Physical adsorption at high relative pressures

Adsorption isotherms and isosteric heat data are presented for various nonpolar gases on oxide, metal, salt and graphite adsorbents. All adsorption isotherms, except those on graphite, were found to cross the P₀-axis at a definite angle before four monolayers are filled, when the adsorption temperature is below the triple point temperature of the adsorbate. Apparently, for all adsorbents but graphite, the electric field emanating from the surface and/or the structure of the surface still strongly affects the molecular interactions in the higher adsorbed layers as P₀ is approached. p ]If the number of adsorbed layers is finite as P₀ is reached, this value of P₀ can not be used in the BET equation for surface area determination. In the case of krypton adsorption or oxides at 77.60°K curved BET plots can be straightened by using P₀ = 4.40 mm mercury. Employing a cross sectional area of a krypton atom of 17.65 Å ² very good agreement will be obtained with the nitrogen BET surface area.