The Chemistry of Adsorption

It is possible to define two kinds of adsorption: physical adsorption (or physisorption) when the adsorbate adheres to the surface because of physical forces, and chemical adsorption (or chemisorption) if the adsorbate is chemically bound to the adsorbent's surface.

Physisorption occurs thanks to van der Waals interactions: these are attractive forces due to weak electrostatic interactions occurring between molecules. If the adsorbate molecules hit the surface with low energy, this can be dissipated as heat by vibration of the lattice of solid; hence, they can be trapped on the surface. If the molecules hit the surface with too much energy, this cannot be dissipated by the adsorbent, and they bounce away.

The change of enthalpy is so low, typically less than 20kJmol—\ that both, adsorbate and adsorbent, do not undergo any change in their chemical status: no new bounds are formed and no change in the energetic state is observed, a part of a low heating of the adsorbent. Indeed, the physical bound is highly labile, and the process can easily revert even simply as a consequence of vibrational motion. It has been estimated that at typical environmental temperature (about 20 °C), lifetime for physisorption is about 10—8s, and only with very low temperature (about —170 °C) lifetime is of the order of seconds. For this reason, physisorption is a relevant process only at low temperature, always under the critical temperature of the adsorbate. Given the nature of the forces that cause physisorption, this is a process that can occur in multilayer: as long as the adsorbed molecules are not able to completely shade the electrostatic potential, new adsorbate can adhere to the adsorbent's surface, even if already covered by other molecules. Of course, the enthalpy required to desorb these new molecules is lower; then the strength of adsorption (and its lifetime) decreases with the increasing of the number of layers of adsorbate already stuck to the surface.

Contrary to physisorption, chemisorption involves stronger forces: indeed in this case adsorbate forms a real chemical bond (usually covalent) with the adsorbent's surface. The change in enthalpy is greater (from 40 to 400kJmol—1, always negative as adsorption is a spontaneous process). Chemisorption can be an activated process, that is, it requires that the adsorbate has a minimum of energy in order to be sorbed. This depends on the presence of an energetic barrier between the physisorbed and chemisorbed state (Figure 1): if this barrier is higher than the energy of the free molecules, then the adsorbate will chemically bond to the adsorbent only if it has more energy than the barrier, otherwise it will be desorbed. In case the barrier is lower than the energy of free molecules, all the molecules physisorbed can quickly form a chemical bond with the adsorbent surface and chemisorption will occur rapidly.

Due to higher enthalpy involved, vibrational motions are not able to break the covalent bonds; hence, lifetime at environmental temperature (20 ° C) is of the order of thousands of seconds (i.e., 1 h), and only at high temperature (100 °C) it decreases to 1 s.

Desorption from chemisorbed state is then always an activated process: indeed, to detach from the surface, molecules need to be supplied with energy to break the bonds. Only a monolayer ofadsorbate can be chemisorbed to adsorbent surface: once if the whole surface is covered by adsorbate's molecules, so that no more sites are available to the bond, then adsorption ends, or yet better, it is at dynamic equilibrium with desorption.

Figure 1 Diagram of energy of adsorption depending on distance (d) of adsorbate molecule from surface. Solid line is for physisorption, involving low enthalpy (AHp), dashed for chemisorption, where the energy required is higher (AHc). On the left is the case of activated chemisorption: Ea,c is the activation energy for chemisorption.

Chemisorption is prepatory to other two important processes: heterogeneous catalysis and ion exchange. Both processes, indeed, need that the substances involved are strictly bound in order to start and complete: catalysts require that substrate is strictly bound on their surface in order to foster the reaction, and the same is true for ion exchanger, which requires a chemical bond with the solute to exchange the ion.

As any other chemical reaction, adsorption is influenced by the physical and chemical conditions of the environment, and in particular by temperature, pH, and redox conditions. Being a surface process, a key factor for adsorption is the surface of the adsorbent: obviously adsorbent with more surface has more capacity to adsorb substances. For this reason, the best adsorbents are porous substances, or more generally the ones with the greater surface per unit volume (e.g., activated carbon and clay).

Equations for Adsorption

As discussed in the previous section, adsorption usually involves low enthalpy and has a short lifetime. This means that adsorption and desorption have low activation energy, and then the amount of molecules adsorbed/ desorbed from the adsorbent is very high: equilibrium is reached fast, if physicochemical environmental conditions do not change.

Hence, adsorption is usually described through a relationship independent from time between the amount of adsorbate attached to the adsorbent and the amount in the environment (expressed as a pressure if it is a gas, a concentration if it dissolved in a solution). In case of dissolved adsorbate,

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