Capillary condensation of hexafluoroethane in an ordered mesoporous silica

A total of 43 adsorption isotherms are reported for hexafluoroethane (HFE) on an ordered mesoporous silica (MCM-41) at 2 K intervals for temperatures from 196 K to 280 K. All isotherms are reversible and, at lower temperatures, exhibit the abrupt step characteristic of capillary condensation in such adsorbents, but at higher temperatures both the extent and slope of the step decrease. The step is absent at temperatures greater than 255 K, here interpreted as the capillary critical temperature for first-order capillary condensation. Equations describing the capillary coexistence boundary and the rectilinear diameter are presented. Pore radii and pore volumes calculated from the isotherms are consistent with the assumption that capillary condensed HFE has essentially the same surface tension and density as bulk liquid over the entire temperature range. We conclude that capillary condensation at T > 255 K occurs by a second-order process.