Universal Singularities of Multilayer Adsorption Isotherms and Determination of Surface Area of Adsorbents

Abstract

The singularity in the adsorption isotherm for macroporous and nonporous adsorbents is considered as a universal function that can be characterized with two parameters: a coefficient of proportionality, K, and an exponent, d. It is shown that the value of Kis proportional to the adsorbent surface area but does not depend on the nature of the adsorbent. This leads to a new method to determine the surface area of an adsorbent, S, that is independent of the form of the adsorption isotherm at low and moderate reduced pressures. Comparison with the BET areas for nitrogen shows that the new method gives the values of Swhich are very close to the BET results if K= 1.47 × 10 −5mol/m 2(for nitrogen). Analysis of adsorption data for macroporous adsorbents shows that the BET isotherm gives systematic deviations and that the experimental amount adsorbed is smaller than the value predicted by the BET equation (even in the range of the best agreement with experiment). These deviations lead to systematic error in the values of Sof about 43%. Using Kequal to K f = 1/σ N A(=1.025 × 10 −5mol/m 2for nitrogen), we are able to eliminate systematic error in the surface area determination. Here σ is the area occupied by one molecule and N Ais the Avogadro number.