Statistical theory of the adsorption of double molecules

Abstract

The adsorption of gaseous atoms upon a solid surface where the sites of accommodation are regularly arranged has been studied extensively by statistical methods, first without interactions between the adsorbed atoms (Fowler 1935) and later with the interactions properly included. The first approach to include the interactions is made by Fowler, using the Bragg and Williams’s approximation (Fowler 1936), and subsequently by Peierls, using Bethe’s approximation (Peierls 1936), both methods having been first employed to investigate the order-disorder transitions in alloys. The main results of these calculations are the same. When the forces between the adsorbed atoms are attractive, there are two different types of isotherms separated by a certain temperature called the critical temperature. For isotherms with lower temperature there exists a range of pressure where several values of the fraction of adsorption 6 may correspond to one value of p. Evidently, all these values correspond to stationary values of the free energy, but only one gives the absolute lowest free energy. So when the point for the lowest free energy shifts from one branch ∅=∅ 1 (p) to another branch ∅=∅ 2 >(p) the measured ∅ will have a discontinuity.