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An important assumption of Langmuir theory is the formation of a monolayer. Because of monolayer formation a saturation in adsorption would reach at higher equilibrium concentration of the adsorbate. This type of adsorption occurs due to short range chemical forces which do not allow penetration through the primary adsorbed molecules. Multilayer formation has been observed when molecules are adsorbed through weak forces (long range forces, normally under physical adsorption) due to cohesive forces exerted by the molecules of the adsorbate.
At high pressure multilayer adsorption takes place. The theory of multimolecular (multilayer) adsorption was developed by Stephen Brunauer, Paul Emmet and Edward Teller and is called BET isotherm. This isotherm derived by them successfully accounts for all types of adsorption.
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It assumes that the surface possess uniform, localised sites and that adsorption on one site does not affect the adsorption on neighbouring sites just as in the Langmuir theory. Also, molecules can be adsorbed in second, third,… and nth layers with the surface available for layer equal to the coverage of the next lower layer.
The rate constants for adsorption and desorption of the primary layer are ka and kd and those of the subsequent layers are all ka' and kd'. The number of sites corresponding to zero, monolayer, bilayer, … coverage at any stage are N0, N1, N2, etc. and Ni in general. The condition for equilibrium of the initial layer is the equality of the rates of its formation and desorption,
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The condition for equilibrium of the next layer is |
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This condition may be expressed in terms of N0 as follows: |
and 
, then 
