WE ALL drink it, and most of us bathe in it and let it drain away without a second thought. But for all its familiarity, water is a very bizarre liquid. Chill most substances and they shrink. Not water: cool it below 4 掳C and it stops shrinking and starts to expand-as anyone who has had burst pipes in winter knows all too well. Most substances are denser in their solid form than as a liquid-but not water. Icebergs float.
The weirdness doesn鈥檛 stop there, either. Water is astonishingly hard to heat, demanding ten times as much energy as the same mass of iron. Its surface tension is higher even than that of syrupy stuff like glycerol. Oh yes, and water is a terrific solvent, embracing a huge range of dissolved substances.
How does water do it? For years, the commonest liquid on the planet has had scientists baffled. Next week, chemists and physicists will converge on the Greek island of Crete to describe their latest attempts to understand this strange substance at a meeting sponsored by the NATO Advanced Study Institute.
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Secrets revealed
There is a growing sense of excitement among the researchers who will be there, because water is beginning to yield is secrets to a combined assault by theorists and experimental scientists.
Most of chemistry focuses on the bonds that hold atoms together to form molecules. But with water鈥檚 simple molecule of one oxygen and two hydrogen atoms the interest lies elsewhere. 鈥淚t鈥檚 the chemical bonds between water molecules that hold the key to many of its properties,鈥� says David Clary of the chemistry department at University College London, one of the organisers of this week鈥檚 meeting. 鈥淭wo water molecules are joined by hydrogen bonds that are at least ten times weaker than typical chemical bonds, meaning that these bonds are strong enough to bind, but weak enough to be easily broken,鈥� Clary says. 鈥淭his allows water molecules to form into all kinds of configurations which would otherwise be impossible. If the hydrogen bonds were just a tiny bit stronger, then water would still be a solid at 100 掳C.鈥� If so, life on Earth would probably have been a non-starter (see 鈥�Why water needs to be weird鈥�).
Working with PhD student John Gregory-plus a thumpingly powerful computer capable of a billion calculations a second-Clary has been studying exactly how those very feeble bonds make themselves felt. 鈥淥ur view has been that if we could understand the behaviour of one water molecule, then two joined together, then three, four, five and so on, we may be able to predict the properties of bulk water,鈥� says Clary. The final triumph would be to come up with the correct value for water鈥檚 well-known properties, like its boiling point and freezing point.
The basic building block of Clary and Gregory鈥檚 computer model is the single water molecule, which chemists have long known to be a V-shaped arrangement with an oxygen atom at the apex and two hydrogen atoms sticking out at around 104 degrees to each other. In bulk water, the propensity for hydrogen bonding means that these V-shaped molecules act like so many drunks at a disco, constantly grabbing any others that pass by, briefly forming a small group or 鈥渃luster鈥� before letting go again and trying their luck elsewhere.
Stable mates
These clusters, which survive for less than a trillionth of a second, are what determine the properties of bulk water. Clary and Gregory have been trying to find out what these clusters are like-and in particular what sort of cluster is most stable, and thus has the biggest influence on the properties of water. In principle, there is a bewildering variety of possible clusters, because the hydrogen and oxygen atoms from different water molecules can come together in a colossal number of ways. To sort out which ones will actually form, Clary and Gregory plugged appropriate sets of numbers into Schr枚dinger鈥檚 equation, the master equation of quantum mechanics. Of the very few clusters that clear this hurdle, one will be the most stable.
Clary and Gregory found their quarry using what are called quantum Monte Carlo methods, in which a computer creates millions of randomly generated configurations of water molecules, checks each one out to seek whether it complies with Schr枚dinger鈥檚 equation and then works out how stable it is.鈥漌e have now found the most stable clusters of two, three, four and five water molecules-and they all join up to form flat, two-dimensional rings鈥�, says Clary. 鈥淏ut when you add just one more water molecule, things change dramatically鈥�.
Gregory and he discovered that when six water molecules come together-forming a 鈥渉examer鈥�-they can snap up and out of two dimensions, and take up three-dimensional shapes. Some are like a book, with the bonds between the hydrogen and oxygen atoms forming the edges of the book and its spine (see Diagram). Others are like a prism, with triangular arrangements of hydrogen and oxygen at each end. But according to the calculations, the most stable hexamer of all is a cage-like structure, like two Egyptian pyramids glued base-to-base. Clary and Gregory believe this hexamer is a vital clue to many of the mysteries of water.
Real life
But computer simulations are one thing-only experiment can decide if the hexamers are real. Enter Richard Saykally of the chemistry department at the University of California in Berkeley. Saykally and his team can put theories such as Clary and Gregory鈥檚 to the test by showing experimentally whether clusters they predict really exist.
The technique they use goes by the name of vibration-rotation-tunnelling spectroscopy. It involves squirting a mixture of water vapour and argon through a nozzle at supersonic speeds, and then probing it with laser light. The technique exploits the fact that the shape of an object affects the way it spins. For example, though a coin set spinning about its centre behaves quite differently from one tossed in the air, the two rotation rates are related through the shape of the coin. Similarly, you can deduce the shape of a water cluster from how it rotates in three dimensions.
Clary and Gregory calculated the three 鈥渞otational constants鈥� characteristic of the cage-like hexamer they predicted would be found in bulk water. Saykally and his team went to work with their supersonic water jet to see if what they found matched the modellers鈥� results.
The rapid expansion as the mixture as it is ejected from the nozzle cools the water molecules to just a few degrees above absolute zero-minimising the thermal 鈥渘oise鈥� that disturbs the clusters. By tuning their infrared laser to different frequencies, the Berkeley team were then able to work out the average rotational constants for clusters in bulk water. The results, published in Nature last summer, were unequivocal: each number was within just 3 per cent of the values calculated for the cage-like hexamer. 鈥淚t鈥檚 a great example of theory and experiment working together,鈥� says Clary.
Further experiments revealed that the average distance between the oxygen atoms in the hexamer cluster is very close to that in bulk water, and the charge distribution is also very similar. All this suggests that the cage-like hexamer plays a key role in determining the overall properties of water. So it should cast new light on some of water鈥檚 mysteries-such as its amazing ability to dissolve compounds. 鈥淥nly three-dimensional structures can explain this,鈥� says Clary. 鈥淔or substances to be dissolved, they have to be encapsulated, and you can鈥檛 do that with flat structures.鈥� Also, the hexamer arrangement creates a highly uneven distribution of electrostatic charge, which tears apart many other molecules into their constituent ions.
What everyone would like is a way to calculate the precise values of properties such as boiling point, freezing point and surface tension from first principles, but this is some way off yet. 鈥淭o work these bulk properties out, we need to look at thousands to hundreds of thousands of clusters, and the computational difficulty increases as the square of the number of clusters you work with,鈥� says Clary. 鈥淥ur current calculations involving just a few clusters already take a few days, so a big effort is needed to make these bigger calculations more efficient鈥�.
Not that water researchers are exactly kicking their heels in the meantime. At the University of Manchester Institute of Science and Technology, Jichen Li is working on the properties of hydrogen bonds among large numbers of water clusters. Working with David Ross at Salford University and colleagues at Birmingham University and the Rutherford Appleton Laboratory, Li has amassed evidence that there may be more than one kind of hydrogen bond, and that this may help explain water鈥檚 weirdness. 鈥淲e鈥檝e been making detailed calculations based on existing experimental data, and these suggest that there are in fact two types of hydrogen bond between water molecules, one of which is stronger than the other,鈥� says Li. 鈥淭he calculations suggested that the ratio of their strength was almost one to two.鈥�
Crystal maze
Li and his colleagues put the theory to the test in experiments last year that used neutrons from the ISIS source at the Rutherford Appleton Laboratory near Oxford to bombard crystals of ice. The neutrons strike the nuclei of atoms in the crystals and set them vibrating. By studying the resultant wobbles of the crystals, Li was able to work out the strengths of the bonds holding them together. Sure enough, he found that there were two different strengths in the bonds binding molecules in the ice.
The next step is to find out if hydrogen bonds have different strengths in liquid water too. Neutron scattering is far less effective as a probe of forces in liquids because the target is not as rigid. Even so, Li says it is providing some tentative evidence that the difference in bond strengths does exist in liquid water.
So what does this have to do with the unusual properties of water? 鈥淭he large temperature range in which liquid water exists might be explained if melting requires the breaking of the weaker bonds only, whereas boiling requires the breaking of the stronger bonds too,鈥� says Li. 鈥淲hen the temperature is high enough to break the strong bonds, water molecules can leave the liquid phase and evaporate.鈥�
Li suspects the different bond strengths might also cast light on a long-standing mystery about living creatures-including humans: the importance of a fixed body temperature. A vast range of biological activity, from the sex of certain reptiles to the malignancy of tumours, seems to depend critically on temperature. But why? Virtually all living organisms are made predominantly from water, and according to Li, the answer may lie in the proportion of strong to weak hydrogen bonds in liquid water. 鈥淭o change [a hydrogen bond] from strong to weak, you have to reach certain critical temperatures, and one of them turns out to be 37 掳C,鈥� says Li. 鈥淭he human body temperature is also 37 掳C. We think that this is significant.鈥� It could be a coincidence-and the idea is certainly speculative. 鈥淚t鈥檚 an interesting deduction,鈥� says Saykally. 鈥淏ut it has not been well-tested and there are other possible explanations.鈥�
What does seem clear is that water has hidden depths that are still far from fully explored. Hints of just how deep these mysteries run emerged while Clary and Gregory were searching for the most stable form of water hexamer. They found two candidates for the most stable water hexamer: the hexagonal cage, and the prism-like structure. At first, the prism structure seemed slightly more stable. But then they took into account effects due to zero-point fluctuations (ZPFs).
A consequence of Heisenberg鈥檚 famous uncertainty principle, which forbids the exact properties of a system being known at any given time, ZPFs are bursts of energy that emerge from nowhere to jostle everything around us-including water clusters. 鈥淲hen we took these into account, the cage-like structure emerged as the most stable form of water cluster,鈥� says Clary. 鈥淏ut if all the structures in liquid water were the prism type, basic properties of water like its boiling point would be quite different.鈥�
If this is right, the weirdness of water could be all down to empty space, which puts waiting for the kettle to boil into a whole new perspective.
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Why water needs to be weird
We all owe our existence to the weirdness of water-as the Harvard biochemist Lawrence Henderson pointed out as long ago as 1913. For example, if water behaved like most materials and contracted on freezing rather than expanding, ice would be denser than water, and would sink to the seabed. There would be none of the insulating layers of ice that conveniently form on the surfaces oceans and lakes, allowing marine and aquatic life to survive in the unfrozen water below. Instead, the ice would sink to the bottom, from where it would inexorably build up until the oceans froze solid.
The enormous specific heat capacity of water means that it takes a lot of solar heating to warm the oceans, and once warmed they are slow to cool. This protects us from sudden climate changes, and allows ocean currents such as the Gulf Stream to carry solar heat from the tropics towards the poles.
The anomalous behaviour of water also plays a key role in life at a more fundamental level. Its relatively high surface tension leads to biological compounds concentrating near liquid surfaces, speeding up biological reactions.
The powerful solvent properties of water are also widely exploited by living creatures: many biologically active compounds are switched on or off by changes in the concentrations of dissolved ions such as sodium and potassium. Individual water molecules, with their tendency to form hydrogen bonds, also stabilise the structure of proteins, whose action depends crucially on their shape.
It鈥檚 not all good news though. That same keenness to form hydrogen bonds makes water a real headache for researchers trying to solve the mystery of the origin of life. Water tears apart the twin strands of DNA, on which life depends for reproduction. While modern forms of life have developed ways of combating this, it is hard to see how molecules such as DNA and RNA could have survived long enough on the primordial Earth to become established as the carriers of genetic information.