Influence of Iron Cations on Tetraethoxysilane Hydrolysis and Gelation Process

The influence of small additives of Fe3+ ions on tetraethoxysilane hydrolysis and subsequent polycondensation of products has been studied by the viscometry and dynamic light scattering methods. Experiments have been carried out at 50°C. Hydrolysis has been carried out at pH 1.5, 2.5, 5.0, or 7.0. The amount of the dopant cations has been varied from 1.5 to 3.8 at %. In the absence of the dopant cations, the gelation time grows with increasing pH from 1.5 to 5.0, while polycondensation occurs without gelation at pH 7.0. If the dopant is added at pH 1.5, the gelation time increases, but, at pH 2.5 and 5.0, it decreases. The gelation time increases with the dopant content at all three pH values. The size of the particles formed during the polycondensation process depends on the pH and the amount of the dopant. The smallest particles with a median diameter of about 10 nm are formed at pH 2.5. It is assumed that the cause of all observed effects is the incorporation of iron cations into the siloxane matrix. The degree of the incorporation depends on the degree of iron cation hydrolysis. This assumption is confirmed by the values of the electrokinetic potential of the studied systems and the dynamics of variations in the zeta-potential with varying pH and dopant content.