Packing, secondary particles

The structure of these pyrogenic silicas has been discussed by Barby [5], particularly with reference to their specific surface area. It was concluded that the initially condensed particles are only about 1 rnn in diameter and that these are so closely packed (high coordination number) to secondary particles of 10 to 30 nm that only a small amount of nitrogen can penetrate the micropores between them. Thus the secondary particles are the ones that are commonly identified in electron micrographs and which determine the specific surface area. They are the primary particles in the voluminous aggregate structure and have a low coordination number of about 3 (see Fig. 8.3). Because of the low level of impurities this type of silica is often used as catalyst support in fundamental studies. 

Until the availability of high-resolution electron microscopy, the structure of silica xerogels remained a matter of conjecture. Now, there is no longer any doubt that the amorphous framework is made up of very small globular units (see Figure 10.6). These primary particles are isotropic and have fairly uniform size of 1-2 nm (with a molar mass 2000 g mol 1). In some xerogels the primary particles are densely packed within secondary particles, whereas in other systems there is a more open arrangement (Barby, 1976). 

Separation. Consider a hypothetical plane parallel to the surface of the cast solution. Assume that the secondary particles are placed on this plane In a two-dimensional lattice of hexagonal close packing (see Figure 4). The lattice coordination number Is... 

Granular silica. In the early 1990s, Japan Storage Battery developed a VRLA battery in which the add was immobilized by closely packed granular silica that was filled between and around the plates [51 53]. The silica consists of fine primary particles and of coarser secondary particles formed by agglomerates of the primary particles. While the fine primary particles immobilize the add, the interstices... 

Macroporous, macroreticular, or isoporous polymer packings exhibit another type of structure (Figure 3.9). As the name implies, these polymers contain so-called macropores (>100 nm) and micropores (<2nm), the latter being inaccessible to large solutes. In other words, macroporous polymer particles constitute an agglomerate made of secondary particles that themselves represent... 

If the primary particles (spherical balls with diameter D) are closely packed in a secondary particle, the acmal crystals occupy 74% of the space in the secondary particle. If the secondary particles are also closely packed, they cannot change in size. The reaction might stop, due to choking of channels through whieh the gas molecules come... 

TiClj (violet) Larger secondary particles random alternation of a-and V- packed particles 80-92 External milling of the ot-and forms, from 1960 onwards... 

An example of a separation primarily based on polar interactions using silica gel as the stationary phase is shown in figure 10. The macro-cyclic tricothecane derivatives are secondary metabolites of the soil fungi Myrothecium Verrucaia. They exhibit antibiotic, antifungal and cytostatic activity and, consequently, their analysis is of interest to the pharmaceutical industry. The column used was 25 cm long, 4.6 mm in diameter and packed with silica gel particles 5 p in diameter which should give approximately 25,000 theoretical plates if operated at the optimum velocity. The flow rate was 1.5 ml/min, and as the retention time of the last peak was about 40 minutes, the retention volume of the last peak would be about 60 ml. 

The secondary structure, the mesopores, is similar to the internal structure of standard HPLC particles. This secondary structure provides the surface for retention. The standard pore size is in the order of 13 nm, resulting in a specific surface area of about 300 mVg. Due to the lower ratio of retentive structure to interstitial space, the retentivity of monoliths and the preparative loadability tends to be significantly lower than the retentivity and loadability of packed beds of 10-nm particles. Since the monolithic columns described here are made from silica, they can be derivatized in the same way and with the same technology as silica-based particles. Also, the useful pH range is the same as for silica-based particles. 

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