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

Abstract

The kinetics of aggregation of tetraethoxysilane (TEOS)-derived silica sols, produced by acid-catalyzed and ultrasound-stimulated hydrolysis, were studied by `in situ' 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 ( R G) 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 R G∼ t 1/ 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.