PORE-SIZE HETEROGENEITY AND THE CARBON SLIT PORE - A DENSITY-FUNCTIONAL THEORY MODEL

We present model isotherms predicted by nonlocal density functional th eory for adsorption of simple fluids in carbon slit pores. The effects of pore size, temperature, and solid-fluid potential interaction stre ngth are examined. Our results are summarized into a classification sc heme based upon regimes of continuous pore filling, capillary condensa tion, and 0 --> 1 layering transitions. The descriptions we have devis ed depart from the IUPAC convention in that the filling behavior, rath er than the physical width of the pore, is used as a guide to classifi cation. Our results suggest that while the magnitude of the solid-flui d interactions dictates the pressure at which pore filling occurs, the type of filling depends primarily upon the ratio of pore width to ads orbate molecular diameter. The critical pore widths that denote the bo undaries between various regimes of filling behavior are strongly depe ndent upon the temperature. To confirm the accuracy of the theoretical results, we compare adsorption isotherms and density profiles calcula ted from nonlocal theory and Gibbs ensemble simulation. The results fr om theory and simulation are found to be in good agreement. We conclud e with a discussion of the problems associated with estimating solid-f luid potential parameters from experiment for use in theoretical compu tations. A comparison of nonlocal theory model isotherms and experimen tal nitrogen uptake measurements on nonporous carbon is presented.