General cluster sorption isotherm: surface area determination

The recently published General Cluster Sorption Isotherm is modified by separating forces lateral and vertical directed to the surface. Calculating the ratio of the infinitesimal contributions of adsorption in both directions as a function of the total adsorption results in an inflection point which is assigned to the position of the monolayer coverage. The method is applied to eight experimental high resolution type II sorption isotherms. The parameters are compared with those of the previous GCSI version and the surface areas are discussed together with those obtained by BET and the excess surface work method (ESW). For N₂ and Ar sorption, the BET method is found to overestimate the actual surface area. Since the GCSI reduces to BET when the sorbate-surface interaction is strong enough, the GCSI derived surface area includes the BET surface as a special case. The application of GCSI for surface area determination requires a sufficiently broad range of experimental data points.