40. ---------------. [ANDERSON, R. B.] Modifications of the Brunauer, Emmett, and Teller Equation. I. Jour. Am. Chem. Soc., vol. 68, 1946, pp. 686-691; Chem. Abs., vol. 40, 1946, p. 3322.
Brunauer-Emmett-Teller theory of multilayer adsorption has been applied extensively to physical-adsorption isotherms. While the simple Brunauer-Emmett-Teller equation provides an excellent method of estimating surface areas, it usually holds only for relative pressures of 0.05-0.40. In almost every case, the amount adsorbed at relative pressures higher than 0.40 is less than that predicted by the simple Brunauer-Emmett-Teller equation. This discrepancy has been explained in 3 ways: (1) By assuming the heat of adsorption in the second layer to be less than the heat of liquefaction of the adsorbate; (2) by assuming that the structure of the adsorbent is such that it will permit adsorption to only a finite number of layers; and (3) by considering the effects of capillary condensation. The author has observed that the simple Brunauer-Emmett-Teller equation can be fitted to many physical-adsorption isotherms in the range of relative pressures of 0.03-0.70 if the relative pressure is multiplied by a constant that is less than 1, usually varying between 0.6 and 0.7. This constant is interpreted to mean that the heat or free energy of adsorption in the 2d to 10th 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 differ from those functions for the liquid has been fitted to isotherms in the range of relative pressures of 0.05-0.98. 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 Brunauer-Emmett-Teller.