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Monday, 15 September 2014

Wondering about water

One day back in the last century (literally) while I was working on my PhD, I went on an expedition into the University’s library archive. I remember it as a dusty cavern with rows of metal shelves creaking under the weight of volumes that smelled of old paper. You had to get a special key to get in, and I remember nervously looking over my shoulder just in case the lights flickered out and my life turned into a cheesy horror movie. I can’t remember what I was actually looking for now, but what I do remember is that I got diverted. In an ancient, dusty volume I found a paper which had been written about a hundred years ago, at the start of the 20th century.

It concerned the structure of water, its bonding and the shape of the molecules. It was a bit of a revelation at the time. I had been drawing the classic ‘Mickey Mouse’ water molecule diagram for years and I’d just never thought about the fact that there was a time when the structure of water was an unknown. Something that people argued over and, indeed, published papers about.

Now, fast forward a few years, and Things We Don’t Know have asked me to write about water clusters, something which is currently at the edges of chemistry. It almost seems meant to be.

Diagram of water molecule H20 showing pairing of electrons, electron sharing between atoms or covalent bonding, and the effective dipole moment of the molecule. Image copyright Things We Don't Know (CC BY 3.0).
Scientists use the "delta" symbol δ to mean “a little bit”. Oxygen is very electronegative, which means it draws the electrons it's sharing with hydrogen (in a covalent bond) towards itself. This leaves the hydrogens slightly positive and the oxygen slightly negative. You might also think it resembles a famous cartoon character, but we couldn't possibly comment. Image ©Things We Don't Know (CC BY 3.0)

Water is important stuff. Without it, life wouldn’t have evolved on this planet. It’s made up of one atom of oxygen and two atoms of hydrogen, joined together with the oxygen in the middle as H-O-H. One of these elements, oxygen, is the second most electronegative element (topped only by its periodic table neighbour fluorine). Electronegativity is a much-abused term in the world of pseudoscience; all it actually means is the ability of an atom to attract electrons in a covalent bond. Hydrogen is far from the least electronegative, but it’s pretty wimpy by comparison. So basically, examine a water molecule and you find that oxygen has greedily dragged the bonding electrons around itself, like a child refusing to share her sweets with the poor, deprived hydrogen atoms.

As a result, although H2O is a covalently-bonded molecule, the oxygen atom is a little bit negatively charged (thanks to its big electron-stealing meanie nature) and the hydrogen is a little bit positively charged, creating something called a dipole.

And THIS means that water is a bit sticky. Those little-bit-negative oxygens are attracted to the little-bit-positive hydrogens on other water molecules, just like opposite poles on a magnet. They join up with each other, forming something called a hydrogen bond, and they don’t want to let go.

These hydrogen bonds lead to all kinds of interesting properties, for example we all take it for granted that water boils at 100°C (under standard conditions, blah blah), but stop for a moment and compare that to H2S, hydrogen sulfide. They have the same number of atoms, joined up in broadly the same way. Sulfur sits directly under oxygen in the periodic table, so they should be similar, right? But no. Quite apart from its distinctive smell of rotten eggs, hydrogen sulfide boils at -60°C. Switch out that sulfur for an oxygen and the boiling point leaps 160 degrees. That’s a pretty impressive effect.

There’s more. Water expands as it freezes. Another thing we all take for granted, despite the fact that things generally expand when they’re heated and contract when they’re cooled. In fact water does contract as it cools, until about 4°C. At this point it starts to expand again, with the result that solid water, ice, is less dense than liquid water. This odd behaviour not only gives you lovely, floaty ice cubes in your frosty drink, it’s also very handy indeed for water-based life forms (and therefore for us, since as far as we know all life started in the oceans) because it means that oceans, rivers, lakes and so on stay liquid under a thick coating of ice even in the coldest temperatures, rather than freezing from the bottom up and thus metaphorically pressing Ctrl-Alt-Delete on any and all life forms every time there’s a cold snap.

Why does this happen? Because water has this sticky property, water molecules don’t hang around on their own. No, they are always joined up to lots of other water molecules. These interactions are pretty strong, meaning a lot of heat energy is required to break them and turn liquid water into a gas, giving water it’s high boiling point. They aren’t random, either. The water molecules join up in a particular pattern, and it’s generally thought to be sort of hexagonal (although other formations have been suggested[1]).

Diagram explaining Hydrogen bonding between water molecules, in which the positive hydrogens are attracted to the negative oxygens of different molecules.
Hydrogen bonding occurs in water when a slightly-positive hydrogen from one molecule is attracted to the slightly-negative oxygen in another. Hydrogen bonds are really strong when compared to attractions between other kinds of molecules, but quite wimpy when compared to covalent bonds(Public domain image)

Which brings us to water clusters. Once chemists understood that water molecules joined up like this, it was only a matter of time before they started to wonder what lots of water molecules joined up would look like. Two seems fairly straightforward, but what about twenty? Or fifty? Or a hundred?

A hypothetical water cluster made from 100 water molecules, forming an icosahedral shape.
A hypothetical water cluster made from 100 water molecules, forming an icosahedral shape. The latest thinking is that hydrogen bonds are forming and breaking all the time, and therefore these structures are continuously forming, breaking apart and reforming in different shapes and sizes. Image credit: Danski14 via wikimedia commons (CC BY-SA)

Do these clusters grow in size forever, or are they limited? What shapes are formed? Do water clusters interact with each other somehow? How long do they hang around? And what happens when we stop thinking about pure water for a moment, and start to think about real life, where water has all kinds of other bits and pieces dissolved within it?

This is where we get to the edges of current science, although scientists have been wondering about water’s strange properties for a very long time. About a hundred and twenty years ago Wilhelm Röntgen suggested that when water reaches its most dense point, at about 4°C, it is actually a mixture of small, hydrogen-bonded, ice ‘crystals’ and individual water molecules[2]. This ultimately led to early ideas of water clusters, the clusters being the groups of ‘joined-up’ water molecules.

Röntgen’s ideas have been refined over the years of course. For starters his model suggested that in liquid water a significant number of water molecules aren’t hydrogen-bonded to others. Later thinking was that hydrogen bonds were distorted in the liquid state, rather than actually broken, and still later we have evidence that hydrogen bonds are breaking and reforming, but incredibly quickly: faster than 200 femtoseconds[3] (that’s 0.0000000000002 of a second, pretty speedy). But the idea of little clusters of water molecules has not gone away, in fact it’s an increasingly interesting area of research.

As technology, particularly in the areas of computing and lasers, has moved on, it’s become possible to look more closely at these clusters. Martin Chaplin, Emeritus Professor of Applied Science at the London South Bank University, has a particular interest in this area. On his website he explains that water clusters form and reform in a kind of endless dance: picking up new partners and dropping old ones. He also explains that once you have about 400 water molecules in a water cluster, ice is likely to form. This isn't much - by comparison, a drop of water contains something like 1,000,000,000,000,000,000,000 water molecules.

Chaplin’s work has particularly focused on smaller clusters containing 280 water molecules, gathering significant evidence for their existence. This model does go some way towards explaining water’s anomalous properties, and it’s also possible to put together a simulation where other molecules combine with this structure, and investigate how they interact with water. For example, he has already looked at sugars. He suggests it might be the case that water clusters cause odd effects, such as higher-than-expected concentrations of solute molecules on the surface of a liquid in otherwise very dilute solutions.

This leads us towards the somewhat murky area of homeopathy, with the idea that water clusters might be the basis for ‘water memory’: the notion that water can somehow retain the properties of substances that were once dissolved within it. Even if this exists, it’s difficult to see how water clusters could really be the basis for it since they are such incredibly short-lived things: if the cluster only exists for a tiny fraction of a second, how can it possibly ‘store’ the shape of a biological molecule? Also, as Chaplin himself points out, even if there does turn out to be a mechanism for water memory, it doesn’t explain how or why the effects are (according to homeopaths) amplified with increasing dilution[4].

So, we know that water clusters, of various sizes, exist. Some have actually been studied, many others simulated. But many unanswered questions remain. It’s not yet fully clear whether these clusters interact with biological systems in any kind of meaningful or significant way, nor really how they interact with each other to make up the sloshy wet stuff in your bathtub. And perhaps most interestingly of all, after all this we still don’t have a model that explains the simple little fact that water is at its most dense at 4°C.



References
why don't all references have links?

[1] Ludwig, R. (2001), "Water: From Clusters to the Bulk". Angew. Chem. Int. Ed., 40: 1808–1827.
[2] W. K. Rontgen, "The structure of liquid water.” Ann. Phys. 1892, 45, 91-97
[3] Smith, Jared D et al. "Unified description of temperature-dependent hydrogen-bond rearrangements in liquid water." Proceedings of the National Academy of Sciences of the United States of America 102.40 (2005): 14171-14174.
[4] Chaplin, Martin F. "The memory of water: an overview." Homeopathy 96.3 (2007): 143-150 doi:10.1016/j.homp.2007.05.006