What is Adsorption?

Adsorption is the process whereby molecules are concentrated on the surface of the activated carbon.

What makes molecules adsorb on activated carbon ?

Adsorption is caused by London Dispersion Forces, a type of Van der Waals Force which exists between molecules. The force acts in a similar way to gravitational forces between planets.

London Dispersion Forces are extremely short ranged and therefore sensitive to the distance between the carbon surface and the adsorbate molecule. They are also additive, meaning the adsorption force is the sum of all interactions between all the atoms. The short range and additive nature of these forces results in activated carbon having the strongest physical adsorption forces of any material known to mankind.

Gas Phase Adsorption - This is a condensation process where the adsorption forces condense the molecules from the bulk phase within the pores of the activated carbon. The driving force for adsorption is the ratio of the partial pressure and the vapour pressure of the compound.

Liquid Phase Adsorption - The molecules go from the bulk phase to being adsorbed in the pores in a semi-liquid state. The driving force for adsorption is the ratio of the concentration to the solubility of the compound.

What compounds are adsorbed ?

All compounds are adsorbable to some extent. In practice, activated carbon is used for the adsorption of mainly organic compounds along with some larger molecular weight inorganic compounds such as iodine and mercury.

In general, the adsorbability of a compound increases with:

  • increasing molecular weight
  • a higher number of functional groups such as double bonds or halogen compounds
  • increasing polarisability of the molecule. This is related to the electron clouds within the molecule