| Abstract
| - To examine the nature of the lower closure point of adsorption hysteresis in ordered mesoporous silicas, wemeasured the temperature dependence of the adsorption−desorption isotherm of nitrogen for three kinds of orderedsilicas with cagelike pores and three kinds of ordered silicas with cylindrical pores. The lower closure point pressureof nitrogen in the cagelike pores with sufficiently small necks, that is, the cavitation pressure of a confined liquid,did not depend appreciably on the cage size in the temperature region far away from a hysteresis critical temperature(Tch) but its cage-size dependence was noticeable in the vicinity of Tch. The lower closure point in the cylindrical poresdepended on the pore size, and its thermal behavior was totally different from that in the cagelike pores. Nevertheless,the hysteresis critical points of nitrogen in the ordered mesoporous silicas, which are defined as a threshold oftemperatures (Tch) and pressure above which reversible capillary condensation takes place in a given size and shapeof pores, fell on a common line in a temperature−pressure diagram regardless of the pore geometries. We considerthis finding as evidence that capillary evaporation in the cylindrical pores follows a cavitation process in the vicinityof Tch in the same way as that in the cagelike pores and also that the low limit of the hysteresis loop that has beenlong recognized since 1965 is due to the occurrence of a vapor bubble in a stretched metastable liquid confined tothe pores with decreasing pressure (cavitation).
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