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

Abstract

A comparative study using small-angle x-ray scattering ͑SAXS͒ 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°C exhibit a mass fractal structure with fractal dimension Dϳ2.4 in the range between the correlation length ϳ5.3 nm and aϳ0.75 nm. An experimental method to probe the mass fractal structure of aerogels from exclusively nitrogen adsorption isotherms has been presented. For aerogels at 500°C, we have found Dϳ2.4 in the range between the pore width 2r ϳ33 nm and 2r a ϳ4.5 nm, which is in notable agreement with the SAXS results (D ϳ2.4, ϳ5.3 nm, aϳ0.75 nm) if we assign the pore width 2r probed by the Kelvin equation in the adsorption method to the Bragg distance 2/q associated to the correlation length 1/q probed by SAXS.

Mass fractal characteristics of silica sonogels as determined by small-angle x-ray scattering and nitrogen adsorption

“…3 An independent method to probe the mass-fractal structure of aerogels, exclusively from the pore-size distribution as obtained from the nitrogen adsorption isotherms, has been suggested so the results were found in good agreement with SAXS results. 3 In this work, a sample series of silica sonogels was prepared using different water-tetraethoxysilane molar ratios ͑r w ͒ in the gelation step of the process in order to obtain aerogels with different bulk densities after the supercritical drying.…”

“…3 An independent method to probe the mass-fractal structure of aerogels, exclusively from the pore-size distribution as obtained from the nitrogen adsorption isotherms, has been suggested so the results were found in good agreement with SAXS results. 3 In this work, a sample series of silica sonogels was prepared using different water-tetraethoxysilane molar ratios ͑r w ͒ in the gelation step of the process in order to obtain aerogels with different bulk densities after the supercritical drying. Interesting structural properties associated with the wet sonogels and the evolution of such a structure in the obtaining of the sonogel-derived aerogels have been inferred from the mass-fractal characteristics as determined from the small-angle x-ray scattering ͑SAXS͒ and nitrogen adsorption data.…”

“…To compare the SAXS data with nitrogen adsorption data, we employed an earlier proposed method 3 for the generation of a mass-fractal structure from the PSD curve by a sequence of approximants, starting from a homogeneous solid with density equal to that of the fused silica a ϳ 2.2 g / cm 3 and probing the resulting bulk density ͑r͒, as each incremental pore volume per mass unit ␦V p ͑r i ͒ is incorporated to the structure. The process can be cast as…”

“…Table II shows the bulk density and the BET-specific surface area S BET of the aerogels. was evaluated from the adsorption data using the total pore volume per mass unit V p through the relation ͑1/ ͒ = ͑1/ a ͒ + V p , assuming the value 2.2 g / cm 3 for the skeletal density a , as frequently quoted for fused silica. It has been pointed out 9 that, in the case of low-density aerogels, the adsorption method applies stress on the sample during measurement leading to an underestimation of pore volume.…”

Mass fractal characteristics of wet sonogels as determined by small-angle x-ray scattering and differential scanning calorimetry

“…For this, we have applied an earlier employed method 28 in an analogy to a method used to generate nonrandom fractals, starting from an initial configuration by a sequence of approximants. Experimentally, we start from a homogeneous solid with density ⌬ S = S − ice , where S is the silica density and ice the density of the ice, assumed as 0.971 g / cm 3 , and follow incorporating to the structure, step by step, each incremental "pore" volume per mass unit of such a solid ͓␦V P ͑r i ͔͒, associated to the "pore" class with radius r i , starting from r i = 0 up to r i = r, and probing the evolution of the bulk density ⌬͑r͒ of such a solid, with respect to the ice density.…”