Understanding Water

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.

As the temperature of water increases, fewer clumps exist but according to the report, they are always there to some degree, in clumps of a similar size. The researchers also discovered that the disordered regions become more disordered as the temperature rises. This more detailed understanding of the molecular structure and dynamics of liquid water at ambient temperatures mirrors theoretical work on "supercooled" water: an unusual state in which water has not turned to ice although it is far below the freezing point. In this state, theorists believe 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 in water at ambient temperatures important for life.

This work partially explains water's unusual properties. According to the report, water's density maximum at 4 degrees Celsius can be explained by the fact that the tetrahedral structures are of a lower density, which does not vary significantly with temperature, while the more disordered regions of a higher density become more disordered and thus less dense with increasing temperatures. 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 also explains the high surface tension that allows insects to walk across it.

According to the report, connecting the molecular structure of water with its bulk properties is important to many 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?" added postdoctoral researcher Congcong Huang, who conducted the X-ray scattering experiments, in the press release. "We must understand the simple before we can understand the complex."

How might this research impact the personal care industry? Considering that water is, 9 times out of 10, the first ingredient listed on a product label, it could immensely impact interactions between materials within a formula as well as the formula's long-term stability. As such, an improved understanding of its structure coupled with the knowledge of other material's structures holds the capacity to formulate for improved efficacy. But to Huang's point, the simple must be understood before the complex.

This research was conducted by scientists from SLAC, Stockholm University, Spring-8, University of Tokyo, Hiroshima University, and Linkoping University. The work was supported by the National Science Foundation, the Swedish Foundation for Strategic Research, the Swedish Research Council, the Swedish National Supercomputer Center and the Japanese Ministry of Education, Science, Sports and Culture through a Grant-in-Aid for Scientific Research.

SLAC National Accelerator Laboratory is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the U.S. Department of Energy Office of Science. SLAC's Stanford Synchrotron Radiation Lightsource is a national user facility which provides synchrotron radiation for research in chemistry, biology, physics and materials science tomore than two thousand users each year.

-SLAC National Accelerator Laboratory

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