Modifications of the Brunauer, Emmett and Teller equation.

It has been shown that the simple equation of Brunauer, Emmett and Teller (B.E.T.) can be fitted to many physical adsorption isotherms in the range of relative pressures of 0.05 to 0.70, if the relative pressure is multiplied by a constant that is less than 1, usually varying between 0.6 and 0.7. The constant is interpreted to mean that the heat or free energy of adsorption in the second to tenth layers is less than the heat or free energy of liquefaction, or that the entropy of adsorption in these layers is more negative than the entropy of liquefaction. A similar equation containing an additional constant denoting the upper limit of the layers in which the heat, free energy or entropy is different from those functions for the liquid has been fitted to isotherms in the range of relative pressures of 0.05 to 0.98. For porous solids equations have been presented for adsorption on solids in which the area available to each succeeding layer is less than the previous one. A new-type equation has been developed for adsorptions limited to n-layers which has better properties than the n-equation of B.E.T.