Abstract

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 A). 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 ofthe 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 A at temperatures above 77.4 K cannot be explained by the capillary critical temperature for a given pore size being achieved as was assumed previousl

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Communications ADVANCED MATERIALSdesiccator under vacuum for 20 min to remove air bubbles trapped in the liquid prepolymer, When features on the surface of the shrunken film were deep, many small air bubbles could not escape but settled down at the interface between the liquid prepolymer and the solid shrunken film. The air bubbles were thus replicated into the replica after the prepolymer was solidified (e.g. the surface texture shown in Fig. 2b-d). We used a UV-curable polyurethane (NOA73, Norland Products, New Brunswick, NJ) and a heatcurable epoxy (F114, Tra-Con, Medford. MA) to replicate the surface of shrunken films. The prepolymcr of NOA73 was irradiated with a UV light (Type 7825-34, Canrdd-Hanovia 450 W medium-pressure Hg vapor lamp, Ace Glass, Tnc., Vineland. NJ) for 45 min, with samples placed at a distance of -1-2 cm from the lamp. The precursor of F114 was cross-linked at 65 "C for 4 h.

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