491 0195-928X/04/0300-0491/0 © 2004 Plenum Publishing Corporation International Journal of Thermophysics, Vol. 25, No. 2, March 2004 (© 2004) Photoacoustic Analysis of the Evolution from Sols to Aged Gels: Comparison of Different Initial Titania Sols 1 1 Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A. M. C. Marchi, 2 R. Castan ˜eda-Guzmán, 3, 4 A. Pérez-Pacheco, 3 S. A. Bilmes, 2, 4 and M. Villagrán-Muniz 3 2 INQUIMAE-DQIAQF, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II, C1428EHA Buenos Aires, Argentina. 3 Photophysic Lab., Centro de Ciencias Aplicadas y Desarrollo Tecnológico, UNAM, A.P. 70-186, C.P. 04510, México D.F., México. 4 To whom correspondence should be addressed. E-mail: castanr@aleph.cinstrum.unam.mx; sarabil@qi.fcen.uba.ar The use of correlation analysis of photoacoustic signal changes during the sol- gel-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 photo- acoustic experiments were done either at constant temperature for monitoring the initial evolution of the system, or under heating at 2°C · min -1 for monitor- ing molecular processes related to drying, sintering, and crystallization. Results obtained from photoacoustic measurements display higher sensibility when compared with those from differential thermal analysis ( DTA) and thermogra- vimetric 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. KEY WORDS: correlation analysis; oscillating reactions; phase transition; photoacoustic; sol-gel; TiO 2 .