Résumé
Understanding of the adsorption process is crucial due to many possible applications such as energystorage or separation. One approach to describe the mechanism of the adsorption is to analyze the equilibrium data(configurations and the energy of interactions) of the appropriate adsorption isotherm type. This allows one to estimatethe specific role of host-guest and guest-guest interactions. Walton et. al. [1] reported a step-like, non-continuousexperimental adsorption isotherms for CO2 in IRMOF-1 (MOF-5). Such a behavior is very unusual for adsorption in othernanoporous materials under similar thermodynamic conditions. We use this example to fully and fundamentally explainthe low-temperature adsorption process of small molecules within the pores of crystalline materials (such as metalorganicframeworks). Based on the Grand Canonical Monte Carlo adsorption simulations we show that the isothermdiscontinuity is an evidence of a structural transformation of the guest which is a result of competition between host-guestand guest-guest interactions in highly symmetrical confinement. Additionally, we prove that even if the measuredisotherm has quasi-continuous form, the adsorption mechanism can be discrete. We propose that the IUPACclassification of isotherms could be supplemented with a new type which corresponds to the adsorption in crystallinepores.