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.
Nilsson and colleagues probed the structure of liquid water using X-ray Emission Spectroscopy and X-ray Absorption Spectroscopy. These techniques use powerful X-rays, generated by a synchrotron light source, to excite electrons within a water molecule's single oxygen atom. Tuning the X-rays to a specific range of energies can reveal with tremendous precision the location and arrangement of the water molecules. In this way Nilsson's team found that water is indeed made up of tetrahedral groups, but clear evidence also emerged for the dominance of a second, less defined structure in the mix.
The idea that liquid water is made up of two structures is not new. German physicist Willhelm Conrad Röntgen, who discovered X-rays in the late 19th century, published a paper proposing that liquid water comprised two different structures—one tetrahedral "ice-like" structure, and another more loosely arranged structure, which helped explain why water behaves in such unusual ways. Now, more than a century later, the current study is giving new life to Röntgen's "two structure" model.
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.
The current study is the most recent addition to a growing body of evidence for a new theory about the structure of liquid water. In 2004, Nilsson and colleagues sparked controversy with a paper published in Science that suggested the tetrahedral model of water was incorrect. Nilsson agrees that the debate is far from settled and that much work remains before a clear picture of liquid water emerges.
The nanoscale has more basic details to be understood.