Stability-Guided Selection of Terpene-Based Hydrophobic Deep Eutectic Solvents for Sustainable Liquid-Phase Processes

Terpene-based hydrophobic deep eutectic solvents (HDESs) are promising alternatives to conventional hydrophobic organic solvents, but their practical applicability depends on stability under processing conditions. Here, sixteen binary terpene-based HDESs composed of menthol, thymol, limonene, linalool, carvone, camphor, and borneol were systematically evaluated using a stability-guided selection approach. Density, viscosity, temperature-dependent flow behavior, volatility-related mass loss, water uptake, and compositional stability after water contact, heating, ultrasound treatment, and UV irradiation were investigated. All systems had densities below 1.0 g cm–3, while viscosity at 25 °C ranged from 1.351 mPa·s for LIM-CARV to 35.506 mPa·s for MENT-THYM. Volatility-related mass loss at 100 °C varied from 3.5 to 39.0%. After contact with a 100-fold excess of water, the water content of the recovered HDES phase ranged from 961 to 30,781 ppm, demonstrating pronounced composition-dependent hydration. Gas chromatography–mass spectrometry (GC–MS) profiling showed that most systems preserved more than 95% of their original components after the applied treatments. THYM-CARV showed the most favorable overall stability profile, with only 3.5% mass loss and ≥99.9% preservation of the original components after thermal, ultrasonic, UV, and water-contact treatments. These results demonstrate that terpene-based HDESs should be selected using integrated structure–property–stability criteria rather than hydrophobicity or component origin alone.