Physical‐Adsorption Isotherm Based on a Triangular‐Site Model

A multilayer physical‐adsorption‐isotherm theory is developed; it assumes that a gas atom cannot be physically adsorbed in an upper layer unless there is a triangular array of three adsorbed atoms in the layer below forming a site for adsorption. Neighbor interactions via Lennard‐Jones intermolecular potentials are included in the calculation of the adsorption energy. The resultant isotherm is linear for low coverages, indicates a horizontal interaction correction to the Langmuir adsorption isotherm when there is only first‐layer coverage, and may yield a series of steps for sufficiently low temperature. Numerical adsorption isotherms for He on glass at 4.28° and 77°K are presented. The former isotherm exhibits distinctive steps when each layer is condensed, whereas the latter isotherm is quite smooth.