Thermal Behavior of Urea-Based Natural Deep Eutectic Solvents Comparing Anhydrous and Hydrated Choline Chloride–Urea Systems

Natural deep eutectic solvents (NADES) are increasingly recognized as environmentally aligned alternatives to volatile organic solvents in green process design. Thermal stability is a critical process parameter governing safe handling, scalability, and integration into extraction and formulation operations. This study compared the thermogravimetric behavior of two urea-based NADES systems — choline chloride:urea (1:1) and its hydrated analogue containing 0.5 molar equivalents of water. Thermogravimetric analysis (TGA) was performed using a simultaneous TG/DSC analyzer (SDT Q600, TA Instruments) under nitrogen atmosphere, heating approximately 2 mg of each sample to 500 °C at 20 °C·min⁻¹ in an open platinum pan. Measurements were performed as single runs per condition. Reported values should therefore be interpreted as representative rather than statistically validated. The hydrated NADES exhibited an initial mass loss of approximately 6.3%, attributed to evaporation of physically bound and adsorbed water between 100–150 °C. Hydration did not markedly alter the onset temperature or overall extent of high-temperature decomposition relative to the anhydrous system. In the DSC signal, however, the non-hydrated NADES displayed more sharply resolved thermal transition features — consistent with a more ordered hydrogen-bond network prior to the onset of decomposition — even though the subsequent degradation behavior of both systems remained comparable. Both systems maintained structural integrity up to approximately 200 °C, beyond which progressive mass loss indicated decomposition of urea-derived fragments and disruption of the eutectic matrix. Choline chloride:urea NADES retains thermal robustness in both hydrated and non-hydrated forms. Hydration modulates low-temperature mass loss without compromising high-temperature decomposition behavior, supporting the process viability of urea-based NADES in green extraction and thermal processing applications.