Aggregation in SiO2 sols from sodium silicate solutions

The structure of the aggregation of SiO2 sols in aqueous solution is investigated by small angle X-ray scattering experiments. In a sodium water glass solution an ion exchange results in an aggregation of the colloids. The aggregation clusters can be characterized by their size, fractal dimension describing the mass distribution and the size of the primary units. When the pH of the gel preparation varies from 2 to 5.8, the gelation time decreases strongly, while the value of the fractal dimension of the aggregation clusters remains constant at about 2.2. However, if the pH exceeds 5.8, the gelation time increases again and the fractal dimension decreases to a value of nearly 1.7. This process is attributable to the increase of the surface charge of the aggregating particles as pH goes up and to the resulting Coulomb repulsion. A corresponding behaviour was found under strong acid conditions. The experimental facts are in contrast to results of computer simulations in the literature.