Silica Gel Desiccant

Desiccants are commonly used to keep products dry and stable. Dry desiccants can absorb moisture from the air either by physical adsorption or by chemical reaction, and thus reduce the humidity in the headspace of sealed containers.

Moisture sorption by silica gel is an example of physical adsorption, and sorption by calcium oxide is an example of a chemical reaction. Different types of desiccants and their moisture sorption capacities can be found in the literature.16 The most commonly used desiccants for solid pharmaceutical products are silica gel, clay, and molecular sieves.

Silica gel is an amorphous form of silica (SiO2 xH2O) and is highly porous. Silica gel particles are composed of an interconnecting network of microscopic pores (capillaries), and therefore have a very large surface area. The mechanisms of moisture adsorption by silica gel include surface adsorption and capillary condensation in the porous network.

Silica gel works well at ambient temperature but may have decreased adsorption rate and equilibrium moisture content at higher temperatures. The moisture in silica gel can be removed by drying at a temperature greater than 110°C. The current USP Chapter <671> recommends pre-drying silica gel desiccant at approximately 150°C to ensure the complete removal of adsorbed water.

Clay is a low cost and efficient desiccant at low temperature. The primary chemical composition of clay includes silica and aluminum oxide, magnesium oxide, calcium oxide, and ferric oxide. Moisture adsorption capacity may be different for different grades of clay desiccant.

Therefore, their moisture adsorption isotherms must be verified during the desiccant selection process. Clay works well at ambient temperature, but at temperatures >50°C, it will likely lose moisture rather than absorb moisture. Similar to silica gel, bentonite clay can be dried at a temperature greater than 110°C.