The Hill-Everett model

Unlike the previous model, where the surface acts through the surface n of its species. Hill and Everett describe surface effects using a physical parameter, namely the force field of the sohd. 

In effect, this model considers that adsorption can be described as a localized condensation process that tends to progressively cover the entire surface when the gas pressure increases. A fluctuation in the fraction of covered surface necessarily induces a fluctuation in the energy of the contact between the two phases. This energy term is the product of an extensive quantity As, which is the contact surface between the two phases, with an intensive quantity Ps, which is comparable with a surface pressure. Ps is identified with the variation of the surface tension coefficient y during the covering, that is to say  

In this case, there are 3 exterior parameters pressure, temperature and surface pressure. Assuming that there is only one independent component, the variance is given by  

There is in effect only one component a solid + a gas - an equilibrium relation. 

At P = Po (saturation vapor pressure), we will assume that the entire surface is covered and that there is no longer any change in the solid s force field the conditions we now have are those of a simple condensation, for which v=l+2-2=l. 

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Thermo(fynamics of adsorption equilibrium in the Hill-Everett model... 

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