FOR most people, making ice is a simple matter of popping a tray of water in the freezer. But not for chemist Eun-Mi Choi and his colleagues at Seoul National University in South Korea. Sub-zero temperatures and novelty moulds are the last things on their minds. They prefer to make their ice by zapping water with electric fields – and they do it at room temperature.
“Ice existing at room temperature – ‘hot ice’ – makes the mind boggle,” says Denys Wheatley, a cell biologist who studies the effects of water on living systems at the University of Aberdeen, UK. “It just shouldn’t happen.”
Choi’s experiments earlier this year have ended a 10-year quest to find out whether hot ice can be made. But they have unwittingly sparked another mystery. According to Choi’s results, the electric field needed to transform lukewarm water into ice is surprisingly small. So small in fact that fields of similar magnitude are found in all sorts of nooks and crannies in nature, from cracks in rocks and clay particles to the crevices of proteins in our bodies. And the search is now on to find out whether hot ice really is lurking in nature.
Advertisement
The story of room-temperature ice began in 1995 with an accidental discovery by materials scientist Jacob Klein at the Weizmann Institute of Science in Rehovot, Israel. He found that organic liquids sandwiched between mica plates only a few nanometres apart could freeze at much higher temperatures than usual.
Soon afterwards it struck him that he might be able to create ice at room temperature by squashing water into a solid. So he spent the next six years trying to do it with water and other liquids. Klein could create solids out of most liquids, but water is no ordinary liquid. While most substances are denser in their solid forms than as liquids, ice is peculiar. Icebergs and ice cubes float because they are less dense than water, taking up more space after freezing than before. Eventually he realised that by packing the water molecules into a tight space he was actively suppressing freezing. So he gave up.
Klein, it seems, had missed one crucial ingredient for making room-temperature ice – an electric field. But just as he was abandoning the project, the hunt was taken up by biophysicists Ronen Zangi and Alan Mark, then at the University of Groningen in the Netherlands. In 2003, they set up computer simulations to see just what would happen to trapped water when an electric field was thrown into the mix.
Because the two hydrogen atoms in a water molecule carry a slight positive charge, and the oxygen atom a slight negative charge, adding an electric field can flip the haphazard arrangement of molecules, making them line up like soldiers. Zangi and Mark’s simulations revealed that this alignment provides enough order for the water to solidify, freezing into what is known as polar cubic ice even at room temperature (see Diagram).
“With a large electric field you could freeze a whole glass of water,” Mark says. But no one had confirmed their predictions – even with tiny amounts of water – until now.
Choi and his team trapped a thin layer of water between a metal plate and a sharp metal tip. Next they applied a weak electric field and gradually tapped on the water, slowly moving the tip downwards. When the tip was just 0.7 nanometres from the metal plate, it hit an obstruction. Choi had struck ice (Physical Review Letters, vol 95, p 085701).
“Hot ice may be hiding in all sorts of unexpected places in nature”
What surprised researchers is that Choi’s team created ice using a field of 106 volts per metre. That may sound high, but it is low enough to be found in nature, revealing the possibility that hot ice may be hiding in all sorts of unexpected places. The crevices of clay particles contain enough charge to harbour specks of ice at room temperature, an effect that could solve a mystery of cloud formation that has perplexed atmospheric scientists for years (see “A cloud never forgets”). And fields across the membranes of nerve cells and on the surfaces of proteins and polysaccharides could be high enough for mini icebergs to be riding around inside cells. “Almost everything we are made of could have a big enough charge to solidify water,” says Klein.
Wheatley believes that the hunt should now begin for icy areas in protein cavities. “Over very small distances on the surfaces of proteins, the charges can be gigantic,” he says. But not everyone shares this enthusiasm. “These fields are present every day in cells and yet this freezing transition has never been observed,” points out Fabio Bruni, a physicist at the University of Rome in Italy who studies water and proteins. Mark agrees that the lack of any reported ice near proteins is disconcerting. Both he and Klein stress that the conditions in Choi’s experiment were highly controlled and may not be reproducible in nature. “We can’t get too excited just yet,” Mark cautions.
Yet signs of hot ice might already have been seen without anybody realising it. Chemists studying the mobility of water have noticed that the molecules become sluggish around some ions that carry two or three positive charges, such as calcium and chromium. It can take an hour for the water molecules in the layer near these ions to move away and be replaced by others. In contrast, water molecules are extremely mobile around ions with single charges, such as potassium and sodium. Perhaps this slow-motion water indicates that it is freezing in a weak electric field.
And a question hovers over the issue of whether Choi’s substance qualifies as ice at all. To do so, it must have a crystal structure, and this has not been shown yet. It might just be a very viscous liquid. But whether hot ice is technically worthy of its name would make no odds in terms of its effect on cells. “To all intents and purposes the behaviour of a true solid and a very viscous liquid are the same,” says Klein.
Wheatley thinks that the consequences of that behaviour could be profound. “Water is the crucible of life – everything else is buzzing round in it. If that environment changes even slightly, it could change 100 different things in the cell,” he says. “It seems that this most common of liquids in our bodies is one of the least understood. It can still surprise us.”
A cloud never forgets
They say elephants never forget. And neither, it seems, do the specks of dust at the heart of cloud formation. Clouds form when supercooled water droplets turn into ice at around -10 °C. But these ice crystals can’t grow in empty space; the water molecules have to latch onto microscopic dust particles in the air – often specks of clay – and the crystals grow around them. But this dust appears to have a curious “ice memory”, which has baffled scientists for years.
“Lab tests show that the first time ice crystals form on a clay particle they need a temperature of around -10 °C,” says Clive Saunders of the University of Manchester, UK. Raising the temperature causes the ice to turn directly into water vapour, leaving the clay particle ready to seed a new ice crystal when the temperature plunges again. Yet the second time round, the ice forms at a higher temperature of about -5 °C. It is as if the dust carries a memory of having formed ice in the past – even though there shouldn’t be any traces of ice left on it.
The mystery could finally be solved if specks of ice can hide out in the electrically charged crevices of the clay particles, even when temperatures rise well above freezing. The newly forming ice might find it easier to grow onto ice that is already there, rather than starting from scratch. So hot ice could explain why it is easier to form ice crystals on clay the second time round.
