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
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