From sol to aerogel: a study of the nanostructural characteristics of TEOSderived sonogels

The nanostructural characteristics of acid-catalyzed sonogels are studied a long the aging process at 60 degreesC in saturated conditions and after the CO, supercritical extraction (aerogel). The structural evolution was studi ed by means of small-angle X-ray scattering (SAXS) and UV-Visible absorptio n techniques. The sonogel exhibits a mass fractal structure in a length sca le between zeta - 1/q(0) similar to 5.3 and a(1) similar to 1/q(m) similar to 0.22 nm, as the length scale probed by SAXS. The apparent mass fractal d imension lightly increases from 2.0 for fresh gel until 2.2 for 14 days agi ng in wet conditions. The UV absorption also increases with the aging time in wet conditions. Both observations are consistent with the syneresis proc ess accompanying the polycondensation progress during aging in saturated co nditions. For long aging times, the wet sonogels show a light transition fr om a mass to a surface fractal. in a very small interval of the length scal e, developing an extremely rough surface with fractal dimension D-S similar to 2.9, The fractal characteristics of the sonogels practically do not cha nge with the alcohol exchange. With the CO2 supercritical extraction (aerog el). the interval in the length scale in which the surface fractal is defin ed increases, while the surface fractal dimension diminishes to D-S similar to 2.5. The mass fractal characteristics are less apparent in the aerogels . (C) 2001 Elsevier Science B.V. All rights reserved.