Structural and vibrational studies of hydrogen bond in supercritical aqueous solutions

The main difference between water and other solvents comes from the special role of the hydrogen bond. More precisely, we study the persistence of hydrogen bond in supercritical aqueous solution, particularly between 647 and 773 K and 22.1 and 40 MPa. A vibrational study by Raman spectroscopy on mixtures of water and heavy water allow us to observe on one side numerous oscillators whose frequency is the one of the isolated molecule and on the other side a few oscillators linked by hydrogen bounds to other molecules. Our study by small angle X-ray scattering on water and water with ions added (such as LiBr and CsBr) shows that those hydrogen bonds should be present in high density inhomogeneities, whose size and contrast increases with the presence of ions and the atomic number of the cation. These ions are still solvated in high density area, thanks to higher 'local' values of the dielectric permittivity. This fact has been demonstrated by a study by X-ray absorption spectroscopy of ZnBr₂ in solvent with a xe value of polarity but variable dielectric permittivity (water, methanol and ethyl acetate). The manuscript ends with a perspective of scientific and experimental study of the oxygen K edge of water by X-ray Raman scattering at high temperature and high pressure. (author)