Capillary hysteresis in nanopores: Theoretical and experimental studies of nitrogen adsorption on MCM-41

Capillary hysteresis in cylindrical nanopores has been studied using MCM-41 as the prime example of a mesoporous material. These materials, due to their regular pore structure, can be considered to be candidates for reference adsorbents for standardizing adsorption measurements and methods for characterization of porous solids. They provide a unique opportunity for verification of theoretical models employed for predicting phase equilibrium in confined geometry. Three samples with monodisperse pore channels have been synthesized and examined using X-ray diffraction (XRD). Nitrogen adsorption isotherms were modeled using nonlocal density functional theory (NLDFT) in a wide range of pore sizes (18-80 Angstrom). Theoretical isotherms for pore channels with sizes corresponding to those identified by XRD were compared with experimental isotherms at different temperatures between 70 and 82 K. The latter have been measured independently on two different adsorption setups. The theoretical thermal dependence of the thermodynamic adsorption-desorption hysteresis predicted by NLDFT is confirmed by the experimental measurements. It is shown that at 77.4 K NLDFT quantitatively predicts equilibrium phase transitions in cylindrical channels of MCM-41. Theoretical and experimental results prove that the nitrogen hysteresis observed at temperatures below 77.4 K is associated with metastability of the adsorption branch of the isotherm. The absence of experimental hysteresis on samples with pore size of about 40 Angstrom at temperatures above 77.4 K cannot be explained by the capillary critical temperature for a given pore size being achieved as was assumed previously.