UNIFORM INTERPRETATION OF GAS-SOLID ADSORPTION

A new form of Gibbs equation includes the differential function psi(Theta) having the form psi(Theta) = psi(P-r) = (Theta/p(r))(dp(r)/d Theta) where p(r) is the relative equilibrium pressure and Theta is the coverage. This relationship is the physical and mathematical base for a uniform interpretation (UI) of gas/solid physical adsorption. The function psi(Theta) is the relative free energy of the surface, therefore, it can be applied to define the thermodynamical correctness of adsorption isotherm equations. Two conditions of correctness have been defined. According to these conditions the change in the free energy of the surface must have finite values at Theta --> 0 and at p(r) --> 1. It has been proven that most of the known and widely-used isotherm equations are, from thermodynamic standpoint, incorrect. The first aim of UI is, with help of the differential equation psi(Theta), to transform the incorrect relationships into correct ones. The second aim is to derive new and correct isotherm equations applicable for practical and theoretical purposes. The third practical applicability of UI is that it provides an exact method to choose the appropriate and correct isotherm equation for describing measured isotherms. The most important role of UI is in the calculations of the specific surface area (a(s)) from measured isotherms of Type I and II. The UI modifies our old models (Langmuir, Fowler-Guggenheim, Sips, Jovanovich, Volmer, deBoer-Hill, BET etc.) applicable to calculate the specific surface area. This modification has two important consequences. The first one means that in the isotherm equations a new parameter, chi, appears and it has great influence on the value of a(s). The second one means that with help of function psi(p(r)) one can distinguish the monolayer and multilayer domains in isotherms of Type I and II. Furthermore, the uniform interpretation needs a new model, so-called ''cloud model'', for interpretation of multilayer adsorption. This model has important theoretical and practical differences in comparison to the well-known BET theory and also provides new possibility to exact calculations of a(s). Based on the UI it is possible to calculate the mixed gas adsorption from the pure isotherms measured on heterogeneous surfaces and to qualify industrial adsorbents for practical use. As a consequence of UI it was possible to define the theorem of ''referable states'' of two dimensional layers (gases) wich corresponds to the ''corresponding states'' of three-dimensional gases. Shortly summarized: the uniform interpretation proposed in this review modifies our old views of classic models of physical adsorption.