Desorption

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.

In chemical separation processes, stripping is also referred to as desorption as one component of a liquid stream moves by mass transfer 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. The general equation for the rate of desorption is:

$R=rN^{x}$

where $r$ is the rate constant for desorption, $N$ is the concentration of the adsorbed material, and $x$ 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 $r$ may be expressed in the form

r=Ae^{{-E_{a}}/{kT}}

where $A$ is the "attempt frequency" (often the Greek letter $\nu$ ), the chance of the adsorbed molecule overcoming its potential barrier to desorption, $E_{a}$ is the activation energy of desorption, $k$ is the Boltzmann constant, and $T$ is the temperature.[1]

Desorption mechanisms

Depending on the nature of the absorbent/adsorbent-to-surface bond, there are a multitude of mechanisms for desorption. The surface bond of a sorbant can be cleaved thermally, through chemical reactions or by radiation, all which may result in desorption of the species.

Thermal desorption

Reductive or oxidative desorption

In some cases, adsorbed molecules are chemically bonded to the surface/material, providing a strong adhesion and limiting desorption. If this is the case, desorption requires a chemical reaction which cleaves the chemical bonds. One way to accomplish this is to apply a voltage to the surface, resulting in either reduction or oxidation of the adsorbed molecule (depending on the bias and the adsorbed molecules).

In a typical example of reductive desorption, a self-assembled monolayers of alkyl thiols on a gold surface can be removed by applying a negative bias to the surface resulting in reduction of the sulfur head-group. The chemical reaction for this process would be:

$R-S-Au+e^{-}\longrightarrow R-S^{-}+Au$

where R is an alkyl chain (e.g. CH₃), S is the sulfur atom of the thiol group, Au is a gold surface atom and e⁻ is an electron supplied by an external voltage source.[2]

Another application for reductive/oxidative desorption is to clean active carbon material through electrochemical regeneration.

Electron-stimulated desorption

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,.[3] 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, fewer molecules are available for desorption, and an increasing number of electrons are required to maintain constant desorption.

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References

Somorjai, Gabor A.; Li, Yimin (2010). Introduction to Surface Chemistry and Catalysis. John Wiley and Sons. Section 4.6.
Sun, K., Jiang, B., & Jiang, X. (2011). Electrochemical desorption of self-assembled monolayers and its applications in surface chemistry and cell biology. Journal of Electroanalytical Chemistry, 656(1), 223-230.
M. H. Hablanian (1997). High-Volume Technology, A Practical Guide. Second Edition. Marcel Dekker, Inc.

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[1] Somorjai, Gabor A.; Li, Yimin (2010). Introduction to Surface Chemistry and Catalysis. John Wiley and Sons. Section 4.6.

[2] Sun, K., Jiang, B., & Jiang, X. (2011). Electrochemical desorption of self-assembled monolayers and its applications in surface chemistry and cell biology. Journal of Electroanalytical Chemistry, 656(1), 223-230.

[3] M. H. Hablanian (1997). High-Volume Technology, A Practical Guide. Second Edition. Marcel Dekker, Inc.