Adsorption de krypton sur Papyex dans le domaine de la monocouche

Among the different samples used for studying physisorbed films on graphitic surfaces, compressed exfoliated graphite, such as Papyex or Grafoil, plays an outstanding role, as it is particularly suitable for scattering experiments. But the compression performed to prepare this kind of sample has been shown to increase the heterogeneity of the surface, which results, as was observed in some cases 6,7,9] in an alteration of the shapes of isotherms, and a slowing down of the adsorption kinetics, with respect to more homogeneous graphitic surfaces.In this work, thermodynamical data characteristic for a physisorbed monolayer of krypton on Papyex have been determined from volumetric measurements performed either at constant temperature or at constant fractional coverage and compared to results obtained on more homogeneous surfaces.The isotherm set between 78 and 90 K is given in Figs. 2 and 3. It is on the whole compatible with the phase diagrams obtained on more homogeneous graphites (Fig. 1). But a slight smoothing of the steps, as well as a slowing down of the adsorption kinetics, which appears mainly for low temperatures and solid two-dimensional (2D) films, are observed as they were before 6, 7,9].Consistency between these results and those obtained by measurements at constant fractional coverage enables us to assume that adsorption equilibrium is reached in the temperature range common to both kinds of experiments.The constants A and B of the equation logP = −(A/T) + B, representing the dependence of the equilibrium pressure on temperature at constant fractional coverage, had been determined from volumetric measurement results, and also compared to values previously obtained in similar conditions on more homogeneous substrates, for different stages of the film formation (Table 1). Values for both types of samples are in good agreement, except for the 2D gas-solid transition (θ = 0.4 − T < 85 K), as could be foreseen from the shapes of isotherms in this domain, namely the bending of the linear part towards higher pressures than on more homogeneous graphitic substrates.The 2D triple point and critical temperatures with very similar values have been proposed before, which were determined in using graphite SPl type10], while from other results it has been concluded that these two characteristic temperatures actually do not exist11]. Owing to the lack of precision of our determinations the existence or the non existence of a 2D liquid domain cannot be asserted.As a conclusion, the main features of the krypton phase diagram being preserved, it can be assumed that the structural properties of the films determined on Papyex are, on the whole, characteristic of adsorption on a uniform surface.

Résumé

Les isothermes d'adsorption de krypton sur Papyex sont déterminées par volumétric dans le domaine de la monocouche, et pour des températures comprises entre 78 et 90, 2 K. Les données thermodynamiques que l'on peut déduire de ces courbes sont comparées à celles obtenues dans des conditions similaires sur des substrats graphitiques plus homogènes. Malgré quelques différences observées concernant la forme des isothermes et la cinétique de l'adsorption, les principales caractéristiques du diagramme de phase du film sont conservées.