A combination of Raman scattering spectroscopy and infrared absorption was applied to investigate the structural evolution of water dissolved in supercritical carbon dioxide under isobaric heating $\text{(T=40–340\hspace{0.05em}°}\mathrm{C},$ $\text{P=250\hspace{0.17em}}\mathrm{bar}\mathrm{).}$ Quantitative analysis of experimental spectra allowed us to determine that at relatively moderate temperatures water dissolved in ${\mathrm{CO}}_2$ -rich phase exists only under monomeric form (solitary water surrounding by ${\mathrm{CO}}_2$ molecules), but hydrogen-bonded species, namely, dimers, begin to appear upon heating. At the same time, the ratio of dimers to monomers concentration increases with further temperature increase and at temperatures close to the temperature of total miscibility of the mixture $\text{(T=366\hspace{0.05em}°}\mathrm{C},$ $\text{P=250\hspace{0.17em}}\mathrm{bar}\mathrm{),}$ water dimers only are present in the ${\mathrm{CO}}_2$ -rich phase.

Raman scattering combined with near- and midinfrared absorption spectroscopies was used to investigate the evolution of the local order in the water rich phase of water-CO2 mixtures under isobaric heating (T=40–360°C,P=250bars). The quantitative analysis of the spectra shows that tetramers and larger oligomers are the main constituents of water at moderate temperatures below 80 °C. As the temperature increases, the dimer and trimer concentrations considerably increase at the expense of larger oligomers. Finally, water dimers are predominant at the highest temperature investigated close to the temperature of total miscibility of the mixture (T=366°C,P=250bars). This result is consistent with our previous investigation [R. Oparin T. Tassaing, Y. Danten, and M. Besnard, J. Chem. Phys. 120, 10691 (2004)] on water dissolved in the CO2 rich phase where we found that close to the temperature of total miscibility water also exists mainly under dimeric form. The current study combined with that mentioned above provides a model investigation of the evolution of the state of aggregation of water molecules in binary mixture involving a hydrophobic solvent in a wide range of temperature.

The local order in aqueous NaCl solutions diluted in supercritical carbon dioxide at constant pressure as a function of NaCl concentration and temperature has been investigated using near and mid infrared absorption spectroscopy. The near IR results have allowed us to estimate the water concentration in CO2 rich phase, whereas the state of water aggregation in CO2 phase was investigated using mid IR spectroscopy. The analysis of the band shape variations of the OD stretching mode of HOD led us to conclude that below 100 °C, water molecules dissolved in CO2 exist only under their monomeric form, whatever the salt concentration is, whereas hydrogen-bonded species, namely, dimers start to appear at higher temperatures. Larger aggregates have a negligible concentration in the range of temperature-pressure investigated. Using near and mid infrared data, we have calculated the concentrations of water species in the CO2 phase. Upon heating, it was found that the concentration of dimers considerably increases at the expense of the monomers and only dimers are detected in carbon dioxide at highest temperatures. Changing the salt concentration affects significantly the concentration of monomers and decreases strongly the dimers population as the solution becomes progressively saturated in salt. In the saturated solution, at 340 °C, the dimer concentration is at least two times smaller than in the binary water–CO2 mixture. These findings are in qualitative agreement with existing thermodynamics data showing that addition of NaCl to the binary H2O–CO2 system shifts the range of partial miscibility of water and CO2 towards higher pressure and temperature.