Structural properties of silica gels prepared from oxalic‐acid‐catalyzed tetraethoxysilane sonohydrolysis

Abstract

About ∼2.1 × 10−3 mol SiO2 cm−3 and ∼88%‐volume liquid‐phase silica wet gels were prepared from oxalic‐acid‐catalyzed tetraethoxysilane (TEOS) sonohydrolysis. Aerogels were obtained by supercritical CO2 extraction. The samples were analyzed by thermogravimetry, small‐angle X‐ray scattering and nitrogen adsorption. Wet gels can be described as mass fractal structures with fractal dimension D ∼ 1.94 and structural characteristic length ξ changing between ∼3.3 to ∼3.0 nm in the studied range of the catalyst concentration. A fraction of the porosity is apparently eliminated in the supercritical process. The values of the BET specific surface SBET, the total pore volume VP and the mean pore size lP of the aerogels were found to change almost randomly around the mean values SBET = 874 m2 g−1, VP = 0.961 cm3 g−1 and lP = 4.4 nm with catalyst concentration variation. These values were not substantially different from those from an equivalent HCl‐catalyzed aerogel. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of poly(vinyl alcohol) additions on the structure of silica xerogels

“…The quantity of V x ml of a 5.5% by weight water-solution of poly(vinyl alcohol) (PVA), prepared from a fully hydrolyzed PVA solid powder sample (Sigma-Aldrich CAS9002 89-5), plus (20 -V x ) ml of water were added to the hydrolyzed sols and the sonication was applied for 2 min more for complete homogenization. V x was varied to 0, 4,8,14, and 20 ml to yield the xerogel samples X0, X4, X8, X14, and X20, respectively. The water dilution was desired in order to obtain a set of sols with different PVA concentrations and approximately the same silica concentration ($1.8 Â 10 À3 mol SiO 2 /cm 3 ).…”

“…The decaying exponent close to 2 has been found to be typical of the mass fractal structure of several silica wet gels [12][13][14], inclusive for silica wet gels prepared with additions of PVA [15]. The SAXS contribution at high-q in the xerogels was then assumed as being produced by a polymeric mass fractal particle formed from Fig.…”

Aerogels Synthesis by Sonocatalysis: Sonogels

“…The coupling between the ultrasound and the heterogeneous mixtures does not seem too good for higher ultrasound power [46]. The oxalic acid catalyzed TEOS hydrolysis peak shows a less pronounced form when compared to the HCl catalyzed one [47].…”

“…The reaction beaker could be surrounded by a thermostatic bath to ensure local temperature control [42,46,47]. TEOS:water:HCl mixtures of constant volume (35 ml) with a molar ratio of 1:5:0.0089 and a pH~1.5 and a bath temperature held at 26.5 C present their exothermic peak after 3-4 min of sonication (20 kHz).…”