Water-carbon dioxide mixtures at high temperatures and pressures:Local order in the water rich phase investigated by vibrational spectroscopy

摘要

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-CO_2 mixtures under isobaric heating (T= 40-360 deg C,P=250 bars).The quantitative analysis of the spectra shows that tetramers and larger oligomers are the main constituents of water at moderate temperatures below 80 deg 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 deg C,P-250 bars).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 CO_2 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.