Perhaps the quickest definition of adsorb is “to accumulate on the surface.” The problem with that definition is that it conjures up misleading thoughts of poor housekeeping and writing your name in the dust. Adsorption is about moisture, not dust. The quick definition is effective in one way though: It makes a clear distinction between “on the surface” adsorption and the very different “becoming imbued or permeated” meaning of a similar word, absorption.
Aside from the confusion between the two words being a dyslexic’s nightmare, adsorption is a common term when working with desiccant dehumidifiers and dryers, and the concept is relatively easy to understand.
Adsorption is a physical process where one substance is attracted to and held by another substance. If you are dehumidifying by adsorption, then water or water vapor is being attracted to and held by a desiccant substance. The water won’t dissolve the desiccant; the desiccant simply holds the water in pores on its surface.
Some solids have the ability to adsorb great quantities of liquids or gases, and these make the best desiccants. These solids really are not all that “solid” but are quite porous. The porosity gives the desiccant a large surface area at the microscopic level. This lets the water move by capillary action from surface tension through the pores into the deeper layers of the desiccant material. The greater the surface area and porosity, the more water it can catch and hold.
Desiccant air dryers work because water is a polar molecule; that means that a molecule of water has a slight electrical charge that will cause it to be attracted or repelled, depending upon the kinds of charges surrounding it. A silica gel desiccant uses these weak inter molecular forces to attract and hold the water.
Molecular sieve desiccants also work by adsorption. In this case, the desiccant has a highly regular crystalline lattice structure that has many open spaces and pores that can hold the water. It is this crystalline structure that gives it a high adsorption rate that is more efficient at reducing humidity than a substance like Montmorillonite clay.
The result of the adsorption process is that a film of liquid or gas forms on the surface of a solid substance. In this way, excess moisture is kept away from products where it could cause harm. The products stay drier, fresher, and free of mold, mildew, rust, or corrosion.