Calorimetric evidence for a bidimensional phase change in the monolayer of nitrogen or argon adsorbed on graphite at 77 K

Abstract

Nitrogen adsorption on a Sterling MT 3100 graphitized carbon black is followed by isothermal microcalorimetry at 77 K. A special feature of the curve of isosteric heats is a distinct peak recorded for a coverage close to 1. Simultaneously, a sub-step is detected on the adsorption isotherm. Both effects are assigned to a bidimensional phase transition of the monolayer, changing from an hypercritical fluid state to a localized state. A similar phenomenon is observed in the case of argon adsorption: here again, a peak on the calorimetric curve and an inflexion on the isotherm are interpreted by a bidimensional fluid → solid transition; nevertheless, in the latter state, argon is not localized at graphite lattice sites.