Isotopic Thermal Diffusion Factor for CO2

Abstract

In continuation of our study of thermal diffusion in common, nonpolar, polyatomic gases, we report the values of the isotopic thermal diffusion factor α0 as a function of temperature for carbon dioxide. We have used a 60% 13C16O2–40% 12C16O2 mixture in a 10-tube metal trennschaukel over an average temperature range of 220–500°K. The experimental α0 values can be represented in this temperature range by the equation, obtained by least squares: α0 = −0.422 + 0.00155T. In this linear variation, α0 is −0.075 ± 0.070 at 239.5°K, and 0.343 ± 0.039 at 502.4°K, the highest average temperature of our measurements. The negative values of α0 below about 250°K confirm our earlier observations made in the case of isotopic N2 and O2 gases. As expected, the agreement between Chapman–Enskog theory and these experiments is poor. Also it has not been possible to correlate the present results with the other transport properties of this gas. Besides the inadequacy of the usual potential models, the plausible reasons for this disagreement are: (a) neglect of the inelastic collisions and internal degrees of freedom in the theory, and (b) incipient dimerization at the lowest temperature.