Definition 

Silica gel is synthetically produced highly porous amorphous silica, in various possible forms. Silica gel is highly porous and has a very large internal surface area of ​​up to 800 m²/g.  Silica gel is produced industrially and can be used in many ways. Silica gel can adsorb many substances. Since silica gel is very hygroscopic, it is also very often used as a desiccant. Generally, the pore volume of silica is greater than 0.2 ml / g and the internal surface area is greater than 400 m2 / g. The pore sizes vary from 1 nm to several hundred nm (nm = Nanometer, 1 nm = 10^-9 m).

The structure of silica gel.Silica gel is found in different modifications. Silica gel is made from SiO₂. The silicium oxide of the silica gel shows a highly porous amorphous structure. In some products part of the SiO₂-molecules are replaced by aluminiumoxide molecules. The Al₂O₃ changes the properties of the silica gel, e.g. at higher levels of aluminium oxide (17%) the silica gel becomes water resistant, whereas the silica gel without Al₂O₃ desintegrates upon contact with water droplets.

Silica gels are also differentiated with respect to their porosity and also in terms of their size. The available pores are distinguished with the help of the pore size distribution: 

• micropores have a radius of less than 1 nm (= 10 angstroms).
• mesopores show a radius of 1-25 nm.
• macropores have a radius of more than 25 nm

Accordingly the fine-pored (microporous) silica gels are called A-type silica gel and open-pore (macroporous) silica gels are called B-type silica gels. 

Production 

An insoluble, clear gel is produced. For the production of waterproof silica gel, aluminum salts can be added to the process. The resulting structure looks like a sponge and is responsible for the porosity of the product. 

Definition of adsorption

Adsorption is described as the enrichment of substances from gases or liquids on the surface of a solid body.  The desorption is the inverse process of adsorption.