Isotopic fractionation between gaseous and condensed carbon dioxide
The isotopic fractionation in the equilibrium liquid-vapour system of carbon dioxide has been measured for both the carbon and oxygen isotopic species in the temperature range 220–303 °K (Fig. 1). The12C16O18O isotopic species is somewhat less, and13C16O2 slightly more volatile than12C16O2, while the fractionation in both cases approaches zero at the critical temperature. The theoretical expression derived for isotopic fractionation in two phase equilibria qualitatively accounts for the observed direction of the enrichment but fails to explain its disappearance at the critical point. A temperature dependence of the binding energy and intermolecular vibration frequency is proposed to bring the theory into accordance with observation. The magnitude of the unknown term in the fractionation caused by internal effects (including hindered rotation) is obtained by subtraction. It is of the same order of magnitude as the other terms.