Revisiting low-pressure gas adsorption narrative in microporous materials: pore-volume effects and discontinuous transport

Gas adsorption in microporous materials is commonly described using continuum thermodynamic concepts, in which pores are assumed to contain a gas phase characterized by well-defined intensive variables such as pressure. In this work, low-pressure gas adsorption is revisited from a discrete molecular perspective by explicitly considering the number of gas molecules that may occupy individual pore volumes. Simple gas molecular counting based on the ideal gas approximation shows that, at ambient temperature and pressures below approximately 1 MPa, micropores with volumes on the order of a few cubic nanometers cannot sustain a stable population of free gas molecules. In this regime, micropores operate in a quasi-vacuum state characterized by vanishing gas molecular occupancy, and adsorption equilibrium cannot be interpreted as the coexistence of adsorbed and vapor phases within the pores. Instead, equilibrium is established between the adsorbed phase and the external gas reservoir located in macropores and in the surrounding fluid. Molecular transport from the gas phase to adsorption sites occurs through discrete transfer events, described here as a discontinuous transport or flight mechanism, rather than through conventional gas diffusion within micropores. These remarks do not affect the result of the adsorption equilibrium as a function of surrounding pressure but they induce a new vision and affect the narrative of the gaseous presence in the pores. Therefore, it requires revisiting the conventional narrative of adsorption in microporous materials.