Desorption is a
phenomenonA phenomenon is any observable occurrence. In popular usage, a phenomenon often refers to an extraordinary event. In scientific usage, a phenomenon is any event that is observable, however commonplace it might be, even if it requires the use of instrumentation to observe it...
whereby a substance is released from or through a surface. The
processIn science, a process is every sequence of changes of a real object/body which is observable using scientific method. Therefore, all sciences analyze and model processes....
is the opposite of
sorptionSorption refers to the action of both absorption and adsorption taking place simultaneously. As such it is the effect of gases or liquids being incorporated into a material of a different state and adhering to the surface of another molecule...
(that is,
adsorptionAdsorption is the accumulation of atoms or molecules on the surface of a material. This process creates a film of the adsorbate on the adsorbent's surface. It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution...
and
absorptionAbsorption, in chemistry, is a physical or chemical phenomenon or a process in which atoms, molecules, or ions enter some bulk phase - gas, liquid or solid material. This is a different process from adsorption, since molecules undergoing absorption are taken up by the volume, not by the surface...
). This occurs in a system being in the state of sorption equilibrium between bulk phase (fluid, i.e. gas or liquid solution) and an adsorbing surface (solid or boundary separating two fluids). When the concentration (or pressure) of substance in the bulk phase is lowered, some of the sorbed substance changes to the bulk state.
In chemistry, especially
chromatographyChromatography is the collective term for a set of laboratory techniques for the separation of mixtures....
, desorption is the ability for a chemical to move with the mobile phase.
Desorption is a
phenomenonA phenomenon is any observable occurrence. In popular usage, a phenomenon often refers to an extraordinary event. In scientific usage, a phenomenon is any event that is observable, however commonplace it might be, even if it requires the use of instrumentation to observe it...
whereby a substance is released from or through a surface. The
processIn science, a process is every sequence of changes of a real object/body which is observable using scientific method. Therefore, all sciences analyze and model processes....
is the opposite of
sorptionSorption refers to the action of both absorption and adsorption taking place simultaneously. As such it is the effect of gases or liquids being incorporated into a material of a different state and adhering to the surface of another molecule...
(that is,
adsorptionAdsorption is the accumulation of atoms or molecules on the surface of a material. This process creates a film of the adsorbate on the adsorbent's surface. It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution...
and
absorptionAbsorption, in chemistry, is a physical or chemical phenomenon or a process in which atoms, molecules, or ions enter some bulk phase - gas, liquid or solid material. This is a different process from adsorption, since molecules undergoing absorption are taken up by the volume, not by the surface...
). This occurs in a system being in the state of sorption equilibrium between bulk phase (fluid, i.e. gas or liquid solution) and an adsorbing surface (solid or boundary separating two fluids). When the concentration (or pressure) of substance in the bulk phase is lowered, some of the sorbed substance changes to the bulk state.
In chemistry, especially
chromatographyChromatography is the collective term for a set of laboratory techniques for the separation of mixtures....
, desorption is the ability for a chemical to move with the mobile phase. The more a chemical desorbs, the less likely it will adsorb, thus instead of sticking to the stationary phase, the chemical moves up with the solvent front.
In chemical
separation processIn chemistry and chemical engineering, a separation process is used to transform a mixture of substances into two or more distinct products. The separated products could differ in chemical properties or some physical property, such as size, or crystal modification or other separation into different...
es,
strippingStripping is a physical separation process where one or more components are removed from a liquid stream by a vapor stream. In industrial applications the liquid and vapor streams can have concurrent or countercurrent flows. Stripping is usually carried out in either a packed or trayed column.-...
is also referred to as desorption as one component of a liquid stream moves by
mass transferMass transfer is the transfer of mass from high concentration to low concentration. The phrase is commonly used in engineering for physical processes that involve molecular and convective transport of atoms and molecules within physical systems...
into a vapor phase through the liquid-vapor interface.
After adsorption, the adsorbed chemical will remain on the substrate nearly indefinitely, provided the temperature remains low. However,as the temperature rises, so does the likelihood of desorption occurring. The general equation for the rate of desorption is:
,
where is the rate constant for desorption, is the concentration of the adsorbed material, and is the kinetic order of desorption.
Usually, the order of the desorption can be predicted by the number of elementary steps involved:
Atomic or simple molecular desorption will typically be a first-order process (i.e. a simple molecule on the surface of the substrate desorbs into a gaseous form).
Recombinative molecular desorption will generally be a second-order process (i.e. two hydrogen atoms on the surface desorb and form a gaseous H
2 molecule).
The rate constant may be expressed in the form
where is the "attempt frequency" (often the Greek letter
Nu , is the 13th letter of the Greek alphabet. In the system of Greek numerals it has a value of 50...
), the chance of the adsorbed molecule overcoming its potential barrier to desorption, is the
activation energyIn chemistry, activation energy is a term introduced in 1889 by the Swedish scientist Svante Arrhenius, that is defined as the energy that must be overcome in order for a chemical reaction to occur. Arrhenius' research was a follow up of the theories of reaction rate by Serbian physicist Nebojsa...
of desorption, is the Boltzmann constant, and is the temperature.