FTIR spectroscopic study on liquid silica solutions and nanoscale particle size determination

Abstract

Silica gel is a very important material in technology. Usually tetraethyl orthosilicate (TEOS) is used as precursor in the sol-gel science. But silica gel can also be formed by liquid silica solutions, like alkali silica solutions and silica sols. Due to their importance in paint technology and as a constituent in building material, we investigated alkali silica solutions (especially potassium water glass) and the silica sol Levasil 300. The gel formation process of inorganic silica solutions is quite different to gel formation from TEOS. Gel formation by TEOS is due to polymerization of monomers, whereas the gel formation process of inorganic silica solutions is due to condensation of dense SiO₂ particles with particle diameters of a few nanometers. Fourier transformed infrared (FTIR) spectroscopy proved to be a powerful tool to obtain information on the structure of these liquid silica solutions and their sol-gel processes. Most information could be derived from the main silica peak at ~1070 cm^-1. This peak can be assigned to the TO₃ vibration mode of silica. In silica solutions and in silica gels this peak is composed of different peaks. Compared to earlier studies an additional peak can be found at ~1040 cm^-1 in potassium silica solutions. By comparison of FTIR spectra of related silica glasses and their liquid solutions the peak at ~1027 cm^-1 can be assigned to vibration modes of SiO₂ on the surface of silica particles. The intensities of these individual peaks contain information on the degree of polymerization and the particle size of the silica particles in the liquid solutions. The information about particle size is limited to nanosized particles between 2 and 6 nm.