Theory of monolayer physical adsorption. II. Fluid phases on a periodic surface

The submonolayer physical adsorption of a simple gas on an exposed single crystal face of an inert solid is treated. A perturbation theory is developed to treat the effect of periodic terms in the gas–solid energy. Monte Carlo computations of the properties of two-dimensional Lennard-Jones and hard-disc fluids are used as a reference system. Isotherms and heats of adsorption are evaluated for a number of values of the gas atom/surface lattice cell size ratio. It is shown that the inclusion of the periodicity has a considerable effect on these properties. In particular, the calculated adsorption isotherms suggest that there is an optimum adsorbate atom size for a given surface symmetry for which the two-dimensional critical temperature is maximized, in agreement with experimental observations.