Separation of the first adsorbed layer from others and calculation of the BET compatible surface area from type II isotherms

In the previous paper it has been proven that a BET compatible specific surface area, a(c)/(s)(N₂, 77), can be calculated from any Type I isotherm measured below the critical temperature. In this paper it is proven that the same calculation can be performed from any Type II isotherms if the isotherm has a pure monolayer domain. In order to distinguish the mono- and multilayer adsorption the relative free energy of the surface as a function of the adsorbed amount, π(r)(n(s)), and the functions ψ(p(r)) and ψ(n(s)) are applied, both defined by the differential expression (n(s)/p(r))(dp(r)/dn(s)). When the multilayer adsorption becomes the dominant process then the function π(r)(n(s)) has a point of inflexion and functions ψ(p(r)) and ψ(n(s)) have maximum values. It has been demonstrated that in most of the Type II isotherms the mono- and multilayer domains can be separated, so the monolayer component isotherm can be calculated by the T (Toth) equation. Therefore, it is possible to calculate the BET compatible specific surface area discussed in detail in the previous paper. It has also been proven that there are Type II isotherms which describe only multilayer adsorption; i.e., the functions ψ(p(r)) and ψ(n(s)) do not have maximum values. In these cases the Harkins-Jura equation should be applied.