A.H. Boonstra

The dependence of the gelation time on the hydrolysis time in a two-step SiO2 sol−gel process

Abstract The gelation time in the basic step of a two-step hydrolytic polycondensation process of tetraethoxysilane (TEOS) in ethanol was strongly influenced by the hydrolysis in the acid step. Generally at short hydrolysis times an extremely long gelation time was measured. Systematic extension of the hydrolysis time results first in a minimum of the gelation time in the basic step, followed by a maximum and subsequently by a gradual decrease in the gelation time. The first decrease in gelation time as a function of the hydrolysis time was ascribed to the formation of silanol monomers during hydrolysis in the acid step. The subsequent increase in gelation time was caused by the decrease of these silanol concentrations, owing to the dimerisation reaction which became dominant because most of the added water had been consumed. For long hydrolysis times an appreciable effect of polymerisation in the acid step resulted in a renewed decrease in gelation time. The effect of parameters such as the hydrolysis temperature and the concentrations of ethanol, water, HCl and NH4OH on the hydrolysis-condensation processes was investigated.

Hydrolysis-condensation processes of the tetra-alkoxysilanes TPOS, TEOS and TMOS in some alcoholic solvents

Abstract The effects of methanol, ethanol, 1-propanol and 2-propanol on the kinetics and the mechanisms of the hydrolysis-condensation reactions of the silanes TPOS, TEOS and TMOS were studied. Also, the influence of the amount of water on the hydrolysis-condensation processes of TMOS and TPOS was investigated. Accordingly, hydrolysis time versus condensation time curves were recorded and 29 Si-NMR investigations were performed. The hydrolyzability of the silanes in an acidic environment was found to decrease in the sequence TMOS > TEOS > TPOS. The effect of the alcohols, in the sequence methanol > ethanol, 1-propanol > 2-propanol, upon the hydrolysis rate of each of the silanes is explained by differences in degree of dissociation of HCl in the different alcohols. Also, the exchange of alkoxy groups, observed when an alcoholic solvent was used with an alkoxy group different from the alkoxy group of the silane, may influence the hydrolysis rate of the silane. Differences in dimerization are ascribed to differences in base strength of the activated silanol complexes with regard to those of the protonated alcohols.

The influence of the addition of alkyl-substituted ethoxysilane on the hydrolysis—condensation process of TEOS

Abstract The effect of methyltriethoxysilane, dimethyldiethoxysilane or phenyltriethoxysilane on the hydrolysis-condensation process of TEOS was investigated using hydrolysis time-condensation time curves. Viscosity measurements and 29 Si-NMR investigations were also performed. It is found that the gelation process of such mixtures is slower than that of TEOS itself although, compared with TEOS, both the hydrolysis rate and the condensation rate of each of these compounds are higher.

The effect of formamide on silica sol-gel processes

Abstract The effect of formamide on the hydrolysis and condensation reactions at 50°C, both in the acid and in the basic step of a two-step sol-gel process, was investigated on mixtures of TEOS, ethanol and water, using 29 Si NMR spectroscopy at −75°C. In the acid step the hydrolysis rate and, especially, the dimerization rate are reduced as a function of the formamide concentration. As a consequence, the gelation time in the basic step is influenced. From density and specific surface area measurements on dried gels we determined at higher formamide concentrations an increase in particle size and an even greater increase in mean pore size. The results obtained are ascribed to a competitive reaction of formamide with the catalyzing protons, which explains the behavior of formamide as a so-called drying control chemical additive (DCCA).

Relation between the acidity and reactivity of a TEOS, ethanol and water mixture

The relation between the hydrolysis time in the acid step and the gelation time in the basic step of a two-step silica sol-gel process of tetraethoxysilane (TEOS), ethanol and water mixtures was investigated using different acids in the concentration range of 10−6 − 10−1 M. The values and positions of the extremes in gelation time were found to depend on the concentration and degree of dissociation of the catalyst used. Decreasing the catalyst concentration resulted in a gradual weakening of the extremes in gelation time. At very low catalyst concentrations, no extremes in gelation time were found and gelation already occurred in the acid step. We also found that by measuring the potential difference in the hydrolyzing mixture continuously, the hydrolysis-condensation process in the acid step could be followed directly. The reaction products formed during this process were investigated with 29Si NMR spectroscopy. The disappearance of the extremes in gelation time at low catalyst concentrations was ascribed to the transition from an acid to a base-catalyzed reaction mechanism for the participating hydrolyzed silicon compounds. The isoelectric point of each silanol determines the potential difference in the hydrolyzing mixture at which the transition from one to the other mechanism takes place.

Effect of hydrolytic polycondensation of tetraethoxysilane on specific surface area of SiO2 gels

The hydrolysis and polycondensation reactions were studied in mixtures of tetraethoxysilane (TEOS), ethanol and water, We observed that the extent of hydrolysis dominates the growth and final size of the compact particles in the resulting silica gel as well as its specific surface area. A high concentration of SiOC2H5 groups at the surface of the particles hampers their growth.

Hydrolysis-condensation reactions in the acid step of a two-step silica sol-gel process, investigated with 29Si NMR at −75°C

Abstract The relation between the hydrolysis time in the acid step and the gelation time in the basic step of a two-step silica sol-gel process was investigated using different HCl concentrations in tetraethoxysilane (TEOS), ethanol and water mixtures with a molar ratio of 1:4:( a + 0.5), where a was in the range of 0.25 and 4. At values of a The hydrolysis-condensation reactions in the acid step at 50°C were observed by 29 Si NMR spectroscopy at −75°C using samples drawn from the hydrolysing mixtures and immediately quenched to temperatures below −100°C. At any desired moment in the course of the hydrolysis, an accurate spectrum with a good signal-to-noise ratio can thus be obtained, corresponding with a well-defined hydrolysis time.

Hydrolysis-condensation reactions of silica gels during autoclave drying

Abstract The effect of the drying temperature, drying time and the presence of water vapour during the autoclave process on the specific surface area ( A ) of gels, prepared by a two-step sol-gel process was investigated for a variety of silica gels. Up to about 300°C a large decrease of A was found only for silica gels, in which an excess of water was present. This decrease was attributed to hydrolysis-condensation reactions at the surface of the silica particles. The decrease at higher temperatures was assigned to organic cracking processes. If an excess of water was added to the autoclave a large decrease of A was found for all the compositions used, depending on the relative amount of water in the autoclave and on the autoclave drying time. Depending on these conditions gels were found consisting of large strongly necked silica particles, as shown in TEM micrographs.

A two-step silica sol-gel process investigated with static and dynamic light-scattering measurements

Abstract The hydrolysis-condensation reactions in mixtures of tetraethoxysilane, ethanol and water were followed in the basic step of a two-step silica sol-gel process with dynamic and static light-scattering techniques. The radii of the particles formed were found to be about 1.5 nm and in agreement with the results found with specific surface area measurements on the corresponding dried silica gels. The relatively high scattering found for mixtures having incomplete hydrolysis in the acid step was ascribed to the formation of large particles caused by prolonged hydrolysis-condensation in the basic step. The decreased light transmission as a function of the decreasing density of dried silica gels was assigned to Rayleigh scattering by pores.

The effect of TEOG on the hydrolysis-condensation mechanism of a two-step sol-gel process of TEOS

The effect of the addition of tetraethoxygermane (TEOG) in the basic step of a two-step silica sol-gel process on the hydrolysis-condensation mechanism of tetraethoxysilane (TEOS) was investigated. Gelation time versus hydrolysis time curves were recorded for mixtures of TEOS, ethanol and water to which TEOG was added. It is found that an ethoxy group of TEOG reacts with silanols under ethanol formation. The rate of this reaction is almost independent of the base concentration. Because unhydrolysed TEOG has four reactive groups, it is efficient in cross-linking chains, which implies that in general its addition results in shorter gelation times. This effect becomes more apparent for mixtures with low water content. At high TEOG: silanol ratios, however, an increase in the gelation time is found to be due to the capturing of silanols by TEOG. The cross-linking effect of TEOG becomes weaker if it is partly hydrolysed before addition to the TEOS mixture.