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

USAXS and x-ray microscopy investigation on silica and precursors of zeolites

Combination of SAXS and USAXS measurements provide an extended q-range (0.006-3.0/nm) to study fractal growth of both aging silica gel as well as precursors of zeolite-A. Mass (silica) and surface (zeolite) fractals are obsd. Scanning transmission X-ray microscopy (STXM) proves to be an extremely useful technique to obtain direct images of wet samples in the 0.1-100 m range, confirming the SAXS/USAXS results on even larger length scale

Gel transfromations in silicas - a combined NMR and SAXS study

H-1 NMR transverse spin-spin relaxation times (T2) of water have been measured to monitor the aggregation and aging processes of low concentration silica gels. Along with H-1 NMR, small-angle X-ray scattering (SAXS) and physisorption measurements; were used to obtain additional information on aging. After acidification of water-glass (silica solution) the primary building units combine to form aggregates with fractal dimensionalities (decreasing T2). Gelation occurs when these growing aggregates form a percolating network, resulting in a minimum value of the T2 relaxation time. During and after these processes, rearrangements of silica take place (aging), a crucial step in the formation of pores. The aging of silica gel can be monitored by changing the relaxation behaviour (increasing T2). With SAXS a densification of the structure is indicated by an observed decrease in fractal dimensionalities and an increase in the radius of the primary building units, indicating a migration of silica from the tips of the aggregates towards the centre of the aggregates. Aging is a slow process compared with the aggregation of the primary particles, but it can be accelerated by adding catalytic quantities of fluorine or by preparation at higher pH or at higher temperatures. By using spin-spin relaxation measurements, reacting aqueous silica systems with a low solid content could be studied in situ. It is shown that hydroxyl and fluoride ions have different impacts on the silica systems, leading to different mechanisms. For aggregation and aging we observed (different) optimal fluorine concentration

Fractal Aggregation and TransforationProcesses in Silica Gels

+92hlm.;24c

Structural development in silica systems

Results obtained with small angle neutron and x-ray scattering are presented of systems with various silica concns. Silica gels with low silica concn. exhibit uniform mass fractal behavior and after prolonged aging, dual mass fractal behavior. A zeolite (silicalite) synthesis precursor gel with a high silica concn. was also investigated and exhibited surface fractal behavior. A model is presented, explaining the occurrence of dual fractal nature after prolonged aging. The change-over from mass fractal behavior to surface fractal behavior is explained, supported by scattering spectra calcd. from simulated aggregate structure

Scanning transmission X-ray microscopy: a new method for the investigation of aggregation in silica

During the preparation of silica by acidification of water glass, primary silica particles form extended and ramified aggregates. The growing aggregates form a gel, a tenuous network of interconnected aggregates. After aging and drying of the wet gel, porous silica is obtained. To study the extremely vulnerable aggregates only noninvasive methods are allowed. Moreover, because of the colloidal scale many methods based on (atomic or molecular scale) spectroscopy are not informative. Scanning transmission x-ray microscopy (STXM), using high- brilliance synchrotron radiation at 3.25 nm (380 eV) and 2.60 nm (480 eV) as an X-ray source, provides a new technique to obtain direct images of wet or solved aggregates at a 50-5000 nm scale, The 50 nm resolution is sufficient to provide excellent images of fractal structures. In this paper the principles of STXM are discussed in relation to investigations of wet gel systems like silica gel. (C) 1997 Academic Pres