Researchers reveal the internal dance of water

Aug 12, 2009 by Kelen Tuttle
This artist's depiction shows two distinct structures of water: in the foreground, tetrahedral low-density water; and in the background, distorted high-density water.

(PhysOrg.com) -- Water is familiar to everyone - it shapes our bodies and our planet. But despite this abundance, the molecular structure of water has remained a mystery, with the substance exhibiting many strange properties that are still poorly understood. Recent work at the SLAC National Accelerator Laboratory and several universities in Sweden and Japan, however, is shedding new light on water's molecular idiosyncrasies and offering insight into its strange bulk properties.

In all, water exhibits 66 known anomalies, including a strangely varying density, large heat capacity and high . Contrary to other "normal" liquids, which become increasingly denser as they get colder, water reaches its maximum density at about 4 degrees Celsius. Above and below this temperature, water is less dense; this is why, for example, lakes freeze from the surface down. Water also has an unusually large capacity to store heat, which stabilizes the temperature of the oceans, and a high surface tension, which allows insects to walk on water, droplets to form and trees to transport water to great heights.

"Understanding these anomalies is very important because water is the ultimate basis for our existence; no water, no life," said SLAC scientist Anders Nilsson, who is leading the experimental efforts. "Our work helps explain these anomalies on the molecular level at temperatures which are relevant to life."

How the molecules arrange themselves in water's solid form, ice, was long ago established: The molecules form a tight "tetrahedral" lattice, with each molecule binding to four others. Discovering the molecular arrangement in liquid water, however, is proving to be much more complex. For over 100 years, this structure has been the subject of intense debate. The current textbook model holds that since ice is made up of tetrahedral structures, liquid water should be similar, but less structured, given that heat creates disorder and breaks bonds. As ice melts, the story goes, the tetrahedral structures loosen their grip, breaking apart as the temperature rises, but all still striving to remain as tetrahedral as possible, resulting in a smooth distribution around distorted, partially broken tetrahedral structures.

Textbook model is incorrect

Recently, Nilsson and colleagues directed powerful X-rays, generated by the Stanford Synchrotron Radiation Lightsource at SLAC and the SPring-8 synchrotron facility in Japan, at samples of liquid water. These experiments suggested that the textbook model of water at ambient conditions is incorrect and that, unexpectedly, two distinct structures, either very disordered or very tetrahedral, exist no matter what the temperature.

In a paper scheduled for publication in the Proceedings of the National Academy of Sciences, the researchers reveal the additional discovery that the two types of structure are spatially separated, with the tetrahedral structures existing in "clumps" made of up to about 100 molecules surrounded by disordered regions; the liquid is a fluctuating mix of the two structures at temperatures ranging from ambient to all the way up near the boiling point. As the temperature of water increases, fewer and fewer of these clumps exist; but they are always there, in clumps of a similar size. The researchers also discovered that the disordered regions themselves become more disordered as the temperature rises.

"One can visualize this as a crowded dance restaurant, with some people sitting at large tables, taking up quite a bit of room - like the tetrahedral component in water - and other people on the dance floor, standing close together and moving slower or faster depending on the mood or 'temperature' of the restaurant; like the molecules in the disordered regions can be excited by heat, the dancers can be excited and move faster with the music," Nilsson said. "There's an exchange when people sitting decide to get up to dance and other dancers sit down to rest. When the dance floor really gets busy, tables can also be moved out of the way to allow for more dancers, and when things cool back off, more tables can be brought in."

This more detailed understanding of the and dynamics of liquid water at ambient temperatures mirrors theoretical work on "supercooled" water: an unusual state in which water has not turned into ice even though it is far below the freezing point. In this state, theorists postulate, the liquid is made up of a continuously fluctuating mix of tetrahedral and more disordered structures, with the ratio of the two depending on temperature - just as Nilsson and his colleagues have found to be the case with water at the ambient temperatures important for life.

Popular notion dispelled

"Previously, hardly anyone thought that such fluctuations leading to distinct local structures existed at ambient temperatures," Nilsson said. "But that's precisely what we found."

This new work explains, in part, the liquid's strange properties. Water's density maximum at 4 degrees Celsius can be explained by the fact that the tetrahedral structures are of lower density that does not vary significantly with temperature, while the more disordered regions - which are of higher density - become more disordered and so less dense with increasing temperature. Likewise, as water heats, the percentage of molecules in the more disordered state increases, allowing this excitable structure to absorb significant amounts of heat, which leads to water's high heat capacity. Water's tendency to form strong hydrogen bonds explains the high surface tension that insects take advantage of when walking across water.

Connecting the molecular structure of water with its bulk properties in this way is important for fields ranging from medicine and biology to climate and energy research.

"If we don't understand this basic life material, how can we study the more complex life materials - like proteins - that are immersed in ?" asked postdoctoral researcher Congcong Huang, who conducted the X-ray scattering experiments. "We must understand the simple before we can understand the complex."

More information: www.pnas.org/content/early/200… /0904743106.abstract

Provided by Stanford University (news : web)

Explore further: New complex oxides could advance memory devices

add to favorites email to friend print save as pdf

Related Stories

More Evidence for a Revolutionary Theory of Water

Jun 30, 2008

The traditional picture of how liquid water behaves on a molecular level is wrong, according to new experimental evidence collected by a collaboration of researchers from the Department of Energy's Stanford ...

Curtain may be closing on scientific water controversy

Jun 27, 2006

The curtain may be ringing down on a scientific controversy regarding the structure of water which arose two years ago. A new study by scientists with the U.S. Department of Energy's Lawrence Berkeley National ...

Physicists reveal water's secrets

Mar 03, 2007

It's essential to all life, and numerous research papers are published about it every year. Yet there are still secrets to reveal about water, that seemingly simple compound we know as H2O.

Supercomputer Study of Water

Feb 27, 2006

Familiar as it is, there's a lot we don't know about water -- such as the structure taken up by liquid water molecules. With a grant of time on one of the fastest computers in the U.S., researchers at UC Davis, ...

Scientists Observe Liquid Water Below Freezing

Jun 24, 2009

(PhysOrg.com) -- Below 0 °C, water turns to ice. But beyond that, or below about -75 °C, the ice may turn back into liquid water. While scientists have previously predicted this phase transition with computer ...

Scientists solve mystery of glassy water

Jan 31, 2008

Water has some amazing properties. It is the only natural substance found in all three states — solid, liquid and gas — within the range of natural Earth temperatures. Its solid form is less dense than its liquid form, ...

Recommended for you

New complex oxides could advance memory devices

Sep 17, 2014

The quest for the ultimate memory device for computing may have just taken an encouraging step forward. Researchers at The City College of New York led by chemist Stephen O'Brien have discovered new complex ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

Alexa
2 / 5 (4) Aug 12, 2009
..with the tetrahedral structures existing in "clumps" made of up to about 100 molecules surrounded by disordered regions...
These clumps may behave like solid metaparticles, thus explaining some anomalous phenomena, like splitting of water by radiowaves.

http://aetherwave...ory.html
JukriS
Aug 13, 2009
This comment has been removed by a moderator.
JukriS
Aug 13, 2009
This comment has been removed by a moderator.
E_L_Earnhardt
1 / 5 (1) Aug 13, 2009
Interesting! Development beginning with "NOTHING"
would be even MORE interesting! "WATER", as basic, is getting there. The "electron" as basic would move it back another step!