Thursday, June 19, 2014

How well do we understand water?

Water is all around us and inside us, it is essential for life as we know it.

It's chemical formula is one that nearly everyone knows regardless of their background. Water has a very simplistic structure as well so what else can we learn about water?

It turns out there is still a lot we can learn about water as was exhibited by two papers that were published in a recent issue of the journal Nature.

Image of the electron localization of water
by JVertrees
One aspect of water which we still can't fully explain is why it floats when it freezes. This characteristic is unlike other molecular substances. Without this characteristic marine life could not survive as lakes, rivers and oceans could freeze solid. To investigate the properties of water as it freezes, scientists at Princeton University used computer modeling. Their results indicate that at very cold temperatures and above a certain pressure water actually splits into two different liquid phases. This finding supports the 'liquid-liquid transition' proposed by Eugene Stanley in 1992. The authors propose that these two liquid phases may be responsible for the areas of low and high density that are known to exist in forming ice. As ice solidifies the low density regions predominate leading to a solid form that is less dense than the liquid form. If this dual nature can be observed in experiments it could improve the predictive ability of our weather and climate models.

The second paper could go a long way toward providing the experimental evidence necessary to study water under different conditions. In this paper scientists at the Stanford Linear Accelerator Center used the Linac Coherent Light Source X-ray laser to collect rapid-fire snapshots of waters molecular structure in the instant before it freezes.

For their experiments the researchers produced a steady stream of very pure water droplets in a vacuum that were directed toward a laser that was pulsing with a femtosecond shutter speed. As the water traveled towards the laser beam some of the liquid evaporated supercooling the remaining water to temperatures below minus 42 F ( -41 C). Their results showed the molecular structure of water is continuously transforming just before freezing and further cooling dramatically accelerates the structural changes observed. It is hoped that by pushing these experiments into colder regions closer to the transition to a solid that they will be able to identity the theories that best explain waters behavior.

Some information for this post was obtained from the Science Daily articles:

Familiar yet strange: Water's split personality revealed by computer model

Scientist take first dip into water's mysterious 'no man's land'

Original articles are:

J.C. Palmer et al Metastable liquid–liquid transition in a molecular model of waterNature, 2014; 510 (7505): 385 

J.A. Sellberg et al  Ultrafast X-ray probing of water structure below the homogeneous ice nucleation temperatureNature, 2014; 510 (7505): 381