Insights on thermodynamic thermal properties and infrared spectroscopic band assignments of gallic acid

Gallic acid (3,4,5-trihydroxybenzoic acid) is a popular nutraceutical found in various natural sources. A confusion regarding its thermodynamic properties, e.g., melting point, can be detected in the reported literature values. Similar issues exist for the assignment of its spectroscopic bands in the region of hydroxyl stretching vibrations. In this study, thermal analysis techniques, infrared spectroscopy and X-ray diffraction were used to study the thermal behavior of gallic acid. It is shown that gallic acid exhibits various thermochemical transitions (solid-solid and solid-liquid transitions). The value of the specific heat of the thermal transition around 90 °C indicates that this effect is not only related to water removal, but to decomposition. The absence of significant/exclusive water removal at 90 °C suggests that water being present in the structure of gallic acid is strongly bounded, while the main pathway for the decomposition around 90 °C seems to be the dehydration through esterification reaction between -COOH and -OH groups of gallic acid. Recrystallization of gallic acid from methanol-heavy water solvent mixture, leads to the incorporation of heavy water in its structure. The comparative evaluation of the recrystallized and raw gallic acid allows for a proper spectroscopic band assignment of various vibrations. The thermal effect around 260 °C is a typical thermochemical transition and not a melting point. The extensive polymorphism of gallic acid and the respective solid-solid transformations are also related to partial decomposition.