Revisiting the Classification of Physisorption Isotherms with Classical Density Functional Theory

Understanding the mechanisms of gas adsorption/desorption and liquid intrusion constitutes a fundamental field that spans physics, chemistry, and engineering and leads to applied problems, such as the characterization of porous materials and porosimetry. The physisorption of gases is typically analyzed at constant temperature for various pressures, and the corresponding isotherms have been classified in standard IUPAC reports, mainly based on empirical considerations. We use classical density functional theory (DFT) to predict the microscopic structure of confined fluids and determine physisorption isotherms for gases, liquids, and supercritical fluids by considering a large range of thermodynamic conditions. The effects of temperature, pore size, and intensity of the fluid-solid interactions are systematically studied. New types of isotherms are identified, and we reinterpret several mechanisms of adsorption and desorption. We propose a new classification of physisorption isotherms organized from a fundamental perspective based on thermodynamic considerations.