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QUIZ: Do You Know the Difference Between Adsorbent & Absorbent? When is an Adsorbent not a Desiccant?
Tuesday 29th September 2015
Often new customers come to us to find solutions to their moisture problems and they may not be totally familiar with the terminology used to describe a number of solutions to solving moisture problems. They may ask for Silica Gel, when actually another kind of desiccant may be more appropriate.
So we would like to take a moment to go over some of the terminology to make it easier to understand the choices in preventing moisture and contaminant damage.
Do You Know The Difference Between a Desiccant and an Adsorbent?
What are Adsorbents?
Adsorbents are materials that remove a contaminant from a fluid (which may be gas or liquid). This contaminant may be moisture, but equally it could be a gas or odor. Put another way an adsorbent material will remove something from the environment and chemically trap it, thus leaving the environment free from that contaminant. The process involves the molecules you wish to trap forming a thin film on the surface of the adsorbent material. If the material has a large surface area, for example if it is made of small grains, then it will be able to adsorb more.
What are Desiccants?
Desiccants are adsorbents, that specifically remove (trap) water molecules (moisture) from the environment. Silica Gel is a well known example of an adsorbent, but there are others. Silica gel can adsorb up to 30% of its own mass in moisture! Note the spelling of desiccant is not dessicant, desicant, dessiccant or decicant, we appreciate it is an easy mistake to make!
What does Absorbent mean?
We've used the word adsorb (note the d) and sometimes this gets confused with the word absorb (with a b). When thinking about absorbing a fluid, it usually implies that the fluid is a bulk liquid, for example some spilled water, and the absorbent material "soaks up" or "drinks in" the liquid. This is often a mechanical process involving capillary action, due to water surface tension. For example, "a plant absorbs moisture through its roots" or "a kitchen towel is used to absorb spilled water".
Adsorb Versus Absorb?
The difference between absorb and adsorb is that to adsorb is to gather something on a surface in a condensed layer involving surface bonding forces. Wheras to absorb is to draw up liquid usually using mechanical or chemical processes. For example carbon will adsorb gases, wheras a paper towel absorbes water.
Does Size Really Matter?
As an interesting sidenote, at nanoscopic particle sizes the difference in behaviour between adsorbents and absorbents really disappears but it's good to understand that there is a difference! Especially when considering what is best for your moisture/contaminant issues.
Some people remember it as "aDsorbes to the siDes" since adsorption uses the surfaces (sides) of the material.
Another interesting consideration is that when dealing with a bulk fluid such as a puddle of water an adsorbent material will often also demonstrate absorbent properties. But when considering moisture molecules in the atmosphere, such as sealed in your packaging, then we are usually dealing with humidity rather than water and the difference between absording and adsorbing is more evident. A paper towel would do little to change the moisture content of the air as it really only absorbs moisture in bulk (as water).
Desiccants are adsorbent but not all adsorbents are desiccants!
Here are some examples of adsorbents.
Sorbatech Adsorbent Products:
Posted by John Logan at 16:49