Gas physisorption studied with density functional theory: hysteresis and the Kelvin equation

We study nitrogen adsorption on oxidic surfaces within the framework of density functional theory. On the basis of a simple free energy functional, with which we calculate nitrogen density profiles and adsorption isotherms, we argue that some experimental hysteresis effects can directly be related to metastable phases: the jumps in the adsorption occur at the spinodal of the metastable branches. For sufficiently large slits or cylinders this occurs at a “universal” relative pressure of 0:42 in the desorption branch. The agreement of the classical Kelvin theory, used to describe hysteresis by the effect of curved menisci, with our present approach is shown to be approximate only