Adsorption and Gas-Particle Partitioning of Organic Compounds

Most particles can adsorb vapor molecules on their surfaces. The interactions between adsorbed molecules and the particle surface are complex, involving both physical and chemical forces. The physics and chemistry of adsorption are quite complicated, and we refer the interested reader to Masel (1996) for detailed coverage of the subject. 

The adsorption process involves first the partial covering of the particle surface with vapor molecules, leading to the formation of a monolayer, occasionally followed by the formation of additional layers. Mono- and multilayer adsorption processes often have different characteristics, as in the former the adsorbed molecules interact directly with the particle surface, whereas in the latter they interact mainly with already adsorbed molecules. 

The adsorption behavior of a surface is generally characterized by an adsorption isotherm, that is, the functional dependence of the amount of gas adsorbed on the gas partial pressure at constant temperature. The frequently used Langmuir isotherm describes the equilibrium adsorption for the formation of a monolayer it is based on the following assumptions  

There are no horizontal interactions among adsorbed molecules. 

One of the most widely used isotherms is based on the BET theory, named from the initials of Brunauer, Emmett, and Teller. The BET theory is an extension of the Langmuir isotherm to include the adsorption of two or more molecular layers assuming, in addition to assumptions 1-3 listed above, that 

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