Structural Development in Silica Systems
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development of the surface area (S) as a function of the radius (R): S - RD, with D, being the surface fractal dimension [12]. These power law exponents and the sizes of the aggregates and constituting particles can be easily deduced from a log-log presentation of the scattering spectra. Computer simulations provided useful information about aggregation processes and the relative roles of diffusion and reaction herein [13,14,15]. Recently also processes accompanied and followed by rearrangements have been studied by simulation [16,17,18]. In this study the structure of silica gel as a function of aging and the effect of concentration on the structure is studied with small angle scattering. The results will be discussed and the proposed model for the concentration effect is supported by computer simulations. EXPERIMENTAL SECTION Small angle scattering SAXS experiments were performed at the synchrotron radiation source at Daresbury Laboratories, United Kingdom, using beam line NCD 8.2. The Q-range covered in the experiments was 0.035-0.28 nm'. Typical aquisition times are 5 min. of X-ray exposure. Accurate and reproducible results are obtained by digitization of the scattered intensities up to 400 data points per measured spectrum. Corrections for background scattering and solvent were performed using the procedure introduced by Vonk [19]. SANS experiments were performed at the Rutherford Appleton Laboratory, ISIS facility, Abingdon, United Kingdom. Pulsed neutrons with wavelengths in the region 2.2-10 A were used in the LOQ diffractometer and were recorded on a 64 cm diameter position sensitive detector ar 4.3 m from the sample. Scattering vectors between 0.005 and 0.23 K' were obtained, providing information on distance scales from roughly 20 to 1000 A in a single measurement. Wavelength dependent corrections for sample transmission and detector efficiency have been included in the data reduction procedure to obtain a composite cross section in absolute units. Corrections have been made for the incoherent scattering of the hydrogen atoms of the template molecule used in zeolite synthesis. Materials Aqueous alkali silica (water glass) was prepared by dissolving silica (Aerosil 380, Degussa AG) and sodium hydroxide (Merck p.a.) in the appropriate ratio in demineralised water. Gels were prepared by the addition of water glass to a solution of hydrochloric acid in polyethylene containers under vigorous stirring at room temperature until the desired pH=4 was reached. The concentration of silica in the prepared reaction mixture is approximately 4 wt%. Gelation was detected when no meniscus deformation was observed on twisting the container. No detectable shrinkage of the silica gel was observed during aging. Zeolite precursors were prepared using Aerosil 380 (Degussa AG) as a silica source. Appropriate amounts of sodium hydroxide (Merck p.a.) and D20 (demineralized) were added as well as the template tetrapropylammonium bromide (Aldrich Chemie 98%), in the following molar ratios: Si0 2/TPA=6.25, Si0 2/NaOH=12.8 and D2 0
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