The spectroscopic technique that separates water molecules lying on the surface reveals how they relax after being excited

A more complete picture of how excited water molecules at an air interface lose their energy has been revealed by RIKEN scientists in a study published in the journal Nature Communications. This discovery will be valuable to better understand the processes that occur on water surfaces.

Water is an anomaly in many ways. For example, its freezing and boiling points are much higher than might be expected, and it is less dense as a solid (ice) than as a liquid.

Almost all of water’s unusual properties derive from the weak bonds that are constantly forming and breaking between neighboring water molecules. Known as hydrogen bonds, these bonds arise because oxygen attracts electrons more than hydrogen. Thus, the slightly negative oxygen in one molecule is attracted to the slightly positive hydrogens in other molecules.

But a small fraction of water molecules—those that lie on the surface—experience hydrogen bonding differently than other water molecules. In their case, the arm that sticks to the air does not form hydrogen bonds.

Until now, no one had been able to discover how the arms of these surface molecules relax after being stretched. This is because it is extremely challenging to isolate the signal from these molecules.

“We have a good understanding of how water molecules behave in the body of liquid, but our understanding of water molecules at the interface is far behind,” says Tahei Tahara of the RIKEN Molecular Spectroscopy Laboratory.

Over the past decade, a team led by Tahara has attempted to remedy this situation by developing highly sophisticated spectroscopy techniques to probe the interactions of water molecules on surfaces.

The team has now developed a technique based on infrared spectroscopy, which is sensitive enough to detect how the oxygen-hydrogen bonds of surface water molecules relax.

Using the technique, the team discovered that oxygen-hydrogen bonds attached to air initially rotate without losing energy. They then relax in a similar way to molecules in the body of liquid forming a network of hydrogen bonds.

“In this sense, there is no big difference between the molecules at the interface and inside the liquid after interacting with their neighbors—both share the same relaxation process,” says Tahara. “These findings present a comprehensive picture of how the extent of oxygen-hydrogen bonds relaxes at the surface of water.”

Tahara and his team now intend to use their spectroscopic technique to look at the chemical reactions taking place at the water interface.