Adsorption is the process by which a gaseous or liquid molecule binds to the surface of a solid material.

The adsorption process is used in many branches of industry, e.g. in the pharmaceutical, food processing and chemical industries.

This phenomenon is caused by weak physical forces such as van der Waals forces and hydrogen bonds.

Adsorption is the accumulation of substances from gases or liquids on the surface of a solid, more generally at the interface between two phases.

In the case of absorption, which is different, the substance penetrates into the interior of a solid or a liquid. The generic term for adsorption and absorption is sorption.

When two or more species adsorb on one surface, this is referred to as coadsorption. This plays a particularly important role in catalysts, where the different types of atoms are adsorbed on the same surface and react there.

Adsorption is a physical process in which substances (usually molecules) adhere to the surface of another substance and attach themselves to its surface. The forces that cause adhesion are not chemical bonds, but merely van der Waals forces.

This form of adsorption is therefore more precisely referred to as physical adsorption or physisorption. In principle, physical adsorption also includes the reverse process, desorption, as a system always strives for an equilibrium between the adsorption and desorption of a substance.

The surface on which adsorption takes place can be the surface of a solid or the surface of a liquid. In general, the surfaces are referred to as interfaces.

The phase from which the substance reaches the interface can be a gas phase or a liquid (solution). The particles reach the interface through their undirected, thermally driven molecular motion. However, the following sections mainly describe the adsorption of gases on solid surfaces.

In addition to absorption and chemisorption, physisorption is an everyday process that takes place on almost all surfaces that are exposed to gases (air) or liquids (water).

Chemisorption is a special case of adsorption in which substances are bound to the surface of a solid by chemical bonds. It differs from physisorption in that it involves the breaking and formation of chemical bonds and can lead to a desorbate, which is formed as the product of a chemical reaction.

Chemisorptions often do not take place in equilibrium and are irreversible; they also usually occur at high temperatures and can be accompanied by an accumulation of substances on the surface. Chemisorptions are important steps that occur in heterogeneous catalysis processes.

In general, the term adsorption should only be used for the everyday processes of physisorption. Special adsorption involving chemical processes should be explicitly referred to as chemisorption.

If, for example, water is absorbed by a surface, this is referred to as physisorption, as no chemical reaction is involved in this process.

In physical adsorption, a substance adheres to a surface through weak forces, similar to adhesion. In chemical adsorption, the substance forms a chemical bond with the surface and becomes part of it.

The adsorption energy in physical adsorption is well below 100 kJ/mol. Chemical bonds within an adsorbed particle remain, but are polarized.