Thermodynamic and crystallographic properties Depending on hydration numbers in tetra-n-butylammonium chloride semiclathrate hydrates

Abstract Tetra-n-butylammonium chloride (TBAC) semiclathrate hydrate crystals with hydration numbers n = (26.4 to 33.2) were prepared from TBAC aqueous solutions over the range of TBAC mole fractions (0.0060 to 0.0500). The thermodynamic and crystallographic properties of the TBAC hydrates were then investigated via ion chromatography, powder X-ray diffraction (PXRD), and differential scanning calorimetry. The crystal system of the TBAC hydrates with n = (26.4 to 33.2) was tetragonal, while the crystal structures (PXRD patterns) slightly varied with the hydration numbers. The dissociation enthalpies per guest mole (TBAC) gradually varied with the hydration numbers, from ∼149 kJ mol−1 at n = 26.4 to ∼187 kJ mol−1 at n = (33.1 to 33.2). These results suggest that the enthalpies per TBAC increased with greater hydrogen-bond formation in the TBAC hydrate crystals. The dissociation temperatures and dissociation enthalpies per H2O of the TBAC hydrates suggest that the hydrate crystals were most thermally stable at around n = 30 and slightly unstable with higher/lower hydration numbers.