pH-sensitive materials based on silica with chemically immobilized methyl red: synthesis and protolytic properties
The purpose of this work is to study the possibility of creating hydrolytically and chemically stable pH-sensitive materials using silica as a carrier and an acid-base indicator that changes color when the pH of medium is varied. Synthesis of pH-sensitive materials was carried out by covalent binding of methyl red dye with aminopropylsilica in the presence of 1,1’-carbonyldiimidazole as a coupling agent. The porous structure, the chemical composition of the surface, and the protolytic properties of the synthesized silica materials were characterized by low-temperature adsorption-desorption of nitrogen, quantitative chemical analysis, thermogravimetry, diffuse reflectance electron spectroscopy, pH-metry. It was proved that chemical immobilization of methyl red on the silica surface does not have a significant effect on the porous structure of initial silica: the isotherms of nitrogen adsorption-desorption and pore size distribution curves confirm retention of wide pore size distribution after chemical modification of silica. The content of the acid-base indicator chemically immobilized on the silica surface was determined by quantitative chemical analysis of synthesized materials, as well as by thermogravimetry and diffuse reflectance electron spectroscopy in the ultraviolet and visible regions. It was demonstrated that diffuse reflectance electron spectroscopy can be a reliable method of quantitatively determining an indicator dye chemically grafted to the silica surface. It was proved that chemically grafted dye retains the ability to protonation of tertiary amino group and formation of resonance azonium ion, which is responsible for changing the color of methyl red. It was found that the protonation of methyl red chemically fixed on the silica surface occurs at higher pH values than for individual indicator dye. Therefore, the color change of grafted dye takes place in less acidic solutions. The proposed synthetic approach can be used to produce regenerable and eco-friendly pH-sensitive materials.