The Difference is on the Desiccant (Surface)
When discussing the function of desiccant (a material used to remove moisture from the air during storage or transport in order to prevent damage to goods) there is a great deal of confusion surrounding how it works. In most cases, desiccant adsorbs moisture rather than absorbs it, and the difference can be unclear. However, AGM’s Managing Engineer Pat Lane is here to explain:
This is not a simple subject because the distinction between absorption and adsorption is a fuzzy one, and many materials that do one also do the other.
First, let’s take a look at definitions of the two processes:
From Webster’s Ninth New Collegiate Dictionary (1988. Print):
“Absorption.” Def. 1a: the process of absorbing or of being absorbed
“Absorb.” Def. 1: to take in and make part of an existing whole. Def. 2a: To suck up or take up such as: a sponge absorbs water, charcoal absorbs gas, and plant roots absorb water.
“Adsorption.” Def. 1: The adhesion in an extremely thin layer of molecules (as of gases, solutes, or liquids to the surfaces of solid bodies or liquids with which they are in contact.
From THE CONDENSED Chemical Dictionary. (10th ed. 1981. Print.):
“Absorption.” Def. 1: In chemical terminology, the penetration of one substance into the inner structure of another, as distinguished from adsorption, in which one substance is attracted to and held on the surface of another.
“Adsorption.” Def. 1: adsorption. Adherence of the atoms, ions or molecules of a gas or liquid to the surface of another substance, called the adsorbent (q.v.) The best-known examples are gas/solid and liquid/solid systems. Finely divided or microporous material presenting a large area of active surface are strong adsorbents, and are used for removing colors, odors, and water vapor (activated carbon, activated alumina, silica gel.)
The Gist of It
Alright, so the two processes sound very similar in terms of definition. To me, however, what distinguishes adsorption from absorption is where it occurs, what forces are involved, and how “thick” the layer is. That is, I believe most adsorbed substances are only a single molecular layer thick on the surface they are adsorbed onto.
How It Works
Many of us use the analogy of a sponge for absorption. The sponge readily sucks up water and then readily releases it when we mechanically squeeze the sponge. However, if a completely dry sponge is wetted, then squeezed out, it still remains damp. This is because at least some of the dampness (although not all) is adsorbed water.
If I wring out a damp sponge with my hands and then place it in a press and squeeze it very hard, I can get more liquid water to run out of it. At some point, however, no matter how hard I squeeze, I won’t get any more liquid water out – yet the sponge will remain damp. This is the adsorbed portion.
Typically, this portion is a single molecular layer thick and coats every bit of the sponge’s surface, including what we don’t see at the microscopic level. For many materials, including the sponge, they are much more porous than we see with our eyes. These pores create a much greater surface area than we expect – or in many cases can even believe to be present.
In the case of desiccant materials, they appear to our eyes as small solids, usually in bead or granule form. But if we were to look at them with a scanning electron microscope, we would see that they are extremely porous with more empty space than solid material, which accounts for their exceptionally expansive surface areas and, therefore, their ability to adsorb significant amounts of moisture.
Now, if we place the sponge in water and then lift it out, it will stop dripping after a few seconds. But, if we look at it closely, we can see liquid water in the “pores” of the sponge. This represents absorption because we can see the liquid water, which is thousands of molecular layers thick.