Characterization of microporous non-supported membrane top-layers using physisorption techniques

Gas physisorption is a generally used technique for the characterization of porous solids. Microporous solids, however, need a different approach compared to mesoporous solids due to enhanced adsorption in pores of molecular dimensions. Therefore, conventional theories and models can not be used. The application of physisorption for microporous solids, and the interpretation of the isotherms is elucidated. Some models presented in literature for pore size assessment in the micropore range are discussed. Based on a number of criteria, the Horváth-Kawazoe model, modified for cylindrical pore geometry, is selected. N₂ adsorption measurments on zeolites were performed to calibrate the method and to show the applicability and physical justification. Typical data obtained from experiments on non-supported microporous silica membrane top-layers show that the pore size distribution of these materials is bimodal, with a strong maximum at an effective diameter of 0.5 nm, and a weaker maximum at 0.75 nm.