Bulk-water textbook models ripped up by new research

New research using high-powered x-rays has revealed that the textbook molecular model of bulk water at ambient conditions is incorrect.

In a paper published in the Proceedings of the National Academy of Sciences, the researchers, from the US Department of Energy's SLAC National Accelerator Laboratory and several universities in Sweden and Japan, revealed the additional discovery that two distinct structures, either very disordered or very tetrahedral, exist no matter what the temperature is.

In all, water exhibits 66 known anomalies, including a strangely varying density, large heat capacity and high surface tension. Contrary to other "normal" liquids, which become denser as they get colder, water reaches its maximum density at about 4ºC. 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."

The current textbook model holds that, since ice is made up of tetrahedral structures, liquid water should be similar, but less structured. As ice melts, 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 tetrahedral structures that are distorted and partially broken.

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. The two types of structure revealed by the research 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 to some degree, in clumps of a similar size. The researchers also discovered that the disordered regions themselves become more disordered as the temperature rises.

This new work explains, in part, the liquid's strange properties. Water's density maximum at 4ºC can be explained by the fact that the tetrahedral structures are of lower density, which 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.

The researchers believe that connecting the molecular structure of water with its bulk properties in this way is very 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 water?" asked postdoctoral researcher Congcong Huang, who conducted the x-ray scattering experiments. "We must understand the simple before we can understand the complex."

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