| Abstract
| - A volume filling of a single micropore and atwo-dimensional (2D) condensation on its walls occur atthe same critical pressure, and the local adsorption behavior may bemodeled by the condensationapproximation. This phenomenon underlies the approach to themicropore volume filling based on thecondensation approximation (VFCA) and the Dubinin-Astakhov (DA)equation. The DA equation followsfrom the VFCA approach, being an approximate form to the generalrelationship. The physical meaningsof the exponent, n, and the characteristic energy,E, with respect to the surface heterogeneity, aregiven.As a whole, n is determined only by the standarddeviation of the micropore widths: the less theheterogeneity,the larger n, approaching to infinity for the homogeneouscarbon. The characteristic energy depends onthe average micropore sizes and its standard deviation. In thecase of the DA equation with n = 2 orn= 3, standard deviations are equal to 0.4915 or 0.3493, respectively,and E depends only upon an averagemicropore size or upon a related adsorption potential. For theindividual micropore, E determines thecritical pressure of a 2D condensation. In the case of wateradsorption on active carbons, the baseheterogeneity due to variation in the pore widths, as perceived bywater molecules, is negligible. Hence,water adsorption may be considered to be an extension of a2D-condensation into practically homogeneousmicropore volumes. It is shown that empirical relationships forcalculating of average micropore sizes andan applicability of the DA equation to the adsorption on nonporoussurfaces may be also explained in theframework of the VFCA approach.
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