The equation of vapor-phase sorption on heterogeneous surfaces with local Guggenheim–Anderson–De Boer model

Recently, we proposed a new four-parametric MGAB equation (modified Guggenheim–Anderson–De Boer equation, GAB) for fitting of experimental isotherms of vapor-phase sorption on geosorbents. In the present study, the theoretical meaning of MGAB equation was elucidated. For this purpose, the properties of the equation of multilayer adsorption on a heterogeneous surface with local characteristics described by GAB model and Zeldovitsh–Roginskiy energy distribution function ρ(ɛ) were derived and the satisfaction of MGAB equation with these properties was shown. MGAB model has two parameters connected with GAB model: v m is the monolayer sorption capacity and k is the constant of the multilayer sorption, as well as two parameters of sorption energy distribution function ρ(ɛ): α, characterizing heterogeneity of ρ(ɛ) and C0, reflecting energy baseline. The influence of parameters α and C0 on the shape of both ρ(ɛ) and model sorption isotherm was established. The applicability of MGAB model for the description of surface heterogeneity of a natural sorbent by example of 13 different sorbate–sorbent systems dried and moistened has been discussed. For these systems equilibrium sorption isotherms were measured, corresponding energy distribution functions ρ(ɛ) were calculated, and obtained α values were compared. On the whole, the differences in α values responded to the real tendencies of heterogeneity changes in these systems.