Statistical Mechanics of Multimolecular Adsorption. IV. The Statistical Analog of the BET Constant a1b2/b1a2. Hindered Rotation of a Symmetrical Diatomic Molecule Near a Surface

Abstract

An approximate study is made of the restricted rotation of a symmetrical diatomic molecule near a surface. This makes possible some calculations concerning the BET constant C=R exp(ε₁—Λ)/kT, and especially the factor R (which is customarily set equal to unity). Cassie has pointed out that if this factor is set equal not to unity but to about 1/50 or even less, the low values of the heat of adsorption usually found on putting R=1 in the analysis of data are accounted for. The results obtained here indicate, unfortunately, that this factor is of the order of, say, 5–10, and that another explanation of the low heats of adsorption mentioned above will have to be found. An alternative method of estimating the area occupied per molecule in a completely filled monolayer is mentioned in an appendix. The temperature dependence of the quantity R is of some experimental interest, and a brief discussion of this dependence as predicted by the present approximate treatment is included.