Adsorption Isotherm and the Distribution Coefficient

The flux qa that combines both the molecular diffusion and mechanical dispersion can be written as 

5 Classical Theory of Diffusion and Seepage Problems in Porous Media 

Note 5.5 (Adsorption, absorption and sorption). The process where a concentration of molecules or ions at an interface between a solid and a fluid becomes incorporated within the solid is referred to as adsorption. Note that at the interface of an electrolyte aqueous solution and a solid, the concentration may become dilute. We refer to this as a negative adsorption. When the adsorbed molecules or ions on the surface can be detached due to changes in the chemical conditions, desorption occurs. 

On a solid-gas interface, we may observe that the gas molecules diffuse into the inner part of the solid. This is referred to as absorption. If both adsorption and absorption take place simultaneously, this process is referred to as sorption.  

Note 5.6 (Van der Waals force). Even for nonpolar molecules, the polar character is generated due to instantaneous deviations in the electron orbit. Because of this electric fleld, the neighboring molecules become polarized, and the energy level of the total system becomes lower if the force is attractive rather than repulsive. Frozen carbon dioxide and crystals of iodine h are examples of crystals formed by van der Waals forces, which are known as molecular crystals. Since the van der Waals forces have no orientation, the molecular crystals occur in a closely-packed structure. The van der Waals force V(r) is inversely proportional to the sixth power of the intermolecular distance r V(r) = —C/r . The van der Waals force is extremely small compared to chemical forces such as ionic bonds, covalent bonds and metallic bonds (i.e., less than 1/100).  

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