Are isotopic mixtures ideal?

Abstract

Isotopic mixtures have long been considered as textbook examples of ideal solutions. High-precision vapor pressure measurements have shown that even these very simple mixtures exhibit deviations from the ideal behavior. The small, but still significant nonideality of isotopic mixtures can be accounted for in terms of the difference in the molar volumes of isotopic molecules. Theoretical analysis demonstrates that the internal vibrations of the molecules significantly contribute to the excess Gibbs energy, and the proper consideration of the volume dependence of molecular vibrations of the component molecules is essential to the understanding of the properties of isotopic mixtures.

An isotopic mass effect on the intermolecular potential

“…For example, changes in intermolecular potentials as a result of mass-dependent differences in electronic structure are often suppressed or ignored. That is, electronic energy and intermolecular potentials are commonly assumed to be the same for all isotopologues [35][36][37][38][39] with the Born-Oppenheimer approximation 40 being invoked to justify this assumption. Under Born-Oppenheimer, the intramolecular potential energy surfaces upon which the nuclei move are independent of isotopic composition.…”