Thermal Properties of Choline Chloride/Urea System Studied under Moisture-Free Atmosphere

Physical chemistry of an archetypal deep eutectic solvent system, choline chloride/urea, was studied using dry components under moisture-free conditions. The phase diagram reveals that the eutectic melting point is 304.95 K (higher than any of the previously reported values to date), and a previously unrecognized phase region in the solid–liquid phase diagram, corresponding to a C1 → C2 transition (351.62 K) from α-choline chloride to β-choline chloride. Viscosities and densities, measured for the eutectic composition as a function of temperature, were compared with all data available in the literature, and discrepancies are discussed. Thermal stability studies reveal that the eutectic composition undergoes a thermal decomposition at temperatures as low as 363.15 K (mass loss rate of 0.411 wt % h–1), which calls for a careful consideration when using these solvents at elevated temperatures.