Influence of iron cations on the hydrolysis of tetraethoxysilane and process of gel formation

The influence of small additions of Fe³⁺ ions on the processes of hydrolysis of tetraethoxysilane and subsequent polycondensation of products was studied using viscosimetry and dynamic light scattering methods. Experiments were carried out at 50^оC, hydrolysis pH was 1.5; 2.5; 5.0 or 7.0, the amount of doping cation varied from 1.5 to 3.8 at. %. In the absence of a doping cation, the gelation time grows with increasing pH from 1.5 to 5.0, and at pH 7.0, polycondensation occurs without gelation. At pH 1.5, the introduction of a dopant increases the gelation time, at pH 2.5 and 5.0 it decreases. With increasing dopant content, the gelation time increases at all three pH values. The size of the particles formed during the polycondensation process depends on the pH and the amount of dopant. The smallest particles with a median diameter of about 10 nm are formed at pH 2.5. It has been suggested that the cause of all the observed effects is the incorporation of iron cations into the siloxane matrix. The degree of incorporation depends on the degree of hydrolysis of iron cations. This assumption is confirmed by the values of the electrokinetic potential of the systems under study and the dynamics of changes in the zeta potential with varying pH and dopant content.