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