Adsorption and Adsorption Complexes

Adsorption is a process of attachment of molecules from gas or liquid phase to surfaces. Molecules residing in gas or liquid phase often have a tendency to stick to surfaces of solid materials or interfaces between liquid and gas phase. Adsorption can be negative and positive. Negative adsorption means that molecules are repelled from the surface and their concentration is lower near the surface, while positive adsorption signifies increase in the concentration near the surface. Adsorption of molecules from gas phase on solid materials surfaces can result in formation of a monolayer and polylayers of molecules.

The process of monolayer adsorption is described by the following equation. 

M + Z ⇄ MZ, 

where M is molecule in gas or liquid phase, Z is an adsorption site on the surface, MZ is an adsorption complex. 

The process of adsorption is characterized by usual chemical thermodynamics quantities - energy of adsorption ΔE_ads, Gibbs free energy of adsorption ΔG_ads, enthalpy of adsorption  ΔH_ads. 

ΔH_ads = Δ_fH(MZ) - Δ_fH(Z) - Δ_fH(M), 

where Δ_fH(MZ) is the enthalpy of formation of the adsorption complex, Δ_fH(Z) is the enthalpy of formation of adsorbent, Δ_fH(M) is the enthalpy of formation of adsorptive.   

The molecules adsorbed can be bonded by weak Van der Waals forces or by stronger chemical interactions. In the first case, the process of adsorption is called physical adsorption or physisorption (ΔH_ads > -10 kJ/mol). In the second case, the process of adsorption is called chemical adsorption, or chemisorption (ΔH_ads < -10 kJ/mol). The enthalpy of adsorption depends on the size of molecules. Large molecules can have ΔH_ads < -10 kJ/mol even if they are bonded by weak van der Waals forces typical for physical adsorption. 

Chemical adsorption results in formation of surface entities with definite chemical structure. Such entities are called adsorption complexes. 

The process of adsorption can be carried out under conditions of constant temperature or constant pressure. The amount of the compound adsorbed depends on its concentration in the gas or liquid phase. The dependence of the amount of compound adsorbed on its concentration under isothermal conditions is called isotherm of adsorption. 

There are several different types of adsorption isotherms.  

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