Some criticism of the BET theory

In spite of the success of the BET theory, some of the assumptions upon which it is founded are not above criticism. One questionable assumption is that of an energetically homogeneous surface, that is, all the adsorption sites are energetically identical. Further, the BET model ignores the influence of lateral adsorbate interactions. 

The dotted lines in Fig. 4.3 indicate the influence of very high adsorption potentials which can account, at least in part, for the failure of BET plots at very low relative pressures (P/Pq 0.05). 

A further criticism of the BET theory is the assumption that the heat of adsorption of the second and higher layers is equal to the heat of liquefaction. It seems reasonable to expect that polarization forces would induce a higher heat of adsorption in the second layer than in the third, and so forth. Only after several layers are adsorbed should the heat of adsorption equal the heat of liquefaction. It is, therefore, difficult to resolve a model of molecules adsorbed in stacks while postulating that all layers above the first are thermodynamically a true liquid structure. The apparent validity of these criticisms contributes to the failure of the BET equation at high relative pressures (P/Pq 0.35). However, in the range of relative pressure leading to coverage near W/ = 1, the BET C values 

Instances have been found in which BET plots were noted to be linear to relative pressures as high as 0.5, and in other cases the linear range is found only below relative pressures of 0.1.The extent to which these deviations from the usual range of linearity reflect unusual surface properties is difficult to ascertain. For example, micropores exhibit unusually high adsorption potentials due to the overlapping potential from the walls of the micropore. Under these enhanced conditions condensation can occur within the pores at relative pressures less than 0.1, and linear BET plots are found at even lower relative pressures. 

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