DOE Pulse
  • Number 307  |
  • March 15, 2010

Researchers rediscover the structure of water


A team of researchers at the Stanford Synchrotron Radiation Lightsource, located at the DOE’s SLAC National Accelerator Laboratory, has found the molecular structure of water to be more complex than recently thought, suggesting that molecular models that went out of fashion decades ago may be in fact more accurate than recent ones.

By recording how SSRL's X-ray beam scattered off a flowing jet of water, lead author Ling Fu, a postdoc at the Centre National de la Recherche Scientifique in France, and SLAC colleagues Arthur Bienenstock and Sean Brennan were able to determine the distances between the water molecules in the jet. As recent models predicted, they saw molecules 0.28 and 0.45 nanometers apart. These measurements confirm the current commonly accepted model, which describes liquid water as a group of water molecules held together in tetrahedral shapes, with the molecule at the center of the tetrahedron separated from four others at the shorter distance and each of these four molecules separated from one another at the longer distance.

Yet the researchers saw some molecules at a third distance as well: 0.34 nanometers. The existence of this third separation length, though not included in the current model, was first seen in 1938. Additional experiments in the 1960s and 1970s first confirmed, but later rejected, that this length exists, concluding that its detection was due to shortcomings in the analysis. As a result, models including this intermediate distance fell out of favor—until now.

That the researchers have now observed this intermediate separation length using modern-day technology suggests that "there's something more going on here" beyond the currently accepted model, Fu said.

These results suggest that liquid water's structure is not completely tetrahedral, but rather has some added complexity. But they do not fully solve the mystery of water's structure because the data taken at SSRL only reveal the distances between water molecules, not the angles of the bonds. "More research is needed to see the complete picture," said Brennan.

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Submitted by DOE's SLAC National Accelerator Laboratory