By Isabelle Robinson, M.Sc.Oct 26 2018Article updated on 5 January 2021
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Water is everywhere. Making up 71% of the Earth’ surface, it runs in our rivers, pours from the skies and sustains all life on the planet. Despite our reliance on the liquid, some of its physical properties have yet to be understood in the scientific community.
Researchers from the University of Zurich, Switzerland, recently published an article in the ‘Journal of Chemical Physics’ reporting their research into water’s hydrogen bonding using THz spectroscopy techniques. It is their hope that the already existing method can explain the more unusual properties of liquid water.
What makes this paper special is the aspect of THz spectroscopy of a liquid. In the THz range, we make spectroscopy explicitly of the intermolecular degrees of freedom of the system in the study, to contrast it to intramolecular degrees of freedom. With THz spectroscopy, we can very directly look at the hydrogen bonding between various water molecules.
Peter Hamm, Researcher, University of Zurich
Traditionally, spectroscopy is a method to measure the interaction between matter and light, in the hopes of deducing its physical properties. The Swiss research team innovatively used the technique to examine the forces between water molecules. THx spectroscopy has a particularly low frequency which enables the researchers to observe the intermolecular forces. The team hopes that this method will allow them to infer something about the liquid’s peculiar properties such as it’s density maximum at 4 degrees Celsius. This is because hydrogen bonding is responsible for much of the unknown properties of water.
During this particular experiment, the team used an ultrashort visible laser pulse to excite a dye molecule dissolved in water. This made it change its charge distribution. Once this was complete, a THz pulse measured the response of the adjacent water molecules and recorded them as a function of time, with 0 being the point of excitation.
"The response we found in the THz frequency range was surprisingly slow. Water is typically considered to be a very fast solvent with a response in the subpicosecond range, but we found a timescale around 10 picoseconds in the THz," Hamm explained. This slow time-scale was attributed to the collective nature of the water response surveyed using THz spectroscopy.
Although, it should be noted that veteran researchers who have been using THz spectroscopy for more than two decades have warned the team not to be too optimistic about the results.
The outcome often has been a bit disappointing because the THz spectra of liquids like water are extremely broad and blurred, and it's very hard to extract information out of that.
Peter Hamm, Researcher, University of Zurich
When asked about the next stage of their research, Hamm enthused that the team plans to use this method to explore water’s properties, structure, and dynamics when below the freezing point, though still in a liquid form.
The special properties of water become significantly more pronounced if one goes to temperatures below the freezing point.
Peter Hamm, Researcher, University of Zurich
References
American Institute of Physics. (2018, June 27). THz spectroscopy could help explain water's anomalies. Retrieved from phys.org: https://phys.org/news/2018-06-thz-spectroscopy-anomalies.html
Saima Ahmed, A. P.-T. (2018). Aqueous solvation from the water perspective. The Journal of Chemical Physics, 148.
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