Solubilization of azo dyes by cetyltrimethylammonium bromide micelles as structure control factor at synthesis of ordered mesoporous silicas

Abstract

Solubilization of anionic azo dyes, methyl red and alizarin yellow, in micelles of cationic surfactant, cetyltrimethylammonium bromide, was studied in aqueous solutions with pH ~ 10 by conductometry and UV–Vis spectroscopy. The electro-conductivity measurements were used to evaluate influence of azo dyes on critical micelle concentration of cetyltrimethylammonium bromide and estimate standard thermodynamic parameters of micellization process. It was found that solubilization of azo dyes by micelles of surfactant is spontaneous process which causes lowering of critical micelle concentration of long-chain quaternary ammonium salt. The phenomenon of azo dyes solubilization by surfactant micelles was confirmed by analysis of the effect of different concentrations of cetyltrimethylammonium bromide on the UV-Vis spectra of azo dyes solutions. Differential spectroscopic technique was employed to obtain quantitative characteristics of azo dyes solubilization in micellar media. The values of binding constant, partition coefficient, and free energy change of partition for azo dyes were calculated. Obtained results proved that methyl red and alizarin yellow incorporate into the CTAB micelle in such a way that their carboxylic groups are lying near by positively charged head group of surfactant and the rest of the molecule is oriented towards center of micelle. Micelles of cetyltrimethylammonium bromide with solubilized azo dyes were used as template in base catalysed hydrothermal-assisted sol-gel synthesis of mesoporous aminosilicas of MSM-41 type. The mesoporous structure of synthesized materials was characterized by low-temperature nitrogen adsorption-desorption and x-ray diffraction analysis. It was proved that incorporation of azo dye additives as solubilizates into the surfactant micelles in sol-gel synthesis leads to the formation of silica materials with improved long-range ordered mesoporous structure.