Diving into Natural Solvents: A Comprehensive Analysis of Deep Eutectic Systems Based on Terpenes and Flavonoids

Terpenes and flavonoids are secondary metabolites present in plants that display interesting biological properties. In this work, we study six nonionic (type V) natural deep eutectic solvents (NADES), in a total of nine studied, based on three terpenes (d,l-menthol, thymol, and carvacrol), used as hydrogen-bond donors, combined with three flavonoids (flavone, flavanone, and 2 '-hydroxychalcone), which act as hydrogen-bond acceptors. These NADES were characterized in terms of their solid-liquid phase diagrams to evaluate their liquid window. The collected data were modeled using the universal quasichemical (UNIQUAC) model, providing a detailed correlation with the nonidealities of these systems. All systems, except for those containing alpha-terpineol, present large deviations from the ideal behavior, which justifies the use of an excess Gibbs energy model such as UNIQUAC. Thermophysical properties, including mutual water solubility, density, and viscosity, were measured. The three flavonoid-based NADES present very similar densities but very different viscosities, with the flavone-based being the most viscous and the 2 '-hydroxychalcone-based being the most fluid, with viscosity increasing with the increase in the flavonoid content. All the studied NADES are hydrophobic, and generally, the water solubility decreases with the flavonoid proportion, with the exception of the flavone:thymol (0.4) NADES. The solubility of water in the selected terpenes and flavonoids is much lower than the solubility of water in the corresponding NADES. For all NADES the eutectic points occurred at small molar fractions of flavonoids. Cytotoxicity assays demonstrate safe use, with a toxicity level like those of starting components.