ADSORPTION

ADSORPTION
Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface
of a solid or a liquid (adsorbent), forming a molecular or atomic film (the adsorbate). It is
different from absorption, in which a substance diffuses into a liquid or solid to form a
solution. The term sorption encompasses both processes, while desorption is the reverse
process.
Adsorption is operative in most natural physical, biological, and chemical systems, and is
widely used in industrial applications such as activated charcoal, synthetic resins and water
purification.
Similar to surface tension, adsorption is a consequence of surface energy. In a bulk
material, all the bonding requirements (be they ionic, covalent or metallic) of the
constituent atoms of the material are filled. But atoms on the (clean) surface experience a
bond deficiency, because they are not wholly surrounded by other atoms. Thus it is
energetically favourable for them to bond with whatever happens to be available. The exact
nature of the bonding depends on the details of the species involved, but the adsorbed
material is generally classified as exhibiting physisorption or chemisorption.
Physisorption or physical adsorption is a type of adsorption in which the adsorbate
adheres to the surface only through Van der Waals (weak intermolecular) interactions,
which are also responsible for the non-ideal behaviour of real gases.
Chemisorption is a type of adsorption whereby a molecule adheres to a surface through
the formation of a chemical bond, as opposed to the Van der Waals forces which cause
physisorption.
Adsorption is usually described through isotherms, that is, functions which connect the
amount of adsorbate on the adsorbent, with its pressure (if gas) or concentration (if liquid).
One can find in literature several models describing process of adsorption, namely
Freundlich isotherm, Langmuir isotherm, BET isotherm, etc. We will deal with Langmuir
isotherm in more details:
Langmuir isotherm
In 1916, Irving Langmuir published an isotherm for gases adsorbed on solids, which
retained his name. It is an empirical isotherm derived from a proposed kinetic mechanism.
It is based on four hypotheses:
1. The surface of the adsorbent is uniform, that is, all the adsorption sites are equal.
2. Adsorbed molecules do not interact.
3. All adsorption occurs through the same mechanism.
4. At the maximum adsorption, only a monolayer is formed: molecules of adsorbate
do not deposit on other, already adsorbed, molecules of adsorbate, only on the free
surface of the adsorbent.
For liquids (adsorbate) adsorbed on solids (adsorbent), the Langmuir isotherm