Thermal Behavior, Density and Viscosity of Terpene-Based Hydrophobic Eutectic Solvent Systems with Alcohols and Carboxylic Acids: Comparison with Tetrabutylphosphonium Bromide (TBPBr)-Based Systems

Hydrophobic eutectic solvent systems (ESSs) were prepared and characterized using temperature-dependent thermophysical and transport property measurements, supported by thermal analysis. The investigated systems comprise terpene-based mixtures, menthol:octanoic acid (1:2) and menthol:decanoic acid (1:1), and thymol-based mixtures, thymol:butanol (1:1), thymol:hexanol (1:1), thymol:octanoic acid (1:1), and thymol:oleic acid (1:1), as well as salt-containing ESSs based on tetrabutylphosphonium bromide (TBPBr), TBPBr:octanoic acid (1:1), and TBPBr:lauric acid (1:1). Density, dynamic viscosity, and electrical conductivity were measured at atmospheric pressure (p = 0.1 MPa) over 293.15–313.15 K. From density data, molar volumes and isobaric thermal expansion coefficients were calculated. The temperature dependence of viscosity was correlated with both Arrhenius and Vogel–Fulcher–Tammann equations. Conductivity results were used to compute molar conductivities, and the coupled conductivity–viscosity behavior was assessed via Walden analysis to quantify deviations from ideal electrolyte behavior and estimate ionicity. Thermal behavior and stability were evaluated by differential scanning calorimetry (DSC) and simultaneous thermogravimetric analysis (TG/DSC). The resulting dataset enables a consistent comparison of volumetric, flow, and ion transport descriptors across fully molecular terpene-based mixtures and TBPBr-containing systems. Overall, the combined transport descriptors, including Walden analysis, provide a practical framework for distinguishing molecular from salt-containing hydrophobic ESS families and support formulation selection for temperature-dependent applications, particularly in biphasic extraction processes.