Silicic acid gels belong to the group of substances with high internal surface, i.e. they possess a large number of small and very small pores. In order to be able to form a picture of the type and appearance of these pores, it is necessary to study the chemical production of silica gel. Dividing the manufacturing process into two phases, the creation of the brine and the conversion of the latter into gel. The manufacture of silica brines in most cases starts from basic materials such as silicate solutions in water. Sodium silicate is the one most frequently used. If an acid, e.g. sulphuric acid, is added to such a solution, low molecular silicic acid molecules are formed. Orthosilicic acid forms the basic building block.
Figure 6.3.1
OH groups, tetragonally arranged around the silica, are referred to as silanol groups and are highly reactive. If the addition of acid is continued, water is split and chain formation of the silicic acid molecules leads to polycondensation. The development of.Si-O-Si links to form polysilicic acids leads to the formation of.the smallest particles, possessing about 3 - 50 times the size of the original molecule. In the further course of the reaction, an increasing number of silanol groups are condensed from the polysilicic acids already formed in the brine. This leads to the creation, accompanied by shedding water and the formation of Si-O-Si links, of molecules which are up to 600 times larger than the particles originally formed in the brine. This leads to an open but continuous structure, a three dimensional network of randomly orientated chains, ribbons and rings of polysilicic acids. The brine solidifies to become hydrogel, a highly porous solid substance.
After washing out the sodium sulphate formed in the course of the reaction, the syneric (expelled) water is driven out of the pores by subjection to heat. The hydrogel becomes activated silica gel, xerogel.
Figure 6.3.2
This drying operation is controlled in such a way that the silanol groups, which cover the total internal surface, are in principle preserved.
Figure 6.3.3
They endow the silica gel with its hygroscopic characteristics and also establish the adsorption qualities in conjunction with the capillary system.
