic solvents (hDESs) were characterised to evaluate their potential applicability in different industries and their environmental impact. Standard physicochemical properties were determined, yielding polarity and density values that were slightly higher for thymol-based hDESs than menthol-based ones, whereas for viscosity, the trend was opposite. Regarding biologically relevant activities, the antioxidative capacity and antimicrobial activity of hDESs were determined. Thymol-based hDESs are more potent as potential antioxidants, especially the one with coumarin as a hydrogen bond acceptor, which had the highest Oxygen Radical Absorbance Capacity (ORAC) value. Antimicrobial activity was assessed on four bacterial strains and one yeast strain. Calculated minimal inhibitory concentrations (MICs) showed that all hDESs possess this activity, and even the antimycotic effect against C. albicans was observed. Furthermore, to ensure the safety of these solvents for human use, in vitro cytocompatibility was determined. hDESs were tested on three human cell lines (HaCaT, CaCo-2, and HeLa), with no cytotoxic effect observed up to 1000 mg L−1. Finally, the environmental impact by the phytotoxicity test and in vitro antioxidative assay on wheat was determined for three selected hDESs, which were found to be slightly toxic, with different effects on plant defence mechanisms against induced antioxidative stress. Overall, the tested terpene-based hDESs demonstrate potential as alternative solvents for various industries, including food production, cosmetics, and pharmaceuticals, with thymol-based variants exhibiting a slight advantage in relation to the parameters evaluated in this study.