In the course of the adsorption process, condensation causes the formation of further layers of molecules on top of the monomolecular layer created by chemical adsorption. This condensation is a purely physical process. It is the effect of two significant forces which result from the bipolar character of the water molecule: Van-der-Waal forces and the surface energy of the water. On the strength of this knowledge Brunauer, Emmet and Teller (BET) arrived at an extension and thus a more generally applicable version of Langmuir’s theory.
Formula 6.3.3.1
In Formula 6.3.3.1, p o is the saturation vapour pressure and C a constant. BET equation takes into account the formation of the monomolecular layer and that of all adsorption layers formed on top of this, using the simplified assumption that the heat of adsorption corresponds to the heat of condensation from the second layer onward.
