Adsorption hysteresis in ink-bottle pore

Abstract

To examine the mechanism of the adsorption hysteresis in ink-bottle pores, we measured the temperature dependence of the adsorption–desorption isotherms of argon, oxygen, and carbon dioxide onto SBA-16 ordered mesoporous material with cagelike pores. The hysteresis loop always shrank with increasing temperature and eventually disappeared at a hysteresis temperature (Th), well below the bulk critical temperature (Tc). When the relative pressures p/p0 of the capillary condensation and evaporation are plotted as a function of reduced temperature T/Tc, all the data including the transition pressures for nitrogen reported previously are represented by a common curve. We also calculated the temperature dependence of the capillary condensation and evaporation pressures of nitrogen under the assumption that adsorption and desorption in an ink-bottle pore may be regarded as the process of the disappearance and formation of a gas bubble in a liquid droplet confined to the pore. A fit between the observed and calculated transition pressures in a wide temperature range was reasonable in light of several assumptions and approximations used. This clearly indicates that the energy barrier for the formation and disappearance of vapor bubbles in the liquid confined to the pores is responsible for the appearance of the adsorption hysteresis and the hysteresis temperature is not concerned with the so-called capillary criticality. At temperatures higher than Th, the reversible capillary condensation takes place, because the energy barrier between a full liquid pore and the vapor coexisting with the liquid film becomes surmountable.