The arrangement of fractal clusters dependent on the pH value in silica gels from sodium silicate solutions

Structural features and thus, optical transparency of silica hydro- and aerogels from sodium silicate solution strongly depend on the preparation parameters. This study is focused on the effect of pH, for values higher than the isoelectric point of silica acid (pH ∼ 2). Previous studies have presented SAXS data characterizing the primary particles and fractal properties in silica gels from a sodium silicate solution. This study presents an analysis focusing on the structural units >50 nm, which is the relevant structure for the process of gelation and for optical transparency. UV–vis–NIR spectra revealed that hydrogel transmission increases with rising pH and decreases with the fraction of SiO2. Scanning electron microscopy (SEM) analyses of the inner structure of the supercritical dried gels showed that the mean cluster size along with the deviation dropped substantially with rising pH. An equation which agrees with the experimental results and describes the correlation between cluster size, fractal dimension, and packing index in mass fractal cluster packages is provided.