On the Thermal Relaxation of CO2 Molecules in Presence of H2O or D2O Molecules

Abstract

Listen

Translate article

None of the various suggestions till now made to explain the large effect of the presence of water molecules on the thermal relaxation of CO2 has proved entirely satisfactory. It seems useful, therefore, to collect more information on the phenomena involved. The aim of this research is to compare the effects on the relaxation of CO2 of the presence of H2O and D2O molecules measuring a part of the dispersion curve for some mixtures. Commercial CO2 was used for this comparison. The mixtures were prepared in a glass bulb and sent into an interferometer, where sound velocity was measured at a constant frequency while the pressure was decreased. The mole fractions of H2O or D2O in the mixtures studied were about 0.0082 and 0.0117. The results show a large deviation of the dispersion curves from the shape of a theoretical one relaxation time curve. The deviations increase with the water content. If this result is confirmed, it would indicate that the water molecules alter profoundly the mechanism of energy exchange in CO2. It is to be observed, however, that we have supposed that during the measurements on each mixture (a few hours) the water adsorbed on the wall of the system did not vary. If this is not the case, the humidity of the mixture would increase as the pressure decreases and this fact could account at least in part for the deviations observed. It is possible, anyway, to compare the results obtained in the mixtures containing the same mole fractions of H2O and D2O. To do this, we have shifted the dispersion curve for CO2 to pass through the experimental points of the two mixtures for various value of γ/ρ (Mc/atmos). From the curves for 0.0082 mole fractions of H2O and D2O the calculated ratio of the shifts of relaxation frequency produced by H2O and D2O is 1.85; from the curves for 0.0117 mole fraction, it is 2.55. These values show that the difference between the effects of H2O and D2O molecules can be essentially accounted for by the change in the mass effect on the probability of inelastic transitions during collisions with CO2 molecules.