Silica gels from aqueous silicate solutions:combined 29Si NMR and small-angle X-ray scattering spectroscopic study

The use of modern spectroscopic techniques in the study of the formation of aq. silica gels is described. The oligomerization process of monomeric silicic acid was studied by silicon-29 NMR spectroscopy; at high pH values cyclic trimeric silicate species were favored compared to the linear structure. Aggregation of primary silica particles of mol. size

A small angle X-ray scattering study on high pH silica precipitations

Silica precipitations were carried out at high pH (7-10) and elevated temperatures (60-90 degrees C) by means of simultaneous dosing of diluted water glass and sulphuric acid into a stirred thermostatted reaction vessel. In order to investigate the development of the vulnerable silica structures during the processes, every 5-10 min small samples were taken from the reaction mixture and analysed using small angle X-ray scattering. It was found that a narrow particle size distribution was maintained throughout the entire preparation procedure (or at least from about 1 h until the end of the process), despite the fact that the primary silica particles were growing continuously. In addition, a continuous decrease in the total number of primary particles was observed, indicating that the primary particles were involved in clustering and aging processes, and that (except for the initial reaction stages) newly dosed silica does not form new particles but is deposited onto the already existing particles. Ostwald ripening, which is regarded as the most important aging mechanism, probably proceeds relatively rapidly under the applied process conditions, leading to a persistent high degree of monodispersit

Influence of aging and dilution on the crystallization of silicate-1

The correlation between the formation of colloidal aggregates and the crystallization of silicalite-l from a clear solution has been investigated with in situ combined small-angle X-ray scattering and wide-angle X-ray scattering, and in situ ultra- small-angle X-ray scattering using synchrotron radiation. Increasing the aging time at room temperature of the synthesis mixture prior to heating to the reaction temperature reduced the induction period and increased the crystal growth rate without affecting the formation of colloidal aggregates. Dilution of the synthesis mixtures did not influence the nucleation and crystal growth rate, and did not affect the formation of colloidal aggregate

Aggregation and aging in silica gel

Aggregation and aging of silica gels, prepared by controlled addition of water glass to hydrochloric acid, and the transformation of an aged gel to a crystalline phase have been studied in-situ using high-brilliance synchrotron radiation. Small angle X-ray scattering (SAXS) has proved to be very informative in readily detecting transformations in the silica gels which can be described using the concepts of mass and surface fractality. The interpretation of SAXS spectra of aged silica gels has been accompanied by computer simulations of aggregation and aging based on a model for the aging mechanism and the calculation of the corresponding structure-factor patterns. Comparison with experimental spectra of aged silica highlights the important role of particle growth on the fractal dimension. Gel transformations during the transition from an amorphous gel to the crystalline phase of silicalite have been successfully monitored, in-situ, both for heterogeneous and for homogeneous preparations, using the combination of small and wide angle X-ray scattering (SAXS-WAXS