Following 18O uptake in scCO2–H2O mixtures with Raman spectroscopy

The uptake of 18O by scC16O2 in mixtures containing liquid H218O was followed with Raman spectroscopy using a specially designed high-pressure optical cell. Characteristic bands from the C16O18O and C18O2 molecules were identified in the supercritical phase and measured in the spectra as a function of time after introducing the liquid H218O into the scC16O2. Temporal dependence indicated the process was diffusion-limited in our cell for both C16O18O and C18O2. The ratio of concentrations of the 18O-labeled CO2 molecules, C18O2/C16O18O, was much higher than a random distribution of the isotopes for the system expected at equilibrium. The results are consistent with previous studies showing both rapid kinetics for oxygen exchange in aqueous solutions and the role of CO2 transport at liquid water interfaces. More importantly, they demonstrate the potential for using Raman spectroscopy with 18O isotopic labeling in scCO2 reaction studies with the recently determined frequency and intensity characteristics of the Fermi dyad peaks from 18O-containing CO2 molecules.

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► 18O uptake by scCO2 from H218O was followed by Raman spectroscopy.
► Raman peaks from the Fermi dyads of C16O18O and C18O2 were identified.
► 18O uptake by scCO2 was diffusion limited as measured in the supercritical phase.
► Slow interfacial process involving 18O-labeled CO2 led to C18O2 enrichment.