Structural characteristics of silica sonogels prepared with different proportions of TEOS and TMOS

Sonohydrolysis of mixtures of tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) with different TMOS/(TMOS + TEOS) molar ratio R was carried out to obtain similar to 2.0 x 10(-3) mol SiO2/cm(3) and similar to 86%-volume liquid phase wet gels. Aerogels were obtained by supercritical CO2 extraction in autoclave. The samples were analyzed by small-angle X-ray scattering (SAXS) and nitrogen adsorption. The structure of the wet gels can be described as a mass fractal structure with fractal dimension D similar to 2.2 and characteristic length increasing from similar to 4.6 nm for pure TEOS to similar to 6.4 nm for pure TMOS. A fraction of the porosity is eliminated with the supercritical process. The fundamental role of the TMOS/(TMOS + TEOS) molar ratio on the structure of the aerogels is to increase the porosity and the pore mean size as R changes from pure TEOS to pure TMOS. The supercritical process increases the mass fractal dimension and shortens the fractality domain in the mesopore region. A secondary structure appearing in the micropore region of the aerogels can be described as a mass/surface fractal structure with correlated mass fractal dimension D-m similar to 2.6 and surface fractal dimension D-s similar to 2.3. (C) 2007 Elsevier B.V. All rights reserved.

Hydrolysis rates of TMOS catalyzed by oxalic acid and stimulated by ultrasound

A simple calorimetric method was employed to study the kinetics of the hydrolysis of the solventless TMOS-water mixtures, under ultrasound stimulation, as a function of the concentration of oxalic acid. The reaction rates were obtained, in relative units, from the measured thermal peak of the reaction as a non-separated function of both the sonication time and the instantaneous temperature of the medium. For concentrations of oxalic acid below 0.01 M, polycondensation reaction starts before complete hydrolysis. For concentrations of oxalic acid above 0.01 M, hydrolysis is complete and, in addition, the inverse of the time, as measured from the starting of ultrasound action until the maximum hydrolysis heat release, was found to be a reasonable relative measure of the average hydrolysis rate constant. The average hydrolysis rate constant was found to be proportional to the square root of the molar concentration of the oxalic acid. This result is in agreement with the literature if we assume small dissociation degree for the catalyst in such a solventless alkoxyde-water medium.

Effect of the temperature on the TMOS sono-hydrolysis and a study of the structure of the resulting aged sonogels

The effect of temperature on the oxalic acid catalyzed sono-hydrolysis of tetramethoxysilane (TMOS) was studied by means of a heat flux calorimetric method. The activation energy of the process was measured as (24.5 +/- 0.8) kJ/mol in the temperature range between 10 and 50 degreesC. The structural characteristics of the resulting sonogels, after long period of aging in saturated conditions, were studied by means of small angle X-ray scattering. The structure can be described as formed by similar to2.7 nm mean size mass fractal-like aggregates (clusters) of primary silica particles of similar to0.3 nm mean size, all imbibed in a liquid phase. The average mass fractal dimension of the clusters was found to be 2.58. The primary particle density was estimated as 2.23 g/cm(3), in good agreement with the value frequently quoted for fused silica. The volume fraction of the clusters, in the saturated sonogels was estimated as about 28%. The moment in which the meniscus of the liquid phase penetrates into the clusters under rapid evaporation process has been detected by an inflection in the first derivative of the curve of weight loss in a simple thermogravimetric test. (C) 2003 Elsevier B.V. All rights reserved.

Study of the hydrolysis of TEOS-TMOS mixtures under ultrasound stimulation

Pure and mixed tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS) were hydrolyzed at 35 degrees C, using oxalic acid as a catalyst and ultrasound stimulation. The hydrolysis reaction was carried out in a specially designed device, in which a heat flow steady state, between the ultrasound source and an external thermostatic bath, was maintained, in the absence of reactions. The exothermic hydrolysis causes a time dependent thermal peak. An induction time is apparent in pure TEOS before the hydrolysis peaks starts, which has been explained by the initial immiscibility gap of the TEOS-water system. The induction time was found to be approximately of the same magnitude as in the HCl catalyzed hydrolysis, in spite of the uncertainty accompanying the peak definition. No induction period is apparent in pure TMOS, so that the hydrolysis starts with its maximum rate. Two independent thermal peaks in the mixed TMOS-TEOS samples were found, both associated to the respective hydrolyses of the pure component. The induction time for the TEOS hydrolysis is decreased as more alcohol (and silanol) is produced in the earlier TMOS hydrolysis. This effect is explained by improvement of homogenization by alcohol.

A study of the hydrolysis pathway in oxalic acid catalyzed reacting TMOS-water mixtures under ultrasound stimulation

The hydrolysis of TMOS in oxalic acid catalyzed reacting TMOS-water mixtures, under ultrasound stimulation, was studied by fitting a simplified dissolution and reaction modeling for samples, the hydrolysis rate of which had been measured in a previous work. The reaction pathway represented in a ternary diagram shows a heterogeneous step for the reaction which gradually progresses until complete homogenization of the system. Besides the water dissolved due to the homogenizing effect of the alcohol, ultrasound maintains a virtual and additional dissolution of water located at the interface between the TMOS and water during the heterogeneous step of the reaction. The mean radius of the heterogeneity represented by water dispersed in TMOS was evaluated as around 150 Angstrom. The oxalic acid concentration accordingly increases the hydrolysis rate constant but its fundamental role on the solubility of water in TMOS could not unequivocally be established.

Calorimetric study of the effect of water quantity on tetramethoxysilane hydrolysis under ultrasound stimulation

The ultrasound stimulated and oxalic acid-catalyzed hydrolysis of tetramethoxysilane (TMOS) was studied by means of a heat flux calorimetric method as a function of the initial water/TMOS molar ratio (r) ranging from 2 to 10. The method is based on the time recording of the hydrolysis exothermic heat peak. which takes place in acidulated heterogeneous water-TMOS mixtures under ultrasonic stimulation, accounting for the instantaneous hydrolysis rate. The hydrolysis rate increases from zero up to a maximum value during the heterogeneous step of the process and then diminishes naturally according to the reactant consumption. The total hydrolyzed quantity was found to be slightly increasing with r. The immiscibility gap of the TMOS- water system in the presence of the hydrolysis products has been inferred from the evaluation of the reacted quantity during the heterogeneous step of the reaction and it has been represented in a ternary diagram in the studied r-range.

Small-angle X-ray and nuclear-magnetic resonance study of siloxane-PMMA hybrids prepared by the sol-gel process

Transparent siloxane-polymethylmethacrylate (PMMA) hybrids were synthesized by the sol-gel process through hydrolysis of methacryloxyproyltrimethoxysilane (TMSM), tetramethoxysilane (TMOS) and polymerization of methylmethacrylate (MMA) using benzol peroxide (BPO) as catalyst. These composites have a good chemical stability due to the presence of covalent bonds between the inorganic (siloxane) and organic (PMMA) phases. The effects of siloxane content, pH of the initial sol and BPO content on the structure of the dried gels were analyzed by small-angle X-ray scattering (SAXS). SAXS results revealed the presence of an interference (or correlation) peak at medium q-range for all compositions, suggesting that siloxane groups located at the ends of PMMA chains form isolated clusters that are spatially correlated. The average intercluster distance - estimated from the q-value corresponding to the maximum in SAXS spectra - decreases for samples prepared with increasing amount of TMSM-TMOS. This effect was assigned to the expected increase in the number density of siloxane groups for progressively higher siloxane content. The increase of BPO content promotes a more efficient polymerization of MMA monomers but has no noticeable effect on the average intercluster distance. High pH favors polycondensation reactions between silicon species of both TMOS and TMSM silicon alcoxides, leading to a structure in which all siloxane clusters are bonded to PMMA chains. This effect was confirmed by Si-29 nuclear-magnetic resonance (NMR) measurements.

Structural study of aged saturated silica gels obtained from tetramethoxysilane sonohydrolysis with different water/tetramethoxysilane molar ratio

The structural characteristics of saturated silica sonogels were studied by means of small-angle x-ray scattering (SAXS) and thermogravimetric analysis (TG), after a long time of aging in saturated conditions. The sonogels were obtained by a sol-gel routine from ultrasound stimulated tetramethoxysilane (TMOS) hydrolysis carried out with the initial water/TMOS molar ratio (r) ranging from 2 to 10. The saturated sonogel structure can be described as composed by mass fractal-like aggregates (clusters) of primary silica particles, all imbibed in a liquid phase. The values of the mass fractal dimension (D) of the clusters was found all around 2.5, while the characteristic size of the clusters (ξ) was found generally increasing with r, going from approximately 2.3 nm (r = 2) to 4.5 nm (r = 10). The volume fraction of the clusters was estimated from the SAXS data. The results were compared to the values of weight loss fraction at the inflection point that has been found in the derivative of the TG curve, which should accounts for the instant in which the meniscus of the liquid phase penetrates into the clusters under a rapid evaporation process as in a TG test.