Super, cool: Liquid, crystal

Crystallizing liquid

A team in Salt Lake City has demonstrated why water can remain in the liquid state even if it is cooled as low as 48 degrees Celsius below its normal freezing point. However, their research also suggests that it is impossible to maintain supercooled water as a liquid at below this temperature.

"The change in structure of water controls the rate at which ice forms," explains Valeria Molinero now, she and colleague Emily Moore have shown that the thermodynamics of water and its rate of crystallization are effectively controlled by the change in the structure of liquid water as it approaches the structure of ice.

We drink water, bathe in it and we are made mostly of water, Molinero say, yet this almost ubiquitous substance still poses several major mysteries about its structure, behaviour, its physics and its chemistry. The University of Utah chemists now believe they have solved at least one of the paradoxes about water, to help explain how the substance can continue to exist as a liquid well below its nominal freezing point. Molinero adds that depending on the temperature and pressure, water ice can actually exist as 16 different crystalline forms in which water molecules cling to each other via well-known, but not fully understood, hydrogen bonds.

Pure, supercooled liquid water at -48 Celsius ultimately succumbs to the cold and solidifies, but it is not simply the drop in temperature that forces the natural change of state. Close to this temperature the molecular structure of water changes physically to form tetrahedral shapes, with each water molecule loosely bonded to four others, according to a new study by Molinero and Moore. The tetrahedral discovery suggests that there is an intermediate structure between liquid and ice. This helps explain the mystery of what determines the temperature at which water is going to freeze, says Molinero. "Intermediate-ice is the name we give to a structure that forms in deeply supercooled water. It is a manifestation of the approach of the structure of the liquid to the structure of ice," Molinero told SpectroscopyNOW.

Icy intermediates

"This intermediate ice has a structure between the full structure of ice and the structure of the liquid," Molinero adds. "We're solving a very old puzzle of what is going on in deeply supercooled water." "If you have liquid water and you want to form ice, then you have to first form a small nucleus or seed of ice from the liquid. The liquid has to give birth to ice," says Molinero. Yet in very pure water, "the only way you can form a nucleus is by spontaneously changing the structure of the liquid," she adds. Molinero says key questions include, "under what conditions do the nuclei form and are large enough to grow?" and "what is the size of this critical nucleus?"

Molinero points out that even though the team has apparently solved one paradox of how liquid water stays liquid below its freezing point, the researchers also hint that tiny amounts of liquid water might exist theoretically even below -48 Celsius when almost all the water has formed either crystalline ice or amorphous water glass. Any remaining droplets of liquid water must survive for only very fleetingly, however; for too short a time for this super-supercooled liquid water to be detected or analysed, unfortunately.

The researchers explain that the discovery is not merely of academic interest. Atmospheric scientists studying climate change and the effects of water and ice particles in the atmosphere require detailed properties in order to help in the validation of their models of atmospheric phenomena and ultimately global warming. "You need that to predict how much water in the atmosphere is in the liquid state or crystal state," Molinero says. "This is important for predictions of global climate." Specifically, such knowledge is important to understanding how much solar radiation is absorbed by atmospheric water and ice. Water vapour, after all, is a most potent greenhouse gas. Liquid water as cold as -40 Celsius has been found in clouds.

Molinero and Moore used computers at the University of Utah's Center for High Performance Computing to simulate the behaviour of supercooled water and to build a computer model using real data that is 200 times faster to compute than previous models. "Computers provide a microscopic view through simulation that experiments cannot yet provide," Molinero says. The computational process on almost 33,000 water molecules took several thousand hours of computer time to determine how the heat capacity, density and compressibility of water changes as it is supercooled, and to simulate how fast ice crystallises.

When asked about the possibility of water perhaps freezing at even lower temperatures Molinero told SpectroscopyNOW: "When water is confined in nanodroplets or in nanopores, the freezing point can be depressed even further. A key result of our work is that it is not so much the temperature but the ordering of the molecules in liquid water that determine how fast ice will form. Conditions that change the structure of water (e.g. salts, nanoconfinement), effectively delay its freezing," Molinero adds.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.