Composition-Dependent Physicochemical Properties and Structure–Property Relationships of Thymol- and Menthol-Based Hydrophobic Eutectic Solvents

Abstract Hydrophobic eutectic solvent (HES) formulations based on neutral, naturally derived components offer broad compositional tunability, but systematic comparisons linking molecular framework and composition to multiple bulk properties remain scarce. Here, thymol-monoethanolamine (Thy-MEA), thymol-ethylene glycol (Thy-EG), and menthol-monoethanolamine (Men-MEA) formulations were prepared at nominal molar ratios of 1:1, 1:2, and 1:3. Density, dynamic viscosity, surface tension, and refractive index were measured from 288.15 to 318.15 K and described using temperature-dependent correlations and a viscous-flow activation model. All four properties decreased with increasing temperature, whereas their composition dependences were property-specific. The thymol-based systems generally exhibited higher densities and refractive indices than Men-MEA, consistent with differences in average molecular packing between the relatively rigid aromatic and more conformationally flexible cycloaliphatic frameworks. Increasing the proportion of monoethanolamine or ethylene glycol reduced the apparent activation energy for viscous flow, indicating a lower barrier to molecular rearrangement. Cross-property comparison further showed that the volumetric, transport, interfacial, and optical responses were not fully concerted. These results establish experimentally supported relationships among molecular constitution, composition, thermal response, and macroscopic liquid behavior in monoterpene-based Type V HES formulations.