CLASSIFICATION OF ADSORPTION BEHAVIOR - SIMPLE FLUIDS IN PORES OF SLIT-SHAPED GEOMETRY

We report calculations based on an accurate (compared to molecular simulations) nonlocal density functional theory for adsorption isotherms for simple fluids on single surfaces and in slit-shaped pores for a wide range of pore sizes, fluid-wall adsorption strengths and temperatures. The calculations are for smooth structureless walls interacting with the (10,4,3) potential, and the intermolecular interactions between fluid molecules are of Lennard-Jones form. The calculations are used to classify the adsorption behavior into seven classes. Six of these correspond to the IUPAC classification scheme (Sing et al., 1985), and the seventh corresponds to capillary evaporation (drying). The range of conditions that give each of these classes are identified. Qualitative comparisons with experiment are made, and we describe some typical phase diagrams for fluids in such pores. All calculations are based on the single pore model. While a distribution of pore sizes can easily be introduced with this model, interconnection of pores is neglected. Although all six of the IUPAC classes are produced with the single pore model, details of the hysteresis loops in classes IV and V are not reproduced.