Silica aggregate

A solid emulsion is a suspension of a liquid or solid phase in a solid. For example, opals are solid emulsions formed when partly hydrated silica fills the interstices between close-packed microspheres of silica aggregates. Gelatin desserts are a type of solid emulsion called a gel, which is soft but holds its shape. Photographic emulsions are gels that also contain solid colloidal particles of light-sensitive materials such as silver bromide. Many liquid crystalline arrays can be considered colloids. Cell membranes form a two-dimensional colloidal structure (Fig. 8.44). 

Table 17.1 also displays the mass-fractal dimension, df, given by the power-law value, P, for the second level (aggregate). The df= 1.63 for A1 is less than that, and is less than 2.37 for A2 and 2.1 for A3. The lower value of df could indicate a more open structure of the A1 sample, " but it could also arise from less interpenetration of the silica aggregates occurring during drying. ... 

The dynamic response of polydimethylsiloxane (PDMS) reinforced with fused silica with and without surface treatment has been discussed in terms of interactions between the filler and polymer [54]. Since bound rubber measurements showed that PDMS chains were strongly attached to the silica surface, agglomeration due to direct contact between silica aggregates was considered an unlikely explanation for the marked increase in storage modulus seen with increasing filler content at low strains. Instead three types of flller-polymer-flller association were proposed which would cause agglomeration, as depicted in Fig. 15. 

The above regions of P(Q) stand out more clearly when P(Q) versus Q is plotted on a log-log scale instead of on a linear scale as shown in Figure 5.11. We illustrate this and the above concepts in Example 5.4 using an experimental study of silica aggregates. 

This is a good place to draw attention to x-ray and neutron scattering since we have already introduced a combination of x-ray and light scattering to examine the fractal structure of silica aggregates in Example 5.4. 

Commercial alumina (ALO-Ex30 and ALO-GB1 from AJKA Alumina Co. Ltd., Hungary), aluminum-hydroxide and silica powders were investigated. Impurity levels of the powders were below 0.5%. In some tests, minor amount of NaNC>3 and Na-silicates were, however, blended into the silica aggregates. The sodium-silicates were used as binding agents of primary particles, while NaNC>3 was applied to release gas (O2 and N2) at temperatures higher than 720 °C. 

From scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (see Fig. 2 and 3) the predominant particle structures of fumed silica are aggregates, that consists of firmly attached and partially fused primary particles. Some characteristic features of these silica aggregates are summarized in Fig. 4. 

Fumed silica aggregates are obviously linear and branched particle structures with a mean size of about 100 to 200 nm. By TEM we derive the size of the partially fused primary particles of about 10 run. This very small particle size correlates well with the high surfaces area of fumed silica which usually is larger than 100 m g as determined by nitrogen adsorption at 78 K according to BET [5]. Adsorption techniques and electron microscopy provide very close values of surface areas. This indicates that fumed silica exhibits a smooth particle surface in the range of nanometers, apparently its surface is free of micropores. 

The ability of fumed silica to support and maintain the free flow of solid-like powders is directly related to the small particle size of its aggregates. The silica aggregates will cover the surface of the powder particle and thus prevent the powder particles to lump together and will additionally act as a ball bearing to let the powder flow (Fig. 6). 

Hydrophilic fumed silica aggregates will very strongly interact in a nonpolar medium by hydrogen-bonds between surface silanol groups of neighbouring particles. For that, hydrophilic silica is an excellent thickener and rheological additiv for nonpolar liquids. Less than 5 wt. % of hydrophilic fumed silica will thicken a liquid alkane or silicone oil to a cuttable rubber-like gel. 

However, PDMS-modified silica aggregates seem to coat nearly the entire toner particle. Their topographical diameter is doubled compared to the HMDS-coated particles, but the diameter estimated from the phase image is very similar to the one foimd for the HMDS-modified ones. The maximum phase shift is reduced compared to the samples above. This indicates that the silica aggregates are covered by a soft material - the silylation layer. This soft material covers not only the particles but also the resin surface in between them. This can be seen by the decreased phase shift in between the silica particles compared to the pure resin. As in sample (Fig. 5b), probably the PDMS silylation layer interacts with the toner resin surface, increasing the overall adhesion. 

This article presents the research on amorphous colloidal silica aggregation. 

Schaefer, D.W. et al.. Fractal geometry of colloidal aggregates, Phys. Rev. Lett., 52, 2371, 1984. Hurd, A.J. and Flower, W.L., In situ growth and structure of fractal silica aggregates in a flame, J. Colloid Interface Set, 122, 178, 1988. 

Figure 5. Fractal dimensionality of aqueous silica aggregates differing in total silica concentration obtained from SAXS spectra. The spectra were recorded after 5 days of reaction (pH 4.0 at 25 °C) in sealed polyethylene bottles. SAXS spectra were recorded at the Synchrotron Radiation Source of Daresbury Laboratories, United Kingdom, on beam line 8.2.

The driving force for structure transformation can be related to the entropy of the structures (22). Very open structures are of highest entropy and thus will strive for a state that has less entropy. Because the structure of the silica aggregates is more or less rigid, the only way to decrease entropy is to gradually dissolve silica at places of highest entropy (i.e., the... 

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