Theoretical studies on the standard but misleading adsorption isotherm II: Modification of the t method to determine the true monolayer capacity and the surface area

Abstract

Although the t curve is one of the most convenient methods for determining the surface area of solids, it may be misleading in that it does not accomodate volume filling for microporous solids or partly lyophobic surfaces. The method is modified to determine the true surface area to avoid volume filling and the partly lyophobic surfaces which may be present in the microporous and partly lyophobic solids. Also V 1- n R plots (where V 1 (ml g -1) is the amount of adsorbate at a definite relative pressure and n R is the number of layers adsorbed on the non-porous adsorbent at the same relative pressure) are derived from V 1- t plots to determine the monolayer capacity. The V 1- n R plot has an advantage over the t curve in surface area determinations because it avoids the calculation of the statistical thickness of the adsorbate, which is criticized by many investigators. Applications of the modified V 1- t and V 1- n R plots on previously studied partly hydrophobic and microporous silica gels give the true surface areas. The present methods could easily be applied to determine the true surface area of any solid, independent of the nature of the adsorbents.