Analysis of the Multilayer Thickness Relationship for Water Vapor and Nitrogen Adsorption

Six silica gels, where each gel had a characteristic calibrated pore size (40-4000 Å), have been characterized both by water vapor and nitrogen adsorption isotherms. From these results, it was concluded that two-thirds of the silica surface is hydrophobic. The selection of wide pores (>300 Å) has enabled the determination of both the water and nitrogen t-curves for porous silica. It was observed that the development of the multilayer was identical in the wide pores irrespective of their size between 300 and 4000 Å. The porous silica t-curve for water coincided exactly with the t-curve of nonporous adsorbents provided that their BET C constants were similar. For nitrogen t-curves, a matching BET C constant ensured similarity only in the monolayer region, above which divergence progressively increased, becoming important close to saturation. The effect of the t-curve choice on the pore size distribution was established. A t-curve displaying reduced adsorption had a tendency to shift the pore-size distribution to lower dimensions, toward the micropore region. As a consequence, the cumulated surface area obtained from the BJH model gave increasingly higher values than the BET nitrogen surface area. However, the pore-size distribution was shifted to higher pore sizes when the selected t-curve was above the natural t-curve. Errors as much as 25% were detected for the mean pore radius and cumulated surface area for certain literature t-curves. A comparison of the water and the nitrogen t-curves indicated a crossover point when the water multilayer thickness became greater than the nitrogen thickness. Such behavior lends support to the cooperative adsorption mechanism for water vapor once a fixed number of water molecules (two layers) are present on the surface. Copyright 1998 Academic Press.