Understanding Water

Posted: August 12, 2009

The US Department of Energy's SLAC National Accelerator Laboratory in Menlo Park, Calif., together with universities in Sweden and Japan, closely examined water and its molecular idiosyncrasies. Acording to SLAC, while water is in abundance in our bodies and our planet, the molecular structure of water has remained a mystery, with the substance exhibiting many strange properties that are still poorly understood. Their recent work, however, offers insight into its unusual bulk properties.

Water, according to an SLAC report, exhibits 66 known anomalies including a varying density, large heat capacity and high surface tension; and contrary to other liquids, which become denser as they get colder, water reaches its maximum density around 4 degrees Celsius. Above and below this, 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 in the press announcement, who is leading the experimental efforts. The way in which molecules arrange themselves in water's solid form (ice) was previously established: the molecules form a tight "tetrahedral" lattice, with each molecule binding to four others. Discovering the molecular arrangement in its liquid form, however, has been much more challenging.

According to the report, for more than 100 years, this liquid structure has been the subject of intense debate and the current textbook model holds that, since ice consists of tetrahedral structures, liquid water should be similar but less structured since heat breaks bonds. As ice melts, the tetrahedral structures are said to loosen their grip, breaking apart as the temperature rises but still striving to remain as tetrahedral as possible, resulting in a smooth distribution around distorted, partially broken tetrahedral structures.

Recently, Nilsson and colleagues directed strong X-rays generated by the Stanford Synchrotron Radiation Lightsource at SLAC and the SPring-8 synchrotron facility in Japan at samples of liquid water and their experiments suggested that the textbook model of water at ambient conditions was incorrect and that, unexpectedly, two distinct structures--either very disordered or very tetrahedral--exist at any temperature. In a paper published on Aug. 10, 2009, in the Proceedings of the National Academy of Sciences, the researchers showed the additional discovery that the two types of structure are spatially separated, with the tetrahedral structures existing in "clumps" consisting of about 100 molecules surrounded by disordered regions; the liquid is a fluctuating mix of the two structures at temperatures ranging from ambient temperatures to near boiling point.