By Edwin Cartlidge | Copyright: New Scientist | Persian translation: Mahmoud Haj-Zamani | Re-typed and re-edited: Sky and Weather Network
Water is the most abundant—and arguably the most important—liquid on Earth. Its importance, however, is not limited to biological phenomena. Compared with other substances on Earth, water has remarkable physical properties, many of which can be regarded as factors that make it particularly compatible with life. [source]
X-ray image of water molecules.
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| Image from the original article |
The universe confronts us with many difficult mysteries, from the nature of dark matter and the origin of the universe to the search for a theory of everything. Yet an equally intriguing puzzle on a much smaller scale can be investigated in an ordinary kitchen. Fill a glass with cold water and drop in an ice cube. The familiar fact that ice floats is the first unusual property. Measure the temperature at different depths and another appears: near the ice the water is close to 0°C, while near the bottom it can be about 4°C. Water reaches its maximum density at about 4°C, an unusual behaviour that distinguishes it from ordinary liquids. [source]
Several of water's peculiarities are crucial to life. Ice is less dense than liquid water, and water near freezing is less dense than water a few degrees warmer, so bodies of water freeze from the top downward. This helps liquid water—and life—persist beneath surface ice, including during ice ages, and is one reason scientists have considered the possibility of life beneath the ice of Jupiter's moon Europa. Water can also absorb a remarkable amount of heat, including through its high latent heat of vaporization, helping moderate climatic variations.
No single theory had, at the time of the source article, provided a universally accepted explanation for all of water's unusual properties. Anders Nilsson of Stanford University and Lars Pettersson of Stockholm University and their colleagues proposed a controversial interpretation extending an idea advanced more than a century earlier by Wilhelm Röntgen, discoverer of X-rays: molecules in liquid water might not be linked in only one structural arrangement but could occupy two markedly different local structures. [source]
Hydrogen bonds and the conventional picture [source] [source]
The key lies in how water molecules interact. Each molecule contains one oxygen atom and two hydrogen atoms. The oxygen side carries a slight negative charge while the hydrogen side is correspondingly positive, allowing neighbouring molecules to attract one another through hydrogen bonds. [source]
Hydrogen bonds are much weaker than the bonds holding the atoms within each water molecule, so they continually break and reform. They are strongest when neighbouring molecules align appropriately. The geometry of H2O allows a molecule to be surrounded by four neighbours in a tetrahedral arrangement. This is the familiar structure of ice. A simple conventional picture of liquid water treats it as having a similar but less rigid network, with additional molecules occupying some of the open space. This helps explain why liquid water is denser than ice and had appeared consistent with X-ray, infrared, and neutron-scattering experiments. [source]
A challenge from X-ray spectroscopy
Some physicists had proposed that under special conditions water separates into two different structural forms, although most researchers assumed only one broad structure under ordinary conditions. About a decade before the source article, Pettersson and Nilsson encountered an unexpected result while using X-ray absorption spectroscopy to study the amino acid glycine. Peaks in an X-ray absorption spectrum can reveal chemical bonding and structure, and a powerful new X-ray source allowed unusually sensitive measurements.
They discovered that the water containing their glycine sample produced a more intriguing spectrum than the amino acid itself. Nilsson recalled that the observation was sufficiently striking that they felt compelled to understand it. In a 2004 paper, they interpreted an unexpected absorption peak as evidence that at any instant about 85 percent of hydrogen bonds in liquid water were weakened or broken—far above the roughly 10 percent predicted by the textbook model cited in the article.
If correct, the result implied a major revision of the conventional picture. Nilsson and Pettersson therefore pursued further X-ray experiments, working with Shik Shin of the University of Tokyo, an expert in X-ray emission spectroscopy. In this method, the article explains, shorter-wavelength X-ray emission corresponds to weaker hydrogen bonding.
The emission spectrum showed two peaks that the team interpreted as evidence of two distinct local structures. They associated the longer-wavelength peak with molecules arranged tetrahedrally and the shorter-wavelength peak with molecules in a different, more weakly bonded environment. The shorter-wavelength feature was stronger, which they regarded as supporting their earlier claim that weakly bonded molecules were more abundant. [source] [source]
As water was heated, the shorter-wavelength peak shifted further toward short wavelengths while the other remained comparatively stable. The team interpreted this as further weakening of hydrogen bonds in the less ordered structure with increasing temperature. They also revisited older experimental results previously considered consistent with the traditional model and argued that these too could fit their interpretation.
How different are the two proposed structures? [source]
To investigate their spatial scale, the researchers used powerful X-rays generated at Stanford's synchrotron radiation facility and measured scattering at different angles. They interpreted the results as showing small regions of tetrahedrally ordered molecules roughly 1–2 nanometres across.
Combining those findings with measurements by Uwe Bergmann at Stanford, they proposed that the ordered structures contain on average about 50–100 molecules and are surrounded by a sea of more weakly bonded molecules. These regions are not static: on timescales shorter than a trillionth of a second, water molecules move between the two local environments as hydrogen bonds break and reform. [source]
Why is water densest at 4°C? [source]
Nilsson and Pettersson's changing balance between two local structures offered a possible explanation. Molecules in the more disordered regions can pack closer together than molecules in the open tetrahedral network. Near 0°C the disordered regions would be relatively uncommon. As the water warms, added thermal energy disrupts some ordered structures, so molecules spend less time in the open tetrahedral arrangement and more time in compact disordered environments, increasing the overall density. [source]
With further heating, however, motion among the more weakly bonded molecules becomes stronger and gradually pushes them farther apart. This thermal expansion then reverses the trend, explaining in the proposed model why density begins to decrease again above approximately 4°C.
Pettersson argued that this two-structure model could account simply for several other anomalous properties of water. Martin Chaplin, a chemist at London South Bank University, was sympathetic to the idea, saying that conventional one-component explanations had to work hard to reproduce the many maxima and minima in water's properties as temperature changes, whereas a two-structure picture could potentially accommodate them more naturally.
Criticism and scientific debate [source]
The 2004 Nilsson–Pettersson paper in Science had been cited more than 350 times by the time of the source article, but substantial skepticism remained. One objection concerned the interpretation of the X-ray spectra, which depended on simulations involving at least 50 interacting water molecules—an extremely complex calculation that could only be solved approximately.
Richard Saykally of the University of California, Berkeley argued that a much more precise theory would be required before such a strong claim could be accepted. In his view, smaller adjustments to hydrogen-bond arrangements within conventional structures could explain the X-ray observations. Michael Odelius of Stockholm University, formerly part of the collaboration, also ceased working with the group because he disagreed with their interpretation of the X-ray emission data. [source]
Another source of skepticism was an early claim in the 2004 paper that weakly bonded molecules form rings and chains. Nilsson and colleagues subsequently became less specific about the exact molecular geometry of the disordered state. Eugene Stanley of Boston University nevertheless argued that this revision did not invalidate the entire hypothesis and regarded the X-ray scattering results as evidence worth considering. [source] [source]
Why the question matters [source]
Nilsson and Pettersson still faced significant opposition, but if a more complete understanding of liquid water's structure could be achieved it might improve knowledge of how proteins and drugs interact with water molecules in the body, potentially assisting drug design. Better understanding of water near tiny pores could also aid desalination and water purification, improving access to clean water.
As Pettersson put it in the source article, our understanding of water is an evolving picture, and further work by multiple research groups would be needed before that picture could be considered complete.