FRACTAL DIMENSION OF MICROPOROUS CARBON ON THE BASIS OF THE POLANYI-DUBININ THEORY OF ADSORPTION - PART 3 - ADSORPTION AND ADSORPTION THERMODYNAMICS IN THE MICROPORES OF FRACTAL CARBONS

Fundamental thermodynamic relations are formulated based on the equati on of physical adsorption on microporous fractal solids, proposed prev iously and derived from the Polanyi-Dubinin theory of volume filling o f micropores. A new adsorption isotherm and corresponding adsorption h eat equations are verified using the experimental data published by Du binin and Polstyanov of benzene and cyclohexane adsorption and adsorpt ion heat on three microporous carbons. The obtained average correlatio n coefficients are compared with those from the original Dubinin-Astak hov (DA) equation. The correlation between the theoretical and experim ental data is satisfactory, especially in the range of relative adsorp tive pressures for which, following Stoeckli, the potential theory is accepted as appropriate. It is shown that, for nearly all the cases, m icropore volumes are similar to those obtained from the original DA eq uation. Fractal dimensions calculated from adsorption data of both sor bates on the same carbon are practically equal and can be treated as c onstants that characterize the micropores of a solid. Average pore dia meters, calculated from the obtained fractal dimensions, and also mini mal and maximal pore widths are similar to those determined by the met hods proposed by other authors, especially those obtained using the eq uation of McEnaney, developed from the analysis of SAXS data. The expl anation of why the approximate adsorption isotherm equation proposed b y Avnir and Jaroniec cannot be applied for the correct determination o f a micropore fractal dimension is given. (C) 1998 Elsevier Science B. V.