Solid-Phase Equilibria of Thymol- and Carvacrol-Based Hydrophobic Eutectic Solvents with Fatty Chains and Their Application to Carotenoid Extraction

Hydrophobic eutectic solvents (HES) based on the terpenes (thymol and carvacrol) were formulated with fatty acids and alcohols and characterized (structural, thermal, hygroscopic, and thermophysical) to assess their performance in extracting carotenoids (bixin, lutein, β-carotene). Solid–liquid equilibrium diagrams demonstrated melting depressions around the eutectic composition, with an increase in eutectic temperature with the fatty acid or alcohol chain length. FTIR and TGA confirm hydrogen bonds and higher thermal stability of the mixtures compared to pure terpenes. HES exhibited low viscosities at 298.15 K, 9–15 mPa s (thymol) and 12–18 mPa s (carvacrol), as well as adjustable density and water absorption governed by chain length and mixture stoichiometry. The carotenoids extraction at 298.15 and 313.15 K yielded recoveries comparable to conventional organic solvents, with particularly high values for bixin and lutein in selected thymol/carvacrol-fatty acid or fatty alcohol systems. COSMOthermX activity coefficients at infinite dilution and novel carotenoid-based micropolarity (MP) and πH indices, derived from UV–vis spectra of bixin, lutein and β-carotene, provided a consistent thermodynamic and solvatochromic provide to rationalize HES selectivity and hydrophobicity. Overall, these results demonstrate that thymol- and carvacrol-based HES are tunable and sustainable media for carotenoid extraction.