The relationship of the geometric factor in the Dubinin-Astakhov isotherm equation with the fractal dimension

The parameter n found in the Dubinin-Astakhov (D-A) equation, which may be expressed as θ=exp[-(A/E)(n)], allows the estimation of the fractal dimensions of microporous adsorbents, such as those investigated in this study. An empirical relationship is shown to exist between n and fractal dimension values when hydrocarbon adsorbates are utilized. The employment of two adsorption isotherms for the same adsorbate, namely those pertaining to the investigated adsorbent and to a reference adsorbent, the fractal dimension of which is known, are sufficient for the evaluation. The values of the fractal dimensions of the zeolites 13X, 5A, 4A, G5 (a faujasite type zeolite), silicalite and two distinct active carbon samples are determined to be possible to be estimated by utilizing the proposed method based on the D-A equation and the results obtained are in accordance with those determined previously by employing the Pfeifer-Avnir and the point-slope methods. The validity of the parameters of D-A isotherm equation in the correlation of full adsorption data is a prerequisite for the evaluation of the fractal dimension values by using the D-A method. The tendency of the n values to become independent of the pressure range above a certain higher pressure value is an indicator of such a validity. Copyright (C) 2000 Elsevier Science B.V.