Can quantum tug-of-war explain water’s weirdness?

An international team of researchers has used a high-speed electron camera to observe the atomic motion of liquid water for the first time.

These observations – which reveal the quantum nature of how hydrogen atoms interact – bring scientists one step closer to understanding the weird and wacky properties of water, like its unusually high surface tension, its large capacity to store heat, and the way it is densest just above freezing point (instead of getting denser as it gets colder, like other liquids).

The results are published in Nature.

Diagram of interactions between water molecules
Hydrogen bonds: hydrogen atoms have a slight positive charge, attracting them to slightly negatively charged oxygen atoms in water. Credit: Hkontro / Wikimedia Commons

“Although this so-called nuclear quantum effect has been hypothesised to be at the heart of many of water’s strange properties, this experiment marks the first time it was ever observed directly,” says co-author of the study Anders Nilsson, a professor of chemical physics at Stockholm University.

“The question is if this quantum effect could be the missing link in theoretical models describing the anomalous properties of water.”

As you may remember from high school science, a water molecule is made up of one oxygen atom (O) and two hydrogen atoms (H). It is the interactions between different H2O molecules – intramolecular forces called “hydrogen bonds” – that give water its bizarre properties. Positively charged hydrogen atoms in one molecule are attracted to more negatively charged oxygen atoms in another. This web of hydrogen bonds holds groups of water molecules together.

Observing these hydrogen bonds is key to understanding how water molecules interact with their neighbours – but it’s a hard process to see, because the hydrogen bonds are small and ephemeral.

“For a long time, researchers have been trying to understand the hydrogen bond network using spectroscopy techniques,” explains Jie Yang, a professor at Tsinghua University in China who led the study. “The beauty of this experiment is that for the first time we were able to directly observe how these molecules move.”

Yang’s study used a high-speed electron camera to capture these elusive bonds.

To begin with, the team set up a microscopically thin jet of liquid water (1,000 times thinner than the width of a human hair) and used infrared laser light to make the water molecules vibrate. They then scattered electrons off the molecules to generate high-resolution snapshots of the molecular movements. This allowed them to build up a stop-motion movie of how the molecules responded to the light.

Read more: Watch closely – molecules in motion

So what did it show?

As a water molecule began to vibrate, its hydrogen atom pulled oxygen atoms from a neighbouring molecule closer – then shoved them away again to expand the space between molecules.

Co-author Kelly Gaffney, from the US Department of Energy’s SLAC National Accelerator Laboratory, explains that this is quantum wave-like behaviour – likely heightened by the low mass of the hydrogen atoms.

“This study is the first to directly demonstrate that the response of the hydrogen bond network to an impulse of energy depends critically on the quantum mechanical nature of how the hydrogen atoms are spaced out, which has long been suggested to be responsible for the unique attributes of water and its hydrogen bond network,” Gaffney says.

These properties are key to many chemical and biological processes, and so better understanding them could help us better understand the origin and survival of life on Earth, the team concludes.

Please login to favourite this article.