CALORIMETRIC STUDY OF THE SOL-GEL SILICA GELATION STAGE - EFFECT OF GELATION PH

This paper reports the results of studies of the thermal behavior duri ng sol-gel synthesis of SiO2, starting from a mixture of tetraethoxysi lane (TEOS), ethanol and water at pH 3, 7 and 9. A heat flow calorimet er was used to study the thermal behavior of mixtures, from the initia l stage of mixing to the formation of gel. When the TEOS gelation reac tion takes place at pH 7, it behaves in unexpected ways; it can be obs erved that the hydrolysis reaction takes place in an oscillatory manne r, with a frequency of 6.5 h, after which an increase in the heat capa city of the reacting mixture is noted. (C) 1997 Elsevier Science B.V.