The traditional picture of how liquid water behaves on a molecular level is wrong, according to new experimental evidence collected by a collaboration of researchers from the Department of Energy's Stanford Linear Accelerator Center (SLAC) in California, RIKEN SPring-8 synchrotron and Hiroshima University in Japan and Stockholm University in Sweden. The team, involving SLAC scientist Anders Nilsson, used advanced X-ray spectroscopy techniques to create a more detailed picture of water's molecular behavior. The results are published as the cover story in the July 30, 2008 edition of the journal Chemical Physics Letters.

Water, by any measure, is rather unique. It behaves unlike any other liquid. It has a tremendous capacity for carrying heat. Its solid phase-ice-is less dense than the liquid. Water also has unusually strong surface tension. These unusual properties are what make water such an essential substance to the existence of life.

In the recent study, Nilsson and colleagues probed the structure of liquid water using X-ray Emission Spectroscopy and X-ray Absorption Spectroscopy. In partial agreement with the traditional model, Nilsson's team found that liquid water is in part made up of tetrahedral groups of molecules. But clear evidence also emerged for the dominance of a second, less defined structure in the mix. Settling the debate about water's molecular structure holds tremendous importance for a range of fields including medicine, chemistry and biology. Current molecular dynamics models, which are used to understand chemical and biological processes, are notoriously limited in their ability to predict water's behavior.

D. Nordlund, M. Odelius, H. Bluhm, H. Ogasawara, L. G. M. Pettersson and A. Nilsson, "Electronic Structure Effects in Liquid Water Studied by Photoelectron Spectroscopy and Density Functional Theory", Chem. Phys. Lett. 460, 86 (2008) doi: 10.1016/j.cplett.2008.04.096