Understanding the Density and Viscosity Behavior of Thymol–Fatty Acid-Based DES: A Combined Experimental and Simulation Approach

This study aims to understand the thermodynamic properties of hydrophobic deep eutectic solvents (DESs) comprising thymol (THY) and the carboxylic acids decanoic acid (C10), undecanoic acid (C11), and dodecanoic acid (C12), utilizing experimental measurements alongside molecular dynamics (MD) simulations. To understand the influence of composition and thermal conditions on the macroscopic properties of the DESs, density (ρ) and viscosity (η) measurements were taken at 293.15 to 353.15 K and 293.15 to 323.15 K, respectively, at mole fractions of THY (XTHY) ranging from 0.1 to 0.9. MD simulations were carried out to elucidate the hydrogen bonding in the hydrophobic DES mixtures to complement the experimental work. The hydrogen bonds between thymol and the acid molecules were computed for a few compositions, enabling the characterization of interactions and bonding. The insights advanced the understanding of the structure–property relationships of DESs and reinforced the rational design of the solvents for thermal energy storage materials and environmentally friendly solvent systems.