Non-ideality of an "ideal'' liquid mixture: (Ar-36+Ar-40)

A mixture of two isotopes of the same rare gas represents the extreme case of molecular simplicity. In this work, the excess molar Gibbs energies for liquid mixtures of (Ar-36 + Ar-40) have been measured for the first time, w ith unprecedented precision. Results show that the excess vapour pressure o f an equimolar mixture is approximately 2.5 Pa in the 85-96 K temperature r ange. The results are well predicted by a modified version of the quantum t heory of isotope effects in mixtures.