Comparative study using small-angle x-ray scattering and nitrogen adsorption in the characterization of silica xerogels and aerogels

A comparative study using small-angle x-ray scattering (SAXS) and nitrogen adsorption has been carried out in the structural characterization of silica xerogels and aerogels, obtained from tetraethoxysilane sonohydrolysis. The specific surface and the mean pore size as measured by both the techniques were found to be in notable agreement in all cases for aerogels and xerogels. According to the SAXS data, aerogels at 500 degreesC exhibit a mass fractal structure with fractal dimension Dsimilar to2.4 in the range between the correlation length xisimilar to5.3 nm and asimilar to0.75 nm. An experimental method to probe the mass fractal structure of aerogels from exclusively nitrogen adsorption isotherms has been presented. For aerogels at 500 degreesC, we have found Dsimilar to2.4 in the range between the pore width 2r(xi)similar to33 nm and 2r(a)similar to4.5 nm, which is in notable agreement with the SAXS results (Dsimilar to2.4, xisimilar to5.3 nm, asimilar to0.75 nm) if we assign the pore width 2r probed by the Kelvin equation in the adsorption method to the Bragg distance 2pi/q associated to the correlation length 1/q probed by SAXS.