main article image
(JJ Harrison/Wikimedia Commons, CC BY-SA 3.0)

Water Seems to Have Not One, But Two Distinct Structures in Its Liquid State

MICHELLE STARR
6 FEBRUARY 2020

Life as we know it wouldn't exist without it, but compared to other liquids, water is just a freaking weirdo. And, as it now turns out, it's even weirder than we thought. Scientists in Japan have demonstrated that water has not one, but two different molecular structures when in its liquid state - one tetrahedral and one non-tetrahedral.

 

That discovery, they say, could have implications for our understanding of living systems, which rely on liquid water.

Water is pretty commonplace on Earth. We have a very soggy planet compared to the rest of the Solar System. We bathe in it; we drink it; we muck about in it in the summer. All life on Earth depends on it. Yet water itself - good old dihydrogen monoxide - is really peculiar stuff.

There's its really weird density. Most liquids become more dense when they cool, resulting in an ice that is denser than the liquid. Water, however, reaches its maximum density at about 4 degrees Celsius (39.2 Fahrenheit).

As the temperature lowers further, it becomes less dense, so at freezing point - around 0 degrees Celsius (32 Fahrenheit) - it is less dense than liquid water and will float on top.

In addition, water has an unusually high surface tension, second only to liquid mercury; its melting and boiling points are unusually high; and that so many other chemicals dissolve in it is also really strange.

In 2018, scientists from the UK and Japan demonstrated that these peculiar properties have to do with the tetrahedral arrangement of water molecules in liquid form. This means that every water molecule is hydrogen-bonded to four others in a rough pyramid shape.

 

But still, how the structure is ordered has remained a topic of debate. One model proposes that water's molecular structure is unimodal - it's tetrahedrons all the way down. The other proposes that the structure is bimodal, consisting of two structures - tetrahedrons and something else.

To try and resolve the issue, industrial scientists from the University of Tokyo conducted computer simulations, and also ran experiments on liquid silica, one of the few other liquids known to have a tetrahedral molecular arrangement as well.

These experiments were based on X-ray diffraction. The way these short wavelengths scatter off the atoms in the molecules in the liquid can be used to infer the arrangement of those molecules.

Specifically, the scientists looked at peaks in the diffraction. And they found that two overlapping diffraction peaks were hidden in what looked like the first diffraction peak.

One of these peaks was consistent with the distance between oxygen atoms in normal liquids. The other was consistent, the researchers were able to show, with the longer distance between oxygen atoms found in tetrahedral molecular arrangements.

"We show the first clear numerical evidence in the structure factor for the dynamical coexistence of the two types of local structures ... supporting the two-state description of liquid water," the researchers wrote in their paper.

This finding could have implications for molecular biology, chemistry, and pharmacology, as well as industrial applications, the researchers said.

The research has been published in the Journal of the American Chemical Society.