Pattern of adsorption isotherms in Ono–Kondo coordinates

Abstract

The Ono–Kondo lattice density functional theory is used to analyze adsorbate–adsorbate interactions for supercritical systems. In prior work, this approach has been used to study intermolecular interactions in subcritical adsorbed phases, and this has included the study of adsorbate–adsorbate repulsions in the regime of adsorption compression. In this paper, we present the general pattern of adsorption isotherms in Ono–Kondo coordinates; this has not been done in the past. For this purpose, experimental isotherms for adsorption of supercritical fluids (including nitrogen, methane, and carbon dioxide) are plotted in Ono–Kondo coordinates. In addition, we performed Grand Canonical Monte Carlo simulations of adsorption for Lennard-Jones molecules and plotted isotherms in Ono–Kondo coordinates. Our results indicate a pattern of isotherms with regimes of adsorbate–adsorbate attractions at low surface coverage and adsorbate–adsorbate repulsions at high surface coverage. When the generalized Ono–Kondo model is used over a wide range of pressures – from low pressures of the Henry’s law regime to supercritical pressures – the slope of the isotherm varies from positive at low pressures to negative at high pressures. The linear sections of these graphs show when the adsorbate–adsorbate interaction energies are approximately constant. When these linear sections have negative slopes, it indicates that the system is in a state of adsorption compression.