Wetting of graphite (0001) by carbon monoxide. A stepwise adsorption isotherm study

Abstract

Sets of stepwise adsorption isotherms of carbon monoxide on exfoliated graphite have been measured volumetrically for coverages ranging from one to five monolayers, at temperatures surrounding the transition temperature between the α and β forms of the bulk crystalline adsorbate. From the analysis of the phase diagram representing the condensation pressures vs. temperature of both the successive monolayers and the bulk crystal phases, it is concluded that the basal face of graphite is incompletely wet by the low-temperature form α-CO and perfectly wet by the high temperature form β-CO. Since the molecules are orientationally ordered in α-CO and disordered in β-CO (plastic phase), this investigation supports the point of view, suggested by previous studies, that orientational order within a crystal phase is generally an obstacle to its spreading over a plane surface. The previous observations of changes in wetting regimes at triple points are briefly discussed in parallel with the present results.