Silica gel is a granular solid with a high specific surface area that can adsorb water. It has a melting point of 1200 degC and an electrical resistivity of 1015 O-cm. It is non-toxic and biodegradable. It is found naturally in nature in the form of sand, clay and rock. The Earth’s crust is 59 percent silica.
The adsorption capacity of silica gel increases with increasing surface area, porosity and mesoporous structure. This is because well ordered mesoporous silica has higher pore diameters and is able to trap more moisture within the pores. It has also been reported that the selectivity of adsorption is improved with the use of mesoporous silica catalysts in comparison to amorphous ones.
In air conditioning systems, the main purpose of using silica gel is to control Relative Humidity. Air will always contain water vapor, but a certain amount can be trapped by the gel to prevent condensation and mould growth. The adsorption capacity of silica is limited to about 50% of the saturation capacity at a given temperature. The goal is to keep Relative Humidity at levels below this limit in order to prevent corrosion and spoilage of the goods.
Silica gel is commonly used as a preservation tool in museums, libraries and other storage facilities to control humidity. It is also used in medical devices such as syringes and drug test kits, as well as in hospitals for sanitation purposes. It is also included in small paper envelopes (usually with a “do not eat” warning) that are placed in packages of dry foods to absorb any excess moisture.