Criticism of the BET Equation

Dependence of the Relative Pressure Corresponding to the Formation of a Statistical Monomolecular Layer on the Constant C Adsorption of Nitrogen at T = 77.4 K 

Source Emmett, P.H., in Catalysis, RH. Emmett, ed., Reinhold, NY, 1954, p. 31. With permission. 

Other groups. The heat of adsorption in a given group of sites is the same while it differs with the value from another group of surface sites. The failure of the model above 0.35 relative pressure is due to the existence of narrow micropores that limit the thickness of the adsorbed film. 

The assumption that the adsorbate has liquid-like properties after the first layer is difficult to reconcile, because both porous and nonporous (planar) adsorbents exposed to a saturated vapor sometimes adsorb strictly a limited amount and not the infinitely large quantity as postulated by BET. Another drawback of the BET model is essentially the coordination number of the molecules in the higher layers. The BET model assumes that each molecule that is adsorbed in any layer after the first layer, gives out its full latent heat of liquefaction, whether it has horizontal neighbors or not, and shall show a coordination number of 12. But in the absence of horizontal neighbors, the coordination number is much less that 12, and, therefore, the heat evolved (i.e., the heat of adsorption) should be only a fraction of the latent heat of condensation. 

the limited validity of the BET equation is due to the shortcomings in the model itself rather than to our lack of knowledge of the various parameters such as the number of layers, the variation of heat of adsorption E, or the evaporation constant b, etc. in the higher layers. 

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