Model for incipient triple point in krypton and nitrogen adsorbed on graphite

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Abstract
An anomaly which occurs in the submonolayer phase diagram of krypton and nitrogen adsorbed on graphite has been ascribed to an ‘‘incipient triple point’’ arising because the low free energy of the solid phase of the adsorbed film prevents the occurrence of a liquid phase. In this paper a detailed thermodynamic model for an incipient triple point is developed and applied to these two systems. The model employs a two-dimensional Ising lattice gas for the fluid phase and a fairly simple phenomenological expression for the free energy of the solid phase, which is assumed to be inert. The parameters are adjusted to fit the experimental heat-capacity data with the use of a method which permits a separate determination of the parameters for the fluid and solid phases. The results are in reasonably good agreement with experiment except for some systematic deviations at higher temperatures and coverages which may reflect the inadequacy of a lattice-gas free energy for the fluid. The parameters of the fluid phase, in particular the metastable critical temperature, are physically reasonable and do not seem to be seriously affected by the influence of substrate imperfections on the experimental results. As the thermodynamic model is a model of the free energy, all thermodynamic properties can be computed once the model parameters are chosen. The changes in the phase diagram and in the specific heat as the parameters are varied, are considered in a simple case in order to explore what happens as a triple point changes into an incipient triple point at a critical endpoint.