Molecular structure in water and solutions studied by photon-in/photon-out soft X-ray spectroscopy

We demonstrate how X-ray emission spectroscopy can be used to elucidate the molecular structure of liquid water, liquid methanol, methanol-water mixtures, as well as cation-water solutions, and to reveal the influence of the intermolecular interaction on the local electronic structure of water molecules. By comparing X-ray emission spectra of the water molecule and liquid water, a strong involvement of the total-symmetric valence-orbital is found in the hydrogen bonding. The local electronic structure of water molecules under different broken hydrogen bonding situations can be separately determined. We find that molecules in the pure liquid methanol in a confined sample cell predominantly persist as hydrogen-bonded chains and rings with six and/or eight molecules of equal abundance. For water methanol solutions the evidence of incomplete mixing is observed at the microscopic level, which provides a new explanation for a smaller entropy increase in the solution due to water molecules bridging methanol chains to form rings. Further, the influences of cations on the water molecular structure have been studied by the X-ray absorption and emission spectra. (C) 2010 Elsevier B.V. All rights reserved.