The different models of adsorption

In order to build a thermodynamic model of physical adsorption, it is important to take note of a few experimental results. The adsorption isotherms acquired through the volumetric or the gravimetric method allow us to make sure that the 

It is important to note that the adsorption process can in no case whatsoever be identified with a simple condensation process. Indeed, condensation is a monovariant process that owes its origin to gas saturation. Saturation is achieved at a pressure P greater than or equal to the saturation vapor pressure Po, which varies with temperature only. On the other hand, adsorption is observed at gas pressure values that are lower than Pq. 

The various thermodynamic models that have been proposed are grounded on such considerations. 

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Fig. 7. The relationships between the different models of adsorption. The various assumptions involved in the classical Langmuir treatment can be considered explicitly in other, more complex models (see Ref. 451 for further details)...

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