Silica particles, fractal structure

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) ... 

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. 

When silica is prepared by acidification of water glass (alkali solution of silica), polycondensation reactions occur between dissolved oligomeric silica species, resulting in (sub)colloidal particles [1]. These primary particles combine to very ramified aggregates, a process described by diffusion or reaction limited cluster-cluster aggregation with power-law dependent density (fractals) [2,3]. After gelation the fractal structure is still preserved at sub-micrometer scale, while at larger scale Euclidean behaviour is observed. 

Sparingly soluble materials are often found precipitated from aqueous solution in the form of amorphous particles of ill-defined structure. Precipitated silica is an obvious extreme example where fractal structures are formed (of dimension about 2.4) [171]. Such materials may be intrinsically hydrophobic or be rendered hydrophobic by surface treatment. The presence of rugosities on the surfaces of such particles can have a profound effect on their antifoam function. 

In contrast, the three- or two-dimensional morphologies of colloidal aggregates via Brownian particle trajectories show a fractal-like structure. One of the most prominent features of the surface deposits formed by the diffusion-limited aggregation mechanism is the formation of isolated treelike clusters (9). In our experiments, the surface morphology of the silica-coated polyethylene composite prepared by... 

Our third applications example highlights the work of Nakano et al. in modeling structural correlations in porous silica. MD simulations of porous silica in the density range 2.2—0.1 g/cm were carried out on a 41,472-particle system using an iPSC/860. Internal surface area, ratio of pore surface to volume, pore size distribution, fractal dimension, correlation length, and mean particle size were determined as a function of the density, with the structural transition between a condensed amorphous phase and a low density porous phase characterized by these quantities. Various dissimilar porous structures with different fractal dimensions were obtained by controlling the preparation schedule and the temperature. 

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