How Do TOPDRY Desiccants Work?
It is important to understand how the desiccant does its job of protecting the product. Most porous adsorbents, such as silica gel, activated clay or molecular sieves rely upon physical adsorption rather than chemical adsorption to accomplish their function.

Physical adsorption involves relatively weak intermolecular forces (van der Waals forces and electrostatic interactions) between the moisture and surface of the desiccant.

Chemisorbents, such as calcium oxide, calcium chloride, involve an actual chemical bond. Physical adsorption of moisture is typically exothermic. The strength of the adsorptive bonds can thus be measured by the heat of adsorption. The higher the heat of adsorption for moisture on the desiccant, the stronger the bonding and the less easily that moisture can be subsequently removed.

In a porous desiccant such as the silica gel used by Dri-Box, water is removed from the airspace by:

1) multi-layer adsorption, which is the attraction of thin layers of water molecules to the surface of the desiccant. Because the desiccant is very porous, the surface area is high and significant amounts of water can be attracted and adsorbed;
2) by capillary condensation in which the smaller pores become filled with water.
Capillary condensation occurs because the saturation water vapor pressure in a small pore is reduced by the effect of surface tension.