Calorimetric study of the sol-gel silica gelation stage: Effect of gelation pH

This paper reports the results of studies of the thermal behavior during sol-gel synthesis of SiO2, starting from a mixture of tetraethoxysilane (TEOS), ethanol and water at pH 3, 7 and 9. A heat flow calorimeter was used to study the thermal behavior of mixtures, from the initial stage of mixing to the formation of gel. When the TEOS gelation reaction takes place at pH 7, it behaves in unexpected ways; it can be observed that the hydrolysis reaction takes place in an oscillatory manner, with a frequency of 6.5 h, after which an increase in the heat capacity of the reacting mixture is noted. (C) 1997 Elsevier Science B.V.