Unveiling the phase behavior of type–II choline chloride/calcium chloride hexahydrate/water deep eutectic solvents via thermal and solid state NMR characterization

Deep eutectic solvents (DESs) have emerged as versatile alternatives to conventional organic solvents and ionic liquids, even though their structure at a molecular–level and their phase behavior often remain poorly understood. In this paper, we investigate the phase transitions of a choline chloride–calcium chloride hexahydrate (1:2) DES. Samples with different hydration levels were examined using an integrated thermal and spectroscopic approach, combining thermogravimetry (TGA), Differential Scanning Calorimetry (DSC, classic and modulated), and solid–state Nuclear Magnetic Resonance (SSNMR). Isotopic substitution with CaCl₂·6D₂O enabled selective probing of water and choline contributions to the mixture dynamics. DSC and 1H Free Induction Decay (FID) analyses permitted to identify the temperature range of the transitions, while variable–temperature 1H and 2H SSNMR provided insight into molecular reorientations dynamics. For the first time in this kind of systems, a glass transition (at −60 °C) was observed, proving that their phase behavior is more intricate than that of a simple eutectic mixture.