Thermoanalytical and spectroscopic characterisation of solid-state retinoic acid.

Thermoanalytical (differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermogravimetric analysis coupled with Fourier transform infrared spectroscopy (TG/FTIR)) and spectroscopic (X-ray diffraction (XRD), ultraviolet–visible (UV–Vis), mass spectrometry (MS) and Fourier transform infrared diffuse reflectance (DRIFT) measurements have been used to characterise solid-state retinoic acid (RA) from a chemico-physical point of view. Between 130°C and 160°C, a phase transition takes place that does not correspond to the transition between the known monoclinic and triclinic phases (DSC and XRD evidence). By annealing in air (in the 130–160°C temperature range and for different times), an exothermic oxidative degradation occurs that, depending on the thermal treatment, competes with the mentioned phase transition (TGA evidence). Spectroscopic techniques (UV– Vis, MS and DRIFT) allow one to conclude that the new solid phase is still constituted by retinoic acid with a different orientation of the side chain. Finally, RA does not undergo stable melting: the fragmentation patterns, both in air and in nitrogen, have been examined by TG/FTIR.