redOrbit Staff & Wire Reports – Your Universe Online
Despite the fact that it covers more than two-thirds of the planet’s surface, and that US households can use as much as 100 gallons of it per day, there are still mysteries to be uncovered about water – as evidenced by a pair of studies published in the June 18 edition of the journal Nature.
In the first paper, researchers from the Department of Energy’s SLAC National Accelerator Laboratory completed the first-ever structural observations of liquid water at temperatures of down to negative 51 degrees Fahrenheit – the so-called “no man’s land” at which some of the more unusual properties of the compounds became extremely amplified.
While scientists have previously established that water can remain in its liquid form in the extreme cold, this study marks the first time that anyone has been able to analyze its molecular structures under such conditions. Using the SLAC’s Linac Coherent Light Source (LCLS) X-ray laser, the study authors have made it possible to study H2O under these exotic conditions, while also gaining new insight into how it behaves in its more natural states.
“Water is not only essential for life as we know it, but it also has very strange properties compared to most other liquids,” lead investigator Anders Nilsson, deputy director of the SUNCAT Center for Interface Science and Catalysis (a joint SLAC/Stanford facility), explained in a statement. “Now, thanks to LCLS, we have finally been able to enter this cold zone that should provide new information about the unique nature of water.”
Water, despite possessing a relatively simple molecular structure, is unusual in several ways, according to Nilsson and his colleagues. For example, its liquid form is more dense than its solid form (hence the reason that ice floats), and it is capable of absorbing tremendous amounts of heat. Furthermore, its density profile makes it so that lakes and oceans do not freeze all the way to the bottom, which prevents fish from dying out during the winter months.
When purified water is supercooled, there is nothing to seed the formation of ice crystals, which means that these traits are enhanced and it can remain liquid at far lower temperatures than is usually possible. The so-called “no mans land” of temperatures ranges from roughly minus 42 to minus 172. For decades researchers have attempted to investigate what happens to water molecules at these temperatures without having to rely on simulations.
“Now the LCLS, with X-ray laser pulses just quadrillionths of a second long, allows researchers to capture rapid-fire snapshots showing the detailed molecular structure of water in this mysterious zone the instant before it freezes,” the SLAC explained.
Using this technology, Nilsson’s team discovered that the structure of a water molecule “transforms continuously” in this temperature range, and that additional cooling causes those changes to “accelerate more dramatically than theoretical models had predicted.”
In the second study, scientists from Princeton University used computer models to study freezing water in the hopes they could discover why ice floats when the majority of liquids crystallize into dense solids that wind up sinking. They discovered that water had a “split personality” of sorts when the temperature is cold enough and the pressure is high enough.
Under those specific conditions, Princeton chemical and biological engineering professor Pablo Debenedetti and his associates learned that water is capable of spontaneously splitting into two different liquid forms, each of which had different densities.
Those forms, the university noted in a statement, can co-exist in much the same way that oil and vinegar do in salad dressing, except that the water separates from itself instead of from a different molecule. If this newly discovered split personality can be replicated in future experiments, it could lead to a better understanding of the behavior of water in the cold temperatures found in high-altitude clouds, where it can exist in a supercooled state before forming snow or hail.
“The new finding serves as evidence for the ‘liquid-liquid transition’ hypothesis, first suggested in 1992 by Eugene Stanley and co-workers at Boston University and the subject of recent debate,” the university said. “The hypothesis states that the existence of two forms of water could explain many of water’s odd properties – not just floating ice but also water’s high capacity to absorb heat and the fact that water becomes more compressible as it gets colder.”
In most liquids, the molecules slow down immensely when they become colder, and eventually form a dense and orderly solid that will sink to the bottom if it is placed in liquid. As previously noted, however, ice floats in water due the unorthodox behavior of its molecules. There are some areas of lower density (fewer molecules contained within a specific volume) and regions of higher density, and as the temperature falls, the lower density regions become so prevalent that they seize control of the mixture and form a solid that is less dense than the original liquid.
“The work by the Princeton team suggests that these low-density and high-density regions are remnants of the two liquid phases that can coexist in a fragile, or ‘metastable’ state, at very low temperatures and high pressures,” the university said. Now that scientists have verified the two different forms of water, it could help them develop a theory that addresses how the liquid behaves at other temperatures, ranging from normal to supercooled.
“The research is a tour de force of computational physics and provides a splendid academic look at a very difficult problem and a scholarly controversy,” said Arizona State University professor C. Austen Angell, who was not involved in the research. “Using a particular computer model, the Debenedetti group has provided strong support for one of the theories that can explain the outstanding properties of real water in the supercooled region.”
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