Uniform silica particles, formation

Numerous techniques have been applied for the characterization of StOber silica particles. The primary characterization is with respect to particle size, and mostly transmission electron microscopy has been used to determine the size distribution as well as shape and any kind of aggregation behavior. Figure 2.1.7 shows a typical example. As is obvious from the micrograph, the StOber silica particles attract a great deal of attention due to their extreme uniformity. The spread (standard distribution) of the particle size distribution (number) can be as small as 1%. For particle sizes below SO nm the particle size distribution becomes wider and the particle shape is not as perfectly spherical as for all larger particles. Recently, high-resolution transmission electron microscopy (TEM) has also revealed the microporous substructure within the particles (see Fig. 2.1.8) (51), which is further discussed in the section about particle formation mechanisms. 

The formation of ordered sphere-packing structures was observed in certain rheological experiments as just described. Due to the extremely uniform size of the particles, an ordered dense packing structure will develop during sedimentation of the Stober silica particles (see Fig. 2.1.12) when the dispersion is either sterically or electrostatically stabilized. The gemstone opal is essentially based on this principle (80-88). A transmission election replica picture is shown in Figure 2.1.13. The uniform... 

Adsorption column preparation and loading. In order to obtain satisfactory results, the tube must be uniformly packed with the adsorbent uneven distribution may lead to the formation of cracks and channels and to considerable distortion of adsorption band shapes. If there is any doubt concerning the uniformity of particle size of the adsorbent powder it should be sifted before use to remove the larger particles fines are removed from the adsorbent using a sedimentation procedure immediately prior to column packing. In this the alumina or silica gel adsorbent is stirred into between five to ten times its volume of the selected solvent or solvent system, allowed to settle for five minutes and the supernatant liquor decanted off the procedure is repeated until the supernatant liquid is clear. 

In addition to these solution-based synthesis methods, uniform spherical particles of surfactant templated silicas and many other inorganic oxides have been prepared via drying of aerosol droplets of inorganic precursor with surfactant in a volatile solvent.This formation method is discussed in more detail in Section 2.7.3. 

The scattering results suggested that the corresponding growth processes in the mixed oxides proceeds by the formation of relatively uniform titania particles, followed by the formation of significantly larger silica particles [52]. 

The conversion of a sol of spherical particles to a uniform gel coctaiuing all the liquid in the sol is not easily understood. When particles collide it is assumed that adhesion can occur but in the case of silica particles there is reason to believe that the attachment is through the formation of Si-O-Si bonds. One reason for thinking so is that the same factors that promote polymerization of moncrr.er and low molecular weight silicic acids also promote the conversion of a sol of coUoidal silica particles to a gel. Thus sols consisting of well-defined spherical particles form gel least rapidly at about pH 2 and the process is accelerated by fluoride ior.s at low pH. 

The question remains why silica sols will still form gels at pH 2, the isoelectric point at which the charge on the particles is presumably zero. It is true that gel formation is slowest at this point, but this can be ascribed to the slowness with which siloxane bonds are formed between particles at this pH. Nevertheless, silica particles do chain together into a uniform gel structure at this pH and the structure is not any different from that formed more rapidly at pH 3-5, for example. Also at the isoelectric point no extreme retardation of gelling has been noted. 

Such a mechanism must have been involved in the formation of 200 nm spheres in a solution of pure silica sol prepared by hydrolyzing SiCU and removing HCl by electrodialysis, as reported by Radezewski and Richter (128b). The purified clear sol contained about 0.5% SiOj and the pH was 6.8. Similarly, uniform porous spherical silica particles up to 1 micron in diameter are formed by the aggregation of primary particles less than 5 nm in size formed by the hydrolysis of ethyl silicate in a water-alcohol-ammonia system as developed by Stdber and Funk (128c). 

Studies on randomness of filler distribution in polymethylacrylate nanocomposite are interesting. In this experiment, siUca particles were formed both before and after matrix polymerization. The results indicated that the concentration of silica was a controlling factor in the stress-strain relationship rather than the uniformity of particle distribution. Also, there was no anisotropy of mechanical properties regardless of the sequence of filler formation. This outcome cannot be expected to be duplicated in all other systems. For example, when nickel coated fibers were used in an EMI shielding application." When compounded with polycarbonate resin, fibers had a much worse performance than when a diy blend was prepared first and then incorporated into the polymer (Figure 7.1). In this case, pre-blending protected the fiber from breakage. 

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