Resolving controversy at the water’s edge

HomeScience ReportsReports and NewsLife Sciences

Resolving controversy at the water’s edge

30.01.2012

High-level spectroscopy and computer simulations of specially diluted liquids reveal the long-debated structure of air–water interfaces

Water (H2O) has a simple composition, but its dizzyingly interconnected hydrogen-bonded networks make structural characterizations challenging. In particular, the organization of water surfaces—a region critical to processes in cell biology and atmospheric chemistry—has caused profound disagreements among scientists.

Figure 1: A ‘snapshot’ from a molecular dynamic simulation reveals that water molecules align at air–water interfaces as coordinated pairs linked by hydrogen bonds. Copyright : 2012 RIKEN

Now, Tahei Tahara and colleagues from the RIKEN Advanced Science Institute in Wako, in collaboration with researchers in Japan and Europe, have uncovered the presence of strongly bonded water pairs at the air–water interface1, rather than previously hypothesized ‘ice-like’ surface structures.

Observing surface water molecules, just a few monolayers thick, requires special experimental techniques that prevent interference by more plentiful bulk particles. One such approach is called vibrational sum frequency generation (VSFG), a laser-based method that selectively vibrates interfacial molecules. Previous VSFG measurements of surface water showed two vibrations that resemble signals recorded from bulk ice and liquid water states. Some scientists have proposed that these vibrations correspond to a partially disordered mix of liquid and four-coordinated ice-like surface structures—a theory at odds with thermodynamic evidence.

... more about:
»Advanced Investigator Grant »HD-VSFG »RIKEN »surface structures »surface water »water molecule »water surface

Other VSFG experiments, however, have suggested that the two vibrations arise from one structure undergoing coupling interactions. To resolve this dispute, Tahara and colleagues turned to heterodyne-detected VSFG (HD-VSFG), a high-level spectroscopic method that detects how the phase of the vibrational signals shifts with respect to the incident laser beam—information that can pinpoint molecular orientation at interfaces.

The researchers then employed a trick using isotopes to account for coupling effects of water molecules: they added the deuterium (D)-bearing compounds HOD and D2O to pure water. By gradually diluting the number of oxygen–hydrogen (OH) bonds in the liquid, these isotopes suppress the interactions between the vibrational modes that normally occur. The remaining ‘stretching’ vibrations that extend and contract OH bonds then provide clear information about the interfacial water structure.
The team’s experiments revealed that as the isotopic dilution progressed, the two OH bands merged into a single peak, which is clear evidence of vibrational coupling within a single structure. After performing molecular dynamic simulations and comparing the results to the HD-VSFG data, a new picture emerged of the air–water interface (Fig. 1): the low-frequency OH vibrations were due to tightly joined pairs of liquid water molecules.

“We were wondering what kind of structure can have strong hydrogen-bonds other than ice at water surfaces,” says Tahara. “When our experiments and [co-author] Morita’s simulation answered the question, rather than surprise I felt that ‘This is it!’ because its structure is quite reasonable.”

The corresponding author for this highlight is based at the Molecular Spectroscopy Laboratory, RIKEN Advanced Science Institute

gro-pr | Research asia research news
Further information:
http://www.riken.jp
http://www.researchsea.com

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Electronic map reveals 'rules of the road' in superconductor

Band structure map exposes iron selenide's enigmatic electronic signature

Using a clever technique that causes unruly crystals of iron selenide to snap into alignment, Rice University physicists have drawn a detailed map that reveals...

Im Focus: Developing a digital twin

University of Texas and MIT researchers create virtual UAVs that can predict vehicle health, enable autonomous decision-making

In the not too distant future, we can expect to see our skies filled with unmanned aerial vehicles (UAVs) delivering packages, maybe even people, from location...

Im Focus: The coldest reaction

With ultracold chemistry, researchers get a first look at exactly what happens during a chemical reaction

The coldest chemical reaction in the known universe took place in what appears to be a chaotic mess of lasers. The appearance deceives: Deep within that...

Im Focus: How do scars form? Fascia function as a repository of mobile scar tissue

Abnormal scarring is a serious threat resulting in non-healing chronic wounds or fibrosis. Scars form when fibroblasts, a type of cell of connective tissue, reach wounded skin and deposit plugs of extracellular matrix. Until today, the question about the exact anatomical origin of these fibroblasts has not been answered. In order to find potential ways of influencing the scarring process, the team of Dr. Yuval Rinkevich, Group Leader for Regenerative Biology at the Institute of Lung Biology and Disease at Helmholtz Zentrum München, aimed to finally find an answer. As it was already known that all scars derive from a fibroblast lineage expressing the Engrailed-1 gene - a lineage not only present in skin, but also in fascia - the researchers intentionally tried to understand whether or not fascia might be the origin of fibroblasts.

Fibroblasts kit - ready to heal wounds

Im Focus: McMaster researcher warns plastic pollution in Great Lakes growing concern to ecosystem

Research from a leading international expert on the health of the Great Lakes suggests that the growing intensity and scale of pollution from plastics poses serious risks to human health and will continue to have profound consequences on the ecosystem.

In an article published this month in the Journal of Waste Resources and Recycling, Gail Krantzberg, a professor in the Booth School of Engineering Practice...

All Focus news of the innovation-report >>>

Top