Phase Behavior in Confinement Studied with a Surface Force Apparatus

Abstract

The free energy of a surface has a major influence on the phase behavior of systems where the area‐to‐volume ratio is large. These include finely dispersed solids and liquids as well as substances confined to meso‐ and microporous solids. In a surface force apparatus (SFA) mica surfaces at or close to contact form a single model pore in which surface energy effects on phase behavior may be conveniently studied. Capillary condensation of liquid from undersaturated vapor is the most familiar example of a surface‐induced shift of bulk phase behavior, and this has been extensively studied with the SFA. I will here describe three additional examples of phase behavior in confinement studied with the SFA. The first is the observation of a surface‐induced phase separation in a binary liquid mixture. The second example concerns the freezing and melting behavior of a long‐chain n‐alkane in confinement. The last example is a quantitative study of capillary condensation below the bulk melting point, i.e., from vapor in equilibrium with solid. These examples illustrate how the SFA may be used to study different aspects of surface effects on phase behavior and how the results complement experiments on phase behavior in porous solids.

Keywords:

• Condensation
• surface force
• confinement
• phase behavior
• melting
• freezing
• phase separation