Thermal Diffusion in Isotopic 16O2–18O2

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Abstract
The isotopic thermal diffusion factor α 0 has been measured for a 90% 16O2–10% 18O2 symmetrical mixture, using both the conventional two‐bulb apparatus and the trennschaukel. The values are in the average temperature range of 125–440°K. We find that α 0 is negative below about 135°K, while at higher temperatures there is good agreement with the literature values ( α 0 = 0.45 at 450°K). The low‐temperature results (below 200°K) are similar to the findings of Raman et al. for 14N2–15N2, and the recent measurements of Stevens et al. on the isotopic mixtures of methane. Theoretical calculations according to the Chapman–Enskog theory using an Exp‐6 potential (α = 17, ε/k = 132.0°K) agree well with the experimental values in the temperature range 260–440°K. At low temperatures the agreement is rather poor. Since even at 78°K as many as five rotational states are excited in this molecule, inelastic collisions must play an important role over the entire temperature range. At our low temperatures, with a fair fraction of the gas at 78°K, the possible incipient polymerization in O2 may, to some extent, be responsible for the negative values of α 0 .