Silica Gels and Powders

Silica gel may be described as a coherent, rigid three dimensional network of contiguous particles of colloidal silica. The formation of silica gel by polymerizatio of silicic acid has been discussed in detail in Chapter 3 and by aggregation of particles of colloidal silica in Chapter 4. 

There are silicas which are borderline between gels and powders. When the bonds between the ultimate colloidal particles are very weak, and can be readily broken by mechanical means, the silica can be classed as a powder that is not a gel. On the other hand, when the colloidal particles form a strong, coherent structure, the silica is classed as a gel even though the size of the gel granules may be as small as a few microns. 

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Due to the variety in porous structure, particle size and surface area, pure silica gels and powders find a very wide range of applications. Variation in preparation methods and parameters allows the tailoring of the substrate properties for specific application needs. The main features in the silica applications are its porosity, active surface, hardness, particle size and the viscous and thixotropic properties. Although most applications are based on a combination of those, a classification according to the main properties of interest may be set up. For references, the reader is referred to the works of Iler6 and Unger7 and to the references cited in chapter 8. 

The hardness and abrasive properties of silica gels and powders are of interest in their use as reinforcing agents in composite materials. 

The introduction to the section Silica Gels and Powders by W. Welsh constitutes an introduction to the study of silica powders. Detailed accounts of the synthetic processes and applications of fumed silicas, silica gels, and precipitated silicas are given by Ferch (Chapter 24) and Patterson (Chapter 32). For scientists, silica powders are of special interest because they offer the opportunity of working with very pure systems with well-controlled ultimate particle size and specific surface area. One of the most important aspects of silica powders is their adsorptive properties. These properties are the subject of the work by Kenny and Sing (Chapter 25), which includes the crystalline zeolitic silica known as silicalite. 

Iler devoted one of the seven chapters of The Chemistry of Silica (1) entirely to the manufacture, characterization, and uses of silica gels and powders . In the 160 pages of Chapter 5, Iler covered so many products, processes, and applications that it is difficult for anyone not already in the field to distinguish between what is of commercial importance and what is of theoretical interest. 

Preparation of Silica Gels and Powders, Geometric Modification of Silicas, and Studies of Structural Characteristics... 

A more detailed discussion of gel formation is reserved for a later chapter on silica gels and powders. Meanwhile, to understand the role of aggregation in the polymerization of polysilicic acid it is necessary to discuss the mechanism by which the molecular weight of silica increases by aggregation of particles or small spherical polymers. 

Current literature relating to various aspects of finely divided silicas is growing rapidly, usually in connection with particular applications or areas of technology. Various aspects of silica gels and powders have been summarized, often in detail, in reviews by various authors (1-lOc). 

Adsorption of Zn(e/i) on silica gels and powders was already mentioned ih Chapter 4 and can be used to determine surface areas of silica powders free from... 

The chemistry of the silica surface is dealt with in Chapter 6. However, a full characterization of silica gels and powders requires methods for determining at least the physical properties of the surfaces in the pores. 

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