Capillary Condensation from Vapors of n-Hexane/Perfluoro-n-hexane Mixtures

Abstract

We have used a modified surface force apparatus to study capillary condensation of near-saturated vapors of n-hexane/perfluoro-n-hexane mixtures in a model pore formed by the gap between two mica surfaces. The measured compositions and mean meniscus radii of curvature of the capillary condensates are in broad agreement with the predictions of the binary Kelvin equations. Our results also show that n-hexane is preferentially adsorbed at the mica/liquid interface of the condensed mixtures, with the extent of adsorption depending on the position in the liquid−liquid phase diagram of the mixture. In addition, liquid−liquid-phase separation was observed in condensates formed from vapors of bulk perfluoro-n-hexane-rich mixtures at, or very close to phase-coexistence. This results in the formation of novel two-phase or “double” condensates, consisting of an inner condensate of a n-hexane-rich phase within an outer condensate of perfluoro-n-hexane-rich phase. Capillary condensation from vapors coexisting with phase-separated bulk mixtures gives rise to condensates of the n-hexane-rich phase. Finally, we have observed that the composition of a mixed condensate can show a significant time-dependence in the initial stages of condensation.