Desorption is a phenomenon whereby a substance is released from or through a surface. The process is the opposite of sorption (that is, either adsorption or absorption). 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 chromatography, 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.
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 H2 molecule).
The rate constant may be expressed in the form
where is the "attempt frequency" (often the Greek letter ), the chance of the adsorbed molecule overcoming its potential barrier to desorption, is the activation energy of desorption, is the Boltzmann constant, and is the temperature.
Electron-stimulated desorption occurs as a result of an electron beam incident upon a surface in vacuum, as is common in particle physics and industrial processes such as scanning electron microscopy (SEM). At atmospheric pressure, molecules may weakly bond to surfaces in what is known as adsorption. These molecules may form monolayers at a density of 1015 atoms/(cm2 ) for a perfectly smooth surface,. One monolayer or several may form, depending on the bonding capabilities of the molecules. If an electron beam is incident upon the surface, it provides energy to break the bonds of the surface with molecules in the adsorbed monolayer(s), causing pressure to increase in the system.
Once a molecule is desorbed into the vacuum volume, it is removed via the vacuum's pumping mechanism (re-adsorption is negligible). Hence, less molecules are available for desorption, and an increasing number of electrons is required to maintain constant desorption.
- Somorjai, Gabor A.; Li, Yimin (2010). Introduction to Surface Chemistry and Catalysis. John Wiley and Sons. Section 4.6.
- M. H. Hablanian (1997). High-Volume Technology, A Practical Guide. Second Edition. Marcel Dekker, Inc.
- Sorption isotherm
- Gibbs isotherm
- Moisture sorption isotherm
- Langmuir equation
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