Water-CO2 interaction in supercritical CO2 as studied by infrared spectroscopy and vibrational frequency shift calculations

Infrared absorption measurements of diluted perdeuterated water in supercritical carbon dioxide are reported. The frequency shift of the anti-symmetric stretching mode (ν3) is analysed using an analytical molecular model to get insights into the water-CO2 interaction. Attractive forces, namely, dispersive and dipole-quadrupole interactions, are found dominant in the stabilisation energy of the water-CO2 pair. Differences between observed and calculated frequency shifts reveal local density enhancements (LDE). The observation of LDE at temperature greater than the critical one (T/Tc ≥ 1.3) allows to conclude that water acts as an attractive solute in the hyper-compressible regime of the solvent.