IT IS the second most abundant molecule in the universe and the most studied material on Earth, yet its properties still defy explanation. Now it seems that at least some of the uncertainty about water can be addressed by considering uncertainty of the quantum kind.
Many of water鈥檚 properties, such as the fact that water containing deuterium or tritium, the 鈥渉eavy鈥� isotopes of hydrogen, freezes above 0 掳 C, have yet to be fully explained, and previous attempts to model such behaviour have been unsuccessful. Now Roberto Car and Joseph Morrone at Princeton University say this is because certain quantum effects have been neglected.
Experiments which probe water molecules by firing neutrons at them show that the hydrogen atoms have a range of positions and momenta relative to the oxygen atom they are bonded to. At this level, Heisenberg鈥檚 uncertainty principle, which says that quantum particles cannot simultaneously have a well-defined position and momentum, comes into play. The resulting uncertainty can affect the weak attractions that are known to exist between hydrogen nuclei in adjacent water molecules. If these nuclei are able to get closer to each other because of the uncertainty principle, the attractions will strengthen, and this can account for some of the strangeness of water, says Car.
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He and Morrone created a computer model of water in which the hydrogen nuclei are replaced by 32 鈥渂eads鈥� that constantly move around, reflecting the uncertainty in their position and momentum. Their results, submitted to Physical Review Letters, show that incorporating the uncertainty principle gives the hydrogen bonds in their simulation similar strengths and positions to those in water.
Besides having the potential to explain some of water鈥檚 anomalous properties, their model could eventually offer a clearer picture of the central role of water in the processes of life, such as protein folding. Car points out that the isotope of hydrogen that water is made from is crucial to life: water containing deuterium or tritium derails many biological processes and is poisonous to most organisms.
Morrone and Car鈥檚 innovation is interesting, says Martin Chaplin, an expert on water at London South Bank University. However, he says there is still a long way to go before we fully understand water. Any model would have to show agreement with properties like melting point, viscosity and density to be truly noteworthy, he says.
