To know the accurate relationship between pore size and the pressure of capillary condensation or evaporation and also to elucidate the pore-connectivity effects on adsorption hysteresis and pore criticality for the ordered mesoporous silicas (MCM-48 and KIT-6) with bicontinuous gyroid structure, we carried out X-ray structural study of MCM-48 and KIT-6 and measurements of the adsorption-desorption isotherms of nitrogen in comparison with MCM-41 and SBA-15 with unconnected cylindrical pores. The results clearly show that the hydraulic diameter describes incorrectly the mean pore size of these materials expressing the strength of confinement under the assumption of cylindrical pores with no networking effects. In addition, the average pore diameter at the middle point of pore intersections still underestimates the strength of confinement compared to the unconnected cylindrical pores of MCM-41 and SBA-15. Despite a large difference in porous structure, the shape and thermal behavior of the adsorption hysteresis for KIT-6, as well as the sorption scanning behavior, were indistinguishable from those for SBA-15, indicating that interconnections among pores of almost the same size do not have a significant effect on the adsorption hysteresis and pore criticality.

To study the freezing/melting behavior of a confined N2, CO, and Kr, we performed X-ray diffraction measurements of N2, CO, and Kr confined inside the cylindrical pores of five kinds of siliceous MCM-41 with different pore radii (R = 1.45−2.9 nm) as a function of temperature. Freezing and melting of these three molecules were almost reversible for all the MCM-41 samples used here. Freezing of the CO confined to the mesopores resulted in the appearance of the orientationally ordered α phase, whereas the N2 solidified into the orientationally disordered phase analogous to the bulk β phase. The solid Kr confined to the mesopores contains a considerable amount of random stacking faults. The freezing-point depressions of N2 and CO relative to the bulk melting point were nearly the same and were slightly larger than that of Kr. Upon cooling, both molecular liquids of N2 and CO confined to the cylindrical mesopores of R = 2.1 nm vitrify, whereas liquid Kr crystallizes within the mesopores of R = 1.8 nm and vitrifies within the mesopores of R = 1.45 nm. The mechanism of vitrification of a confined liquid upon cooling is considered.