Curtain may be closing on scientific water controversy
Teresa Head-Gordon and Margaret Johnson, bioengineers with Berkeley Lab's Physical Biosciences Division, and the University of California Berkeley/San Francisco Joint Graduate Group in Bioengineering, characterized the static structural organization of liquid water by analyzing data which was collected by Head-Gordon's research group in 2002 using the ultrabright x-ray beams at Berkeley Lab's Advanced Light Source (ALS). They found that while the "rings and chains" alternative model of liquid water may exist for the briefest of instants, the average structure is that of the familiar tetrahedral network. "I think that most scientists who work in water, liquids or disordered systems will find our paper very convincing," said Head-Gordon. "For some, it will be convincing enough so that it should end the controversy." Water covers 70 percent of the Earth's surface and makes up 60 percent of the human body. Despite water's ubiquitous presence in our lives, it remains a mystery. Whereas most substances contract when they solidify, water expands, making it less dense as a solid than as a liquid. Our lives depend upon liquid water but, given its light molecular weight, water at room temperature should be a gas. The key to understanding the strange but vital properties of liquid water is to fully understand its structure. A single water molecule is V-shaped, but because the oxygen atom is more electronegative than the hydrogen atoms, the electrons in the molecule tend to gather towards the oxygen end, creating a slightly negative pole there and a slightly positive pole on the hydrogen side. The polarity of each water molecule results in a weak attraction between it and other water molecules, called a hydrogen bond. In the traditional scientific picture of water in the solid ice state, every individual water molecule forms four hydrogen bonds — two that are electron acceptors and two that are electron donors – through which it connects to its nearest neighbors. The result is a network of tetrahedrons. When ice melts, these bonds may become distorted and up to 20-percent of them broken. Despite these thermal distortions, liquid water still retains its tetrahedral network. This tetrahedral structure, coupled with strong hydrogen bonding, has long been thought to be the source of liquid water's unusual properties. Two years ago, however, scientists at Stanford University reported a series of experiments, using x-ray absorption spectroscopy and x-ray Raman scattering techniques, that indicated a radically different molecular arrangement for water. They reported that in the liquid state, more than 80 percent of the hydrogen bonds between water molecules were broken. On the average, they found each liquid water molecule formed only two hydrogen bonds — one electron donor and one electron acceptor. From this they concluded that in the liquid state, water molecules form a network of large rings or chains, rather than tetrahedrons.
|X-ray scattering pattern of liquid water at room temperature and pressure obtained at Beamline 7.3.3 of the Advanced Light Source at the the Lawrence Berkeley National Laboratory (Photo: LBNL)|
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