THEORETICAL INTERPRETATION OF ADSORPTION BEHAVIOR OF SIMPLE FLUIDS IN SLIT PORES

Nonlocal density functional theory is used to interpret and classify the adsorption behavior of simple fluids in model materials having slit pores. A systematic study is reported for a wide range of the variables involved: temperature, pressure, pore width H, and the intermolecular parameter ratios epsilon(sf)/epsilon(ff) and sigma(sf)/sigma(ff). Adsorption isotherms, isosteric heats of adsorption, and phase diagrams are calculated. The isotherms are related to those of the 1985 IUPAC classification; the range of variables corresponding to each of the six isotherm types is determined, and the underlying factors leading to each of the types are elucidated. In addition to the six types of the 1985 classification, a seventh type is identified, corresponding to capillary evaporation. A similar study and classification is reported for the heats of adsorption and phase transitions (capillary condensation and layering transitions) in pores. Since the materials studied do not exhibit either heterogeneity or networking, the conditions leading to phase transitions are clearly seen. Where possible, qualitative comparisons with experimental observations are made. The theoretical classification reported here should provide a useful framework against which to interpret experimental data.