Proton donor–acceptor property of matrix during the sol–gel reaction

The UV–VIS absorption spectrum of thymol blue (TB), which was used for an in situ probe for the proton donor–acceptor property of the surrounding matrix, has been observed as a function of time during the sol–gel–xerogel reactions of tetraethyl orthosilicate under HCl catalyzed and uncatalyzed conditions. The value of relative abundance, RA Z , of zwitter ion species was calculated. In the ethanol-containing system, RA Z increased for 7 h, then decreased until 114 h had passed, and finally increased gradually. These dynamic changes of the RA Z were commonly observed for all the systems. The final RA Z values were expected larger than 0.7 irrespective of the initial chemical composition. The characteristic increase and decrease in RA Z are ascribed to the increase and decrease in proton-donating ability from the surrounding matrix to encapsulated TB molecules. A general acidity function, H − , of the matrix was estimated.