Defying Decomposition using Simultaneous Fast Scanning Calorimetry and micro-XRD: Effect of the Halide Anion on The High-Temperature Propert

Deep eutectic solvents (DESs) are mixtures of two or more components that are characterised by unexpectedly large melting point depressions. Fusion properties are key for the prediction and theoretical description of the DES’s liquid window. Tetraethyl ammonium halide salts (Et4NX) are common DES constituents, but their fusion properties are inaccessible under typical experimental conditions owing to thermal decomposition. Additionally, Et4NX salts show extensive polymorphism, making phase assignment from calorimetric data challenging. Here, we propose to use ultra-fast scanning calorimetry combined with micro-XRD to elucidate the melting point by overcoming decomposition and resolve the high-temperature crystal structure of Et4NX (X = Cl−, Br−, or I−) and its mixtures. From these measurements we expect to gain additional insight in the large differences in phase behaviour exhibited by Et4NX + glutaric acid mixtures upon changing the halide anions.