Classification of Gibbs adsorption isotherms

The current IUPAC classification of gas–solid adsorption isotherms has two deficiencies: it is incomplete and it gives the incorrect impression that adsorption isotherms are always monotonically increasing functions of pressure. However, there are many isotherms that are not monotonic and show maxima. The reason for this deficiency is that the IUPAC classification is based conceptually on ideas about the `absolute' adsorption rather than on the Gibbs adsorption. It is shown here that supercritical gases show adsorption isotherms which are fundamentally different from those in the IUPAC classification. There are experimental data for hydrocarbons, inert gases, nitrogen, carbon oxide, carbon dioxide, nitrogen oxides, and others on microporous and macroporous adsorbents for supercritical temperatures showing nonmonotonic isotherms with maxima. It also is shown that the Ono–Kondo lattice model is able to predict all known types of adsorption behavior. By changing two energetic parameters (energies for adsorbate–adsorbate and adsorbate–adsorbent interactions), one can obtain smooth or stepped multilayer adsorption isotherms for macroporous adsorbents. It also is possible to describe adsorption on microporous adsorbents by imposing the appropriate boundary conditions. The Ono–Kondo model also predicts steps in isotherms similar to those observed in high resolution measurements for nitrogen and oxigen on zeolites. The monolayer version of the Ono–Kondo theory gives all known types of monolayer adsorption isotherms.