For sure you have heard about absorption, but what about adsorption? The lesser-known process of adsorption is the method that is central to Cotes' dehumidifiers for dealing with humidity. But let us take it from the beginning.
Both absorption and adsorption are sorption processes. Sorption is a physical and chemical process through which one substance attaches to another. The main difference is that while absorption involves the mass transfer of particles into another material (one substance absorbing another), adsorption takes place with the adhesion of particles onto the surface of a susbtance.
COMPARING ABSORPTION AND ADSORPTION
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What is absorption?
Absorption occurs when molecules pass into a bulky material. The particles diffuse or dissolve into another substance (which absorbs them), forming a solution. Once dissolved, the molecules cannot be easily separated from the absorbent.
An everyday example of this is a paper towel absorbing water - imagine water evenly permeating a paper towel as it soaks it up. The ratio of absorption depends on factors like concentration levels of the substance being absorbed, exposed surface area, and pressure.
What is adsorption?
Adsorption is a process in which atoms, ions, or molecules from a substance adhere to a surface (of the adsorbent). This is classified as an exothermic process because energy is released when the adsorbed substance sticks to the surface of the adsorbent substance. The rate of the process depends mainly on the surface area and temperature: lower temperatures promote adsorption.
Silica gels are used to absorb moisture, and thus reduce humidity. The name silica gel most likely sounds familiar to you, and thus reduce humidity. Cotes' adsorption dehumidifiers have a rotor with a honeycomb surface coated with silica gel, which attracts water molecules and keeps them "trapped".
Learn more about how Cotes adsorption dehumidifiers work:
To summarise:
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|
ABSORPTION |
ADSORPTION |
|
Definition |
Assimilation of particles throughout the bulk of the solid or liquid. Mass transfer of liquid particles into solids. |
Accumulation of the molecular species at the surface. Mass transfer of liquid particles onto solids. |
|
Phenomenon |
A bulk phenomenon |
A surface phenomenon |
|
Heat exchange |
Endothermic process |
Exothermic process |
|
Temperature |
Not affected by temperature |
Favoured by low temperatures |
|
Rate of reaction |
Occurs at a uniform rate |
Steadily increases and reaches an equilibrium |
|
Concentration |
Concentration eventually becomes even throughout the material. |
Surface concentration differs from internal concentration. |
References:
- Cussler, E. L. (1997). Diffusion: Mass Transfer in Fluid Systems (2nd ed.). New York: Cambridge University Press. ISBN 978-0-521-45078-2.
- Fomina, M., & Gadd, G. (2014). Biosorption: current perspectives on concept, definition and application. Bioresource Technology, 160, 3-14. https://doi.org/10.1016/j.biortech.2013.12.102
- Ruthven, Douglas M. (1984). Principles of Adsorption and Adsorption Processes. John Wiley & Sons. University of New Brunswick, Fredericton, Canada. ISBN: 978-0-471-86606-0
