Theoretical Isotherms for Physical Adsorption at Pressures below 10−10 Torr

Theoretical isotherms for the physical adsorption of argon, nitrogen, and helium on a heterogeneous surface in the pressure range from ${10}^{\text{−10}}$ Torr down to pressures indefinitely low are presented. The calculations are based on adding a single assumption to a model that was successful in correlating experimental data for argon, nitrogen, and helium over a wide range of adsorbents, temperatures and pressures above ${10}^{\text{−10}}$ Torr. The results are compared with an alternative model that postulates a homogeneous surface and are found to be significantly different. It is concluded that experiments should be possible to establish which, if either, of these approaches is useful.