In the liquid state, water molecules actually have two different structures

From the oceans to the cells passing through the atmosphere, water is omnipresent on Earth. In the liquid state, it is a solvent necessary for life. Which makes it one of the most studied molecules by chemists. However, there is still much to discover about it. By studying it through X-rays, a team of researchers recently discovered that water molecules adopt two different structures in the liquid state. A result that could have important implications in terms of biochemistry and industry.

Researchers at the University of Tokyo have used calculation methods and analysis of recent experimental data to demonstrate that water molecules have two distinct structures in the liquid state.

The team studied the X-ray scattering through water samples and showed a hidden bimodal distribution under the first diffraction peak resulting from the tetrahedral and non-tetrahedral arrangements of the water molecules.

This work can have important implications in many scientific and technological fields, but especially with regard to living systems, such as proteins and cellular structures, which are strongly affected by the surrounding water molecules. Given the omnipresence of water on our planet and the central role it plays in all known life, it can be difficult to believe that there is still something to learn about this molecule.

An unusual liquid because of its structure and molecular arrangements

A simple molecule composed of only two atoms of hydrogen and one of oxygen, water still hides fundamental mysteries that remain to be elucidated. For example, water has unusually high melting and boiling points and expands even when it freezes (unlike most liquids that contract). These and other unusual properties make it very different from almost all other liquids, but also allow life as we know it to exist.

Water has a tetrahedral molecular arrangement where a molecule of H2O is linked to 4 other molecules via hydrogen bonds. Credits: Qwerter / Wikimedia Commons

The strangeness of water can be better understood by thinking of the unique interactions between H2O molecules - the hydrogen bond. Water tends to form four hydrogen bonds with its four neighbors, which leads to tetrahedral arrangements. Such arrangements can be largely deformed under thermal fluctuations. However, the question of whether the distortion leads to the coexistence of separate tetrahedral and non-tetrahedral arrangements has remained controversial.

Liquid water: X-rays reveal that it actually has two structures

Now, scientists at The University of Tokyo have combined computer simulations and the analysis of scattering experimental data to find the "structure factor" of water -- the mathematical function that represents the paths of dispersed X-rays when they scatter off the hydrogen and oxygen atoms. The analysis showed two overlapping peaks hiding in the first diffraction peak of the structure factor. One of these peaks corresponded to the distance between oxygen atoms as in ordinary liquids, while the other indicated a longer distance, as in a tetrahedral arrangement. "The combination of new computational methods and analysis of recent X-ray scattering data allowed us to see what was not visible in previous work," first author of the study Rui Shi explains.

Analysis of the X-ray diffraction by the oxygen and hydrogen atoms has shown a double peak indicating the existence of two distinct molecular structures. Credits: Rui Shi and Hajime Tanaka

One of these peaks corresponds to the distance between the oxygen atoms, as in ordinary liquids, while the other indicates a longer distance, as in a tetrahedral arrangement. This discovery can have enormous implications in many scientific fields. Knowing the exact structural order of water is essential for a complete understanding of molecular biology, chemistry and even many industrial applications.


Direct Evidence in the Scattering Function for the Coexistence of Two Types of Local Structures in Liquid Water

Rui Shi, Hajime Tanaka

J. Am. Chem. Sac

Publication Date:January 21, 2020

Post a Comment

Previous Post Next Post