Physical Properties of Isotopically Substituted Classical Fluids

The observed differences in physical properties between isotopically substituted classical fluids are treated in terms of the theorem of corresponding states. It is shown that these differences are at least partially due to changes in the intermolecular potential energies of interaction. When the potential function of an isotopically substituted molecule is conformal with that of the unsubstituted molecule, the differences in the physical properties of the fluids can be quantitatively related to the differences in the depth and spatial extension of the potential functions, and to the changes in the internal degrees of freedom of the molecules. It is shown that the measured properties of perdeutero benzene-benzene and perdeutero cyclohexane-cyclohexane are consistent with these equations, and that the values of the parameters chosen to fit the theory to the data are reasonable. The limits of applicability of this treatment and its possible extension to less simple systems are discussed.