Silica particles, fractal

Any discussion of fumed silica particle structure has to take into account this enormous difference. The approach of mass fractal dimension may provide a rough but helpful estimation. A real mass fractal is limited by the size of the cluster as an upper limit and the size of the particles as a lower limit. Then, the density of the cluster pduster be calculated from the true density of the particle Pparticio fhe ratio of the cluster size c/ciuster to the particle size /particle and the mass fractal dimension /) of the cluster (Eq. 2) ... 

This lack of micropores is further confirmed by SAXS and nitrogen adsorption data. Again a very helpful concept is fractality, in this case the surface fractal dimension D. A smooth, non-porous surface is described by a surface fractal dimension of = 2.0, but a totally porous body will reach a of up to nearly 3. SAXS and adsorption measurements on fumed silica result in a surface fractal dimension very close to 2.0 [6j. The fact of a nonporous smooth surface of the fumed silica particles is a most... 

The power-law exponent /x is around 4.0-5.0. A similar result is obtained for the yield stress in shear cr, except the exponent is smaller, coated silica particles. Such power laws have been derived from theories that model the gel as a network of interconnected fractal clusters (BuscaU et al. 1988 Shih et al. 1990 Potanin et al. 1995). 

Perspectives. Ordered mesostnictured and mesoporous silica has been known for little more than ten years. Tremendous progress has been made with respect to precise control of the structure, texture, and chemical fimctionality of the surface of these materials. His lecture surveyed the synthesis of such materials, with a focus on organically ordered mesoporous materials. Quite a number of contributions dealt with amorphous fumed silica, its Physical-Chemical Features and Related Hazard Risk Assessment (M. Heinemann), the description of fractal aggregates (C. Batz-Sohn), and the Characterization of Size and Structure of Fumed Silica Particles in Suspension (F. Babick). E. Brendle reported on Adsorption of Water on Fumed Silica, and in a second paper he summarized research on Methylene Chloride Adsorption on Pyrogenic Silica Surfaces. 

Keywords dynamic light scattering, fractal aggregates, filmed silica, particle size, simulation... 

For amorphous silica layers the s)mthesis process is similar to that used for mesoporous membranes, except that now solutions of ultra small, polymeric silica particles, with fractal dimensions smaller than 1.5-2.0, are used as precursors. These are produced with a set of specific synthesis conditions (e.g. high acidity to control the relative rates of the hydrolysis and condensation reactions). 

Jullien, R. and Botet, R., Aggregation and Fractal Aggregates, World Scientific, Singapore, 1987. Berry, M.V. and Percival, I.C., Optics of fractal clusters such as smoke. Opt. Acta, 33, 577, 1986. Ereltoft, T., Kjems, J.K., and Sinha, S.K., Power-law correlations and finite-size effects in silica particle aggregates studied by small-angle neutron scattering, Phys. Rev. B, 33, 269, 1986. 

In the inhibition period aluminum cations react with the silica oligomers and subcolloidal particles to form isolated patches of alumino-silicates on the surface of the small silica particles. From the review work of Iler (Her 1979), it is known that the reaction of aluminum cations with silicate oligomers is fast compared to the clustering of particles. After the inhibition period, the size of the aggregates slowly increases. Increasing the concentration of aluminum cations in the solution involves the formation of more homogeneous alumino-silicate particles. At a concentration of 0.10 mol% aluminum in the silicate solution, the fractal dimensionality... 

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