Statistical Mechanics of Multimolecular Adsorption. IV. The Statistical Analog of the BET Constanta1b2/b1a2. Hindered Rotation of a Symmetrical Diatomic Molecule Near a Surface

摘要

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 constantC=Rexp(egr;1mdash;Lgr;)/kT,and especially the factorR(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 puttingR=1 in the analysis of data are accounted for. The results obtained here indicate, unfortunately, that this factor is of the order of, say, 5ndash;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 quantityRis of some experimental interest, and a brief discussion of this dependence as predicted by the present approximate treatment is included.