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 stu died by 'in situ' measurements of small-angle X-ray scattering (SAXS) at th e temperatures 40 degreesC, 60 degreesC and 70 degreesC. The results were a nalyzed in terms of the evolution with time (t) of the SAXS intensity probi ng the mass fractal characteristics of the system, the average radius of gy ration (Rc,) of the clusters and the number of primary particles per cluste r. The aggregation process yields mass fractal structures which exhibit a s cattering exponent (alpha) practically equal to 2, in the probed length sca le range (5.3 nm < 1/q < 0.22 nm), beneath and even far beyond the gel poin t. This suggests that a is a direct measure of the real mass fractal dimens ion (D) of the structure. The precursor sol (pH = 2) exhibits I nm mean siz ed clusters with mass fractal dimension D similar to 1.9. Increasing the pH to 4.5, the cluster mean size and the number of primary particles per clus ter increase but the system keeps a more opened structure (D similar to 1.4 ). In the first aggregation stages, D increases up to similar to2 by incorp orating primary particles to the clusters without changing their mean size. From this stage, the aggregation progresses following a thermally activate d scaling law well described by R-G similar tot(1/D) in all cases. This is indicative of a diffusion-controlled cluster-cluster aggregation process. T he activation energy of the process was found to be 91.7 kJ/mol. (C) 2001 E lsevier Science B.V. All rights reserved.