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In this study, the oxidative degradation of Methyl Blue (MB) triphenylmethane dye, in aqueous solution by Fenton oxidation process was investigated. The UV-Vis spectral changes of Methyl Blue dye during discoloration process were studied. Kinetic study was done to find out the effect of pH on discoloration rate of dye solution.

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.

The food-colouring dye tartrazine is a significant additive and in the same time a biologically active material. Thermal behaviour of trisodium (4E)-5-oxo-1-(4-sulphonatophenyl)-4-[(4-sulphonatophenyl)hydrazono]-3-pyrazole carboxylate, tartrazine or E102 as it is also known, was studied in relation with the physical, chemical, thermal and biophysical (interaction with proteins, antioxidant and phytotoxicity) properties. Tartrazine structure and microstructure have been determined by X-ray diffraction and scanning electron microscopy investigations. Tartrazine has a monoclinic crystalline structure (space group C2/c). Electronic transitions and atomic bonds behaviour in molecules and fluorescent excitation of the compound have been identified by spectroscopic methods: Fourier-transformed infrared, UV–Vis, atomic fluorescence and Raman spectroscopies. Tartrazine absorbs in the visible region at 436.36 nm and shows laser fluorescence of all its elements. Chemical (acidity) and optical properties (refractive index and electric susceptibility) were determined for different concentrations on a sensitive range of temperatures. Thermal analysis measurements of tartrazine revealed thermal stability until 200 °C in air, while in the inert environment (argon) the stability is increased to 300 °C. Biological and biophysical activities and properties of tartrazine have been evidenced through its interaction with animal proteins, comparison with the standard gallic acid and phytotoxicity towards wheat strains/roots, when several concentrations of tartrazine solutions were used. Tartrazine has moderate biological activity when interacting with proteins, obtained antioxidant activity above a threshold concentration of 0.25% and presents plant growth phytotoxicity.

Azoic dyes are compounds of interest from the point of view of their potential applications, such as the use of liquid crystals in optoelectronic and organic electroluminescent devices, or may be employed as template matrices for producing high-aspect ratio inorganic nanomaterials. Herein, 4-[(4-chlorobenzyl)oxy]-3,4′-dichloroazobenzene azoic dye, known as CODA, is selected as a choice compound among such materials due to its liquid crystalline properties and may be further used as nanostructured material in various applications. Thermokinetic study of CODA azoic dye thermal decomposition in air flow atmosphere was performed by employing thermogravimetric data; the kinetic parameters of the two decomposition steps were obtained under non-isothermal linear regimes, by means of multi-heating rates methods. Differential and integral “model-free” kinetic methods like Friedmann, Kissinger–Akahira–Sunose and Ortega, the invariant kinetic parameters method and the Perez-Maqueda et al. criterion (by Coats–Redfern and differential equations) were used. The kinetic study reveals very different behaviour related to the two decomposition steps of CODA, with complex processes composed of more than one kinetic mechanism for each of those, as indicated also by the Gotor et al. master plot method. Modern devices incorporating such materials tend to use them as thin films due to their specific properties; the CODA thin films were deposited on silicon substrates by matrix-assisted pulsed laser evaporation technique, using a Nd:YAG laser working at the wavelength of 266 nm. The preservation of the CODA compound after the transfer on the substrates was confirmed by Fourier transform infrared spectroscopy, while the morphology and topography of the deposited materials and of the thin film surfaces were investigated by atomic force microscopy and optical microscopy.

The thermal stability and the adsorption properties have been investigated for three benzothiazole compounds: 2-mercaptobenzothiazole (MBT), N-cyclohexyl-2-benzothiazole sulfenamide (NCBSA), and 2,2′-dibenzothiazole disulphide (BTD), reported in our early studies as corrosion inhibitors for carbon steel in different media. The electrochemical results were used to calculate the degree of surface coverage (θ). The adsorption mechanism of the three inhibitors was discussed according to the free energy of adsorption ( $$ \Delta G_{{\text{ads}}}^{\circ } $$ ) value obtained from Temkin adsorption isotherm, this being the best way to quantitatively express the adsorption process of their molecules on carbon steel surface. Thus, a mixed type mechanism involving the synergism between physisorption and chemisorption was proposed. The thermal analysis curves showed that, for the occurred events up to 470 °C, mass losses take place with endothermic effects followed by the total oxidation of the residue with an exothermic effect around 520 °C. Consequently, their effectiveness follows the order: BTD > NCBSA ≥ MBT, while the thermal stability ranges as follows: NCBSA

The azo-ester, namely 4-(phenyldiazenyl)phenyl 2-furoate (PPF) by a coupling reaction between 4-(phenyldiazenyl)phenol and 2-furoyl chloride in the presence of pyridine was obtained. For characterization of this compound UV–Vis, FTIR, and thermal analysis were used. PPF was investigated as corrosion inhibitor for carbon steel in saline waters using potentiodynamic polarization. Morphology of the surface before and after corrosion was examined by optical microscopy. Potentiodynamic polarization shows that the corrosion current densities decrease and values of polarization resistance and inhibition efficiency increase with PPF concentration reaching a maximum of 89.6 %, at 0.1 mmol L−1.

Using the Williamson method, a new dye 4-[(4-chlorobenzyl)oxy]-3,4′-dichloroazobenzene (CODA) with liquid crystalline properties was synthesized. The structure and the thermal behavior of CODA were investigated by means of nuclear magnetic resonance, X-ray diffraction, differential scanning calorimetry, and light polarized optical microscopy techniques. The thermophysical processes were monitored by heating–cooling cycles, but the formation of liquid crystal phases were exhibited only for small values of the cooling rates. For the first heating–cooling cycle, the melting and the solidification processes, thus the characteristic temperatures, are shifted to higher values when compared to the following cycles.

Thermal analysis of 2-allyl-4-((4-(4-methylbenzyloxy)phenyl)diazenyl)phenol dye was performed in air flow. The compound thermal behavior was investigated using TG, DTG and DSC techniques, under non-isothermal linear regime. Kinetic parameters of the two decomposition steps were obtained by means of multi-heating rates methods. Isoconversioanl methods (KAS and FWO), Invariant Kinetic Parameters method and Perez-Maqueda et al. criterion (by means of CR and FW equations) were used.

Thermal analysis of 4-[(4-chlorobenzyl)oxy]-4′-chloro-azobenzene dye, exhibiting liquid crystalline properties, was performed in dynamic air atmosphere. The compound behavior was investigated using TG, DTG, DTA and DSC techniques, under non-isothermal linear regime. The evolved gases were analyzed by FTIR spectroscopy. Kinetic parameters of the first decomposition step were obtained by means of multi-heating rates methods, such as isoconversioanl methods, IKP method and Perez-Maqueda et al. criterion.