The process by which a solid surface concentrates fluid molecules by physical forces is known as adsorption (whereas absorption is a process whereby fluid molecules are taken up by a liquid and distributed throughout that liquid).
Activated carbon adsorption: the London dispersion force
The elemental force causing physical adsorption on activated carbon is the London dispersion force, a form of Van der Waals force, which results from intermolecular attraction. Thus, in the case of adsorption, carbon and the adsorbate are chemically unchanged. However, in the process known as chemisorption, molecules react chemically with the carbon surface (or an impregnant on the carbon surface) and are held by chemical bonds which are much stronger forces compared to London dispersion forces. The London dispersion force is an intermolecular interaction that exists between all molecules (both polar and non-polar), but it is extremely short-ranged. It is responsible for condensation of most gases to liquids, and the reason why higher-molecular-weight gases have higher boiling points. London forces are:
- Additive: The observed London force is the sum of all individual interactions of the adsorbate molecule and the neighbouring graphite plates composing the carbon structure. The magnitude of the adsorption force will be related to the number of carbon plates, or the density of carbon, near the adsorbate molecules.
- Non-specific: London forces exist between all molecules. Therefore, all molecules adsorb on activated carbon to some extent, depending on their vapour pressure and solubility at the carbon temperature.
- Temperature independent: The London forces are unaffected by temperature, and thus the adsorption force field will be constant with temperature. (However, the carbon adsorption capacities will still be sensitive to the changes with temperature in vapour pressure or solubility of the adsorbing molecules).
- Short-ranged: The magnitude of the London force is very sensitive to the separation of the adsorbate molecule from the graphite plate. The London force can be considered negligible with a separation greater than about two molecular layers. Therefore, the adsorption forces will only be significant if the gaps or voids within the carbon structure (pore widths) are less than four or five molecular layers.
The first three characteristics of London forces are shared by another well-known force, namely gravity. London forces, and consequently the carbon adsorption forces, are analogous to gravitational forces. However, London forces are much shorter-ranged, and operate on a molecular rather than astronomical scale.
DESOTEC: adsorption on the move!
At DESOTEC, we manage to harness activated carbon’s full adsorption capacity. This way, we can provide you with high-performing purification systems for gases, liquids, air and water thanks to the unique adsorption treats of our high-quality activated carbon. Adsorption on the move!