Calorimetric Study of Sol and Gel Formation.

A sensitive, large sample volume calorimeter has been used to study the sol-gel transition in several alkoxide gel forming systems. The apparatus permits monitoring reactions associated with formation of the sol through its gelation. The metal oxide precursor, its concentration with respect to water, ambient temperature, and catalyst were varied and their effects observed. Results of the study support a model of gel formation in which hydrolysis and condensation reactions are occurring simultaneously. Additionally, in acid catalyzed systems it is apparent that particle formation (approx. 5 nm diameter particles) is occurring along with the condensation reactions. The results help to explain the large discrepancy in gelation times between various silicate sol-gel systems.