Photoacoustic Analysis of the Evolution from Sols to Aged Gels: Comparison of Different Initial Titania Sols

The use of correlation analysis of photoacoustic signal changes during the solgel- aging evolution of titania sols prepared from the hydrolysis of titanium n-butoxide in alcoholic, aqueous, and micellar media which provide different initial conditions and different evolution pathways is proposed. The photoacoustic experiments were done either at constant temperature for monitoring the initial evolution of the system, or under heating at 2°C·min−1 for monitoring molecular processes related to drying, siIntering, and crystallization. Results obtained from photoacoustic measurements display higher sensibility when compared with those from differential thermal analysis (DTA) and thermogravimetric analysis (TGA). The evolution of these systems shows instabilities below and above gelation, as well as during thermal treatment. These stabilities are minimized when the thermal treatment is performed on gels aged several months, and can be associated to molecular rearrangements or oscillating chemical reactions. For all the systems studied, the sensitivity of the correlation analysis recovered from photoacoustic signals permits detection of a transition in the 100 to 150°C range.