Phase behaviour in slits—when tight cracks stay wet

Abstract

The phase behaviour of fluids confined to a single-slit pore can be conveniently studied with a simplified surface force apparatus. Such investigations are possible because multiple-beam interferometry with back-silvered mica surfaces allows the refractive index of the intervening medium to be determined. Adsorption isotherms at (effectively) single surfaces can be measured, and the capillary condensation transition occurring between two surfaces at small separations studied as a function of the relative vapour pressure. The shape of the adsorption isotherm of cyclohexane on mica is shown to be in qualitative agreement with that determined by ellipsometry. The capillary condensation transition in tert-bytanol is found to be well described by the coalescence of adsorbed films due to van der Waals forces for films thicker than about 4 nm. For thinner adsorbed films the transition occurs for larger than expected slit widths, for both tert-butanol and cyclohexane. At temperatures below the bulk melting point capillary condensates remain liquid because of surface energy constraints. The maximum size of liquid condensates of tert-butanol and neopentanol is inversely proportional to the temperature below the freezing point, and the relationship gives information on solid-liquid interfacial energies.