A SAXS study of kinetics of aggregation of TEOS-derived sonogels at different temperatures

The kinetics of aggregation of tetraethoxysilane (TEOS)-derived silica sols, produced by acid-catalyzed and ultrasound-stimulated hydrolysis, were studied by `in situʹ measurements of small-angle X-ray scattering (SAXS) at the temperatures 40°C, 60°C and 70°C. The results were analyzed in terms of the evolution with time (t) of the SAXS intensity probing the mass fractal characteristics of the system, the average radius of gyration (RG) of the clusters and the number of primary particles per cluster. The aggregation process yields mass fractal structures which exhibit a scattering exponent (α) practically equal to 2, in the probed length scale range (5.3 nm <1/q<0.22 nm), beneath and even far beyond the gel point. This suggests that α is a direct measure of the real mass fractal dimension (D) of the structure. The precursor sol (pH=2) exhibits 1 nm mean sized clusters with mass fractal dimension D∼1.9. Increasing the pH to 4.5, the cluster mean size and the number of primary particles per cluster increase but the system keeps a more opened structure (D∼1.4). In the first aggregation stages, D increases up to ∼2 by incorporating primary particles to the clusters without changing their mean size. From this stage, the aggregation progresses following a thermally activated scaling law well described by RG∼t1/D in all cases. This is indicative of a diffusion-controlled cluster–cluster aggregation process. The activation energy of the process was found to be 91.7 kJ/mol.