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: A Chip with Blood Vessels

Biochips have been developed at TU Wien (Vienna), on which tissue can be produced and examined. This allows supplying the tissue with different substances in a very controlled way.

Cultivating human cells in the Petri dish is not a big challenge today. Producing artificial tissue, however, permeated by fine blood vessels, is a much more...

Im Focus: A Leap Into Quantum Technology

Faster and secure data communication: This is the goal of a new joint project involving physicists from the University of Würzburg. The German Federal Ministry of Education and Research funds the project with 14.8 million euro.

In our digital world data security and secure communication are becoming more and more important. Quantum communication is a promising approach to achieve...

Im Focus: Research icebreaker Polarstern begins the Antarctic season

What does it look like below the ice shelf of the calved massive iceberg A68?

On Saturday, 10 November 2018, the research icebreaker Polarstern will leave its homeport of Bremerhaven, bound for Cape Town, South Africa.

Im Focus: Penn engineers develop ultrathin, ultralight 'nanocardboard'

When choosing materials to make something, trade-offs need to be made between a host of properties, such as thickness, stiffness and weight. Depending on the application in question, finding just the right balance is the difference between success and failure

Now, a team of Penn Engineers has demonstrated a new material they call "nanocardboard," an ultrathin equivalent of corrugated paper cardboard. A square...

Im Focus: Coping with errors in the quantum age

Physicists at ETH Zurich demonstrate how errors that occur during the manipulation of quantum system can be monitored and corrected on the fly

The field of quantum computation has seen tremendous progress in recent years. Bit by bit, quantum devices start to challenge conventional computers, at least...

All Focus news of the innovation-report >>>

Top