Adsorption and absorption are two sorption processes through which one substance attaches to another. The main difference between them is that adsorption is the adhesion of particles onto a substance, while absorption involves mass transfer into another material. But, adsorption and absorption involve other differences as well.
Here is a comparison of adsorption and absorption, a closer look at their definitions, and examples of each process.
Comparing Adsorption vs Absorption
Usually, when people think about adsorption and absorption, they consider the mass transfer of liquid particles onto (adsorption) or into (absorption) solids. But, these processes can involve plasma, gases, liquids, or dissolved solids where the ions, atoms, or molecules are adsorbed or absorbed by liquids or solids. While both sorption processes share this similarity, they differ in several ways:
| Adsorption | Absorption |
|---|---|
| Accumulation of particles onto a substance surface | Accumulation of particles throughout another substance |
| Surface phenomenon | Bulk phenomenon |
| Exothermic process | Endothermic process |
| Favored by lowering temperature | Not affected by temperature |
| Rate steadily increases until it reaches equilibrium | Occurs at a uniform rate |
| Surface concentration differs from internal concentration | Concentration eventually becomes the same throughout the material |
Adsorption Definition and Examples
Adsorption occurs when ions, atoms, or molecules adhere to a surface. The substance adsorbed onto the surface is called the adsorbate. The substance with the surface is called the adsorbent. Adsorption is an exothermic process because energy is released when the adsorbate sticks to the adsorbent. The rate of the process depends largely on surface area and temperature. Low temperature promotes adsorption because particles with less thermal energy have less kinetic energy and are more likely to stick to surfaces from covalent bond formation, hydrogen bonding, or other intermolecular forces.
Examples of adsorption include:
- Water adsorbing onto silica gel
- Contaminants adsorbing onto activated charcoal
- Particles adsorbing onto zeolites
- Silver adhering to glass and forming a mirror surface
- Non-stick coatings on pans
- Adsorption chillers used with refrigerants
- Viruses adsorb onto cells and surfaces
Uses of adsorption include water purification, cooling water for air conditioners, heterogeneous catalysts, surface treatments, and ion exchange columns.
Absorption Definition and Examples
Absorption occurs when ions, atoms, or molecules pass into a bulky material. These particles (the absorbate) diffuse or dissolve into the absorbent substance. A familiar example is a paper towel picking up water. Eventually, water evenly permeates the paper. Absorption occurs passively (diffusion) or actively (facilitated diffusion or active transport) and is an endothermic process. Absorption rate depends on several factors, include concentration, exposed surface area, and pressure.
Examples of absorption include:
- A paper towel absorbing water
- Hair absorbing water
- Oxygen from air dissolving into water
- Sodium hydroxide absorbing carbon dioxide from air
- Cells absorb water and nutrients from their surroundings
Uses of absorption include spill clean-up, hydration, and digestion.
Note that there is another definition of absorption in science, referring to the interaction where matter absorbs energy from light.
References
- Crini, Grégorio; Badot, Pierre-Marie (2010). Sorption processes and pollution : conventional and non-conventional sorbents for pollutant removal from wastewaters. Besançon: Presses universitaires de Franche-Comté. ISBN 978-2848673042.
- Cussler, E. L. (1997). Diffusion: Mass Transfer in Fluid Systems (2nd ed.). New York: Cambridge University Press. ISBN 978-0-521-45078-2.
- IUPAC (1997). Compendium of Chemical Terminology (the “Gold Book”). Blackwell Scientific Publications. doi:10.1351/goldbook
- McMurry, John (2003). Fundamentals of Organic Chemistry (5th ed.). Agnus McDonald. ISBN 0-534-39573-2.