Capillary condensation and adsorption in cylindrical and slit-like pores

The nature of adsorption of simple fluids confined in model pores is investigated by means of a density functional approach. For temperatures T corresponding to a partial wetting situation a first-order phase transition (capillary condensation) from dilute ‘gas’ to dense ‘liquid’ occurs at relative pressuresp/psrttclose to those predicted by the macroscopic Kelvin equation, even for radii R, or wall separations H as small as 10 molecular diameters. In a complete wetting situation, where thick films develop, the Kelvin equation is, in general, not accurate. At fixed T the adsorption T,(p) exhibits a loop; Tmjumps discontinuously at the first-order transition, but the accompanying metastable portions of the loop could produce hysteresis similar to that observed in adsorption measurements on mesoporous solids. Metastable thick films persist to larger p/psatin slits than in cylinders and this has repercussions for the shape of hysteresis loops. For a given pore size the loop in Tmshrinks with increasing T and disappears at a capillary critical temperature T r p (< T,).If T > TEaPcondensation no longer occurs and hysteresis of Tm will not be observed. Such behaviour is found in experiments. A prewetting (thick-thin film) transition can occur for confined fluids. The transition is shifted to a smaller value of p/psat than that appropriate to prewetting at a single planar wall. Whereas the magnitude of the shift is very small for slits, it is substantial for cylinders and this leads to the possibility of finding a triple point, where ‘liquid’ and thick and thin films coexist, in cylindrical pores whose radii may not be too large for investigation by experiment or computer simulation. Adsorption of super- critical fluids ( T > T,, the bulk critical temperature) in cylinders is mentioned briefly.