Azorubine: physical, thermal and bioactive properties of the widely employed food, pharmaceutical and cosmetic red azo dye material

Azorubine (E122), an azoic dye with the chemical formula C20H12N2Na2O7S2, has a wide range of applications in the food and cosmetic industries. Physical properties of azorubine were investigated by correlating the optical properties (refractive index, electric susceptibility and optical anisotropy) with the acidity study and biological activity. The optical study of azorubine shows that its refractive indexes are decreasing when the temperature of the solution is increased. The optical anisotropy of azorubine was observed/studied under polarized light at room temperature (RT). Azorubine exhibits the phenomenon of birefringence in results crystallites by drying and crystallization at RT of aqueous solution with concentration of 5%. THz spectroscopy identified the THz spectral “signature” of azorubine at a few wavelengths. The azorubine solution has, at a temperature of 24 °C, a weak alkaline character, corresponding to a pH = 7.14, and as the temperature increases, the acidity of the sample increases, the pH reaching value of 6.70 at the temperature of 55 °C. Five spectroscopic techniques were used to identify spectral properties of azorubine in powder and solution form: Fourier-transform infrared (FTIR), UV–Vis, Raman, laser fluorescence and THz spectroscopy. The morphology and chemical composition of azorubine were studied by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDXS). Thermal analysis effectuated in air indicates the evaporation of absorbed and physically bonded water up to 200 °C, after which the material possesses thermal stability up to 300 °C. Oxidative decomposition takes place in three exothermic steps between 300 and 687 °C; also, a high-temperature oxidation of the Na-based residues increases the mass above 815 °C. Azorubine has moderate biophysical activity when interacting with proteins (bovine serum albumin and collagen); the antioxidant activity of azorubine is increasing with increasing its concentration, while the phytotoxicity study indicates an overall inhibition of the wheat growth with increasing the concentration of azorubine and feeding time.