Characterization and thermal decomposition kinetic study of promising antioxidante nitroxides

Nitroxides are a class of molecules with a promising potential in the cosmetic industry due to their antioxidant actions that allow their use in the treatment of skin aging by inhibiting the action of free radicals and, consequently, reducing the effects of skin aging. In this work, three nitroxides: TEMPOL, BENZITEMPOL and TSTEMPOL were characterized in terms of physical–chemical aspects, employing several analytical techniques, such as: Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetry (TG) and ultra-performance liquid chromatography. With the results of the XRD and FTIR, it was possible to identify the crystalline nature of these molecules, in addition to confirming the presence of the expected functional groups for the structure of these compounds. Thermoanalytical techniques made it possible to know the thermal behavior of these three nitroxides and for this purpose four heating ratios (25; 10; 15 and 20 °C min −1 ) were used, in the temperature range of 25–900 °C for TG and 25–400 °C for DSC, both under a 50 mL min −1 nitrogen atmosphere. TEMPOL, BZTEMPOL and TSTEMPOL had a melting point of 72.2, 121.0 and 108.4 °C, respectively and the compound that had the greatest thermal stability was TEMPOL at 152 °C. Also, the kinetic of TEMPOL thermal decomposition was also studied to identify the activation energy, frequency factor and mechanism of the process. This study was conducted using Kissinger–Akahira–Sunose (KAS) isoconversional method along with artificial neural network. The activation energy according to KAS method is about 65 kJ mol −1 along the entire decomposition progression, indicating a single step process. The neural network results showed that the process was conducted following a majority combination of F1 and D3 kinetic models, suggesting that the thermal decomposition occurs after fusion as a first order process and with diffusion of material from the bulk to the surface interface, as a tridimensional diffusion process.