Protonation of Patented Blue V in aqueous solutions: theoretical and experimental studies

Abstract

Abstract. The acid-base properties of the Patented Blue V dye were studied by spectrophotometry and tristimulus colourimetry. The mechanism of protonation of Patented Blue V has been investigated with semiempirical andDFTmethods. The quantum chemical calculations of total energy defined themost stable isomer for each protonated form in water solution. In addition to thermodynamic parameters, the condensed Fukui function and molecular electrostatic potential were successfully used as reactivity descriptors for the determination of the most favorable site for protonation. Moreover, for the explanation of the structure of the most deprotonated form of the dye in highly basic medium, the weak intramolecular interactions were investigated with the reduced density gradient function. The semi-empirical calculations of absorbance spectra explained the changing of the colour of the dye for the different protonated states. It has been shown that the dominant form of the Patented Blue V was the electro-neutral form, and the molar absorptivity (ε639 = 1.06×105 dm3·mol−1·cm−1) increases with the increase of the dielectric permittivity of the solvent. The replacement of polar solvents by less polar ones is causing a bathochromic shift of the absorption band of the dye, the value of which is correlated with the value of the Hansen parameter.