Silica Gel

Definition of 'silica gel'
Silica gel is an amorphous form of silicon dioxide, which is synthetically produced in the form of hard irregular granules (having the appearance of crystals) or hard regular beads. A porous structure of interlocking cavities gives a very high surface area (up to 800 square meters per gram). Silica gel is manufactured industrially and easy to handle. It can adsorb a wide spectrum of substances; it is mainly used to adsorb water vapour, i.e. it is able to hold (water) molecules on its inner surface and therefore is called 'desiccant'.
The pore volume of silica gel is generally larger than 0.2 ml/g, the inner surface larger than 400 m2/g. The pore width varies from 1 nm up to a few hundred nm (nm = nanometer, 1nm = 10-9 cm).

Structure of silica gel
Silica gel is a highly porous form of silica or a mixture of aluminum oxide and silica. Despite its name, silica gel is a solid. The silicate is streaked with a structure of pores of various sizes.
The pores are classified according to their diameter*:
− Micropores have a radius less than 1 nm (= 10 Angstrom).
− Mesopores have a radius of 1-25 nm.
− Macropores have a radius larger than 25 nm.

According to the different average pore diameters of the silica gels they are categorized into fine-porous, so called A-type silica gels, and silica gels with large-pores, B-type silica gels.

Definition according to CEFIC (Conseil Européen des Fédérations de l'Industrie Chimique)

Production
Silica gel is produced from sodium silicate and sulphuric acid, dependent on the end product sometimes aluminium oxide is added to make the material water-resistant. During the process micro-crystal openings are generated that contain a large number of at random distributed pores with different sizes.
Silica gels are the result of the classical reaction of an alkali silicate with a mineral acid. Sulphuric acid is the most commonly used acid, although other mineral acids such as hydrochloric acid, nitric acid or phosphoric acid can be used. Sodium or potassium silicate may be used as the alkali silicate, with sodium silicate being preferred. The acid is added to the alkali silicate solution until a pH of about 3 to 5 is reached. The resulting product is solid silica which includes the liquid phase. That is, the silica fully includes the water within its pores. For this reason that the solid phase contains the liquid phase, these silica materials have been termed silica hydrogels, with the dried silica being termed a silica gel. The mode of drying will determine the pore size structure and the final size of the end product.

Definition of adsorption
The enrichment of substances at the phase boundary layer is generally called adsorption. The adsorption onto the solid phase can occur from the surrounding gas or liquid phase. Desorption is the inversion of adsorption occurrences.

Physical adsorption
The adsorption can be discerned in physical adsorption and chemisorption. The physical adsorption is mainly driven by van-der-Waals forces. The chemical properties of the adsorbed substances are unchanged. The physical adsorption is reversible, i.e. the adsorbed substances can be loosened from the surface and turned into their original state under certain conditions.

Adsorbed substances
Organic and non-polar substances can generally be adsorbed by silica gel. These substances are also adsorbed by activated carbon, whereas the kinetic reaction with activated carbon occurs faster than silica gel. On the other hand for the polar water molecule silica gel is the more selective and efficient adsorbant as compared to other adsoring media like activated carbon. Therefore silica gel is mainly considered as desiccant, a product that removes water from air and other gases. There is no chemical reaction, no by products or side effects. Even when saturated with water vapour, silica gel still has the appearance of a dry product, its shape unchanged. Silica gel is one of the oldest and most popular desiccant and adsorbent used for a wide number of industrial and consumer applications.

Representation of adsorption
By means of an adsorption-isotherm the grade of adsorption is represented as a function of the concentration of the substance, which has to be adsorbed. The adsorption-isotherm describes the equilibrium between:

− substance in the air, which has to be adsorbed (rest concentration)
− adsorbed substance at the silica gel surface (max. loading at the same rest concentration).
In general the maximum loading increases with higher concentration. When an equilibrium of equal pressure is reached, no more adsorption occurs.
Thus the higher the humidity of the surrounding air, the greater the amount of water that is adsorbed before equilibrium is reached.