PHASE-BEHAVIOR IN SLITS - WHEN TIGHT CRACKS STAY WET

The phase behaviour of fluids confined to a single-slit pore can be co nveniently studied with a simplified surface force apparatus. Such inv estigations are possible because multiple-beam interferometry with bac k-silvered mica surfaces allows the refractive index of the intervenin g medium to be determined. Adsorption isotherms at (effectively) singl e surfaces can be measured, and the capillary condensation transition occurring between two surfaces at small separations studied as a funct ion of the relative vapour pressure. The shape of the adsorption isoth erm of cyclohexane on mica is shown to be in qualitative agreement wit h that determined by ellipsometry. The capillary condensation transiti on in tert-butanol is found to be well described by the coalescence of adsorbed films due to van der Waals forces for films thicker than abo ut 4 nm. For thinner adsorbed films the transition occurs for larger t han expected slit widths, for both tert-butanol and cyclohexane. At te mperatures below the bulk melting point capillary condensates remain l iquid because of surface energy constraints. The maximum size of liqui d condensates of tert-butanol and neopentanol is inversely proportiona l to the temperature below the freezing point, and the relationship gi ves information on solid-liquid interfacial energies.