Thermodynamics of gas adsorption on fractal surfaces of heterogeneous microporous solids

Thermodynamic equations are derived for physical adsorption of gases and vapors on fractal surfaces of heterogeneous microporous solids. The influence of surface geometry on gas adsorption on a microporous solid is studied by analyzing the dependence of the differential molar enthalpy ΔH, the immersion enthalpy ΔH_im, and the differential molar entropy ΔS on the fractal dimension D, which is used to characterize the surface irregularity. It is shown that - ΔH, - ΔH_im, - ΔS, and the average adsorption potential Ā increase as the fractal dimension of the surface accessible for adsorption increases. The dependence of these thermodynamic quantities on the fractal dimension D is attributed to the fact that the fraction of small micropores in microporous solids increases as the fractal dimension D increases.