Hybrid isotherms for adsorption and capillary condensation of N2 at 77 K on porous and non-porous materials

A simple hybrid isotherm, which is an addition of Frenkel–Halsey–Hill isotherm to the Redlich–Peterson (RPFHH) isotherm, is proposed, taking into account the theoretical limitations of these models. The hybrid isotherm was fitted to N2 adsorption isotherm (at 77 K) on porous and non-porous materials that involve only an adsorption step or an additional step due to condensation of the gas molecules on large pores. The RPFHH isotherm was found to be the successful in representing the N2 adsorption isotherm of any type of porous and non-porous materials over the whole range of relative pressure range. The RPFHH isotherm can be adjusted to identify the adsorption process besides the condensation process, allowing to determine the theoretical sorption capacity. The surface area of the materials predicted by the RPFHH was in general in good agreement with the values obtained by the BET method.