Calculation of the vapour pressure ratio of Ne, A, Kr, and Xe isotopes in the solid state

Abstract

In order to discuss the validity of the anharmonic Einstein model developed byHenkel, the vapour pressure ratio of neon isotopes is recalculated, following Johns’s method, by making use of a Lennard-Jones (6–12) potential and of different couples of parametersɛ andσ. Results are compared with the recent experimental data byRoth andBigeleisen. The dependence of lnpm/pM from temperature is found to be scarcely sensitive to the choice of molecular parameters and is in good agreement with experimental results. As to the absolute values of lnpm/pM at any given temperature one does not find as good an agreement as claimed byJohns. It is shown how the uncertainty in the determination of molecular parameters cannot explain the whole disagreement between experiments and theory. Calculations for argon are carried on in the same scheme. Results are lower than preliminary experimental results byBoato andScoles. A brief discussion is given to suggest why difference in vapour pressure between two separated isotopes could be slightly different from the same quantity deduced by measuring the fractionation factor of an isotopic mixture. A possible influence of the isotopic composition of an isotopic solid mixture on the fractionation factor is also pointed out. Finally the lnpm/pM for krypton and xenon is calculated by means of the quantum theorem of corresponding states.