Capillary condensation of nitrogen in MCM-48 and SBA-16

To examine the effects of connectivity among pores of almost the same size and of different size and geometry on adsorption hysteresis and pore criticality, we measured the temperature dependence of the adsorption-desorption isotherm of nitrogen onto MCM-48 and SBA-16 materials, respectively, with well-defined three-dimensional networks of cylindrical pores and cagelike pores, over a wide temperature range of T/T-c = 0.43 to 0.90. The results for MCM-48 strongly suggest that interconnections among pores of almost the same size and geometry do not have a significant effect on the adsorption hysteresis and pore criticality. For SBA-16 with well-defined ink-bottle pores, capillary condensation in the hysteretic isotherms takes place near the equilibrium, whereas capillary evaporation from large cavities is delayed. The classical concept of pore blocking is not supported by the experimental isotherms for SBA-16, because the delayed desorption is not concerned with emptying of the small channels. The hysteresis shrinks with increasing temperature and eventually disappears well below the bulk critical temperature, in disagreement with the classical concept of pore blocking.