Water has numerous anomalous properties, many of which remain poorly understood. One of its intriguing behaviors is that it exhibits the so-called temperature of maximum density (TMD) at 3.98 degrees Celsius (39.16 degrees Fahrenheit). In a new study published in the journal Physical Review Letters, researchers at New York University provide experimental evidence for previously unknown abrupt changes in proton (H+) transfer kinetics in water at this temperature.

Liquid water is known to be an excellent transporter of its own autoionization products; that is, the charged species obtained when a water molecule (H 2 O) is split into protons and hydroxide ions (OH-).

This remarkable property of water makes it a critical component in emerging electrochemical energy production and storage technologies such as fuel cells; indeed, life itself would not be possible if water did not possess this characteristic.

Water is known to consist of an intricate network of weak, directional interactions known as hydrogen bonds.

For nearly a century, it was thought that the mechanisms by which water transports the H+ and OH- ions were mirror images of each other — identical in all ways except for directions of the hydrogen bonds involved in the process.

Current theoretical models and computer simulations, however, predicted a fundamental asymmetry in these mechanisms.

If correct, this asymmetry is something that could be exploited in different applications by tailoring a system to favor one ion over the other.

Experimental proof of the theoretical prediction has remained elusive because of the difficulty in directly observing the two ionic species.

In the new study, New York University’s Professor Alexej Jerschow, Professor Mark Tuckerman and Dr. Emilia Silletta devised a novel experiment for nailing down this asymmetry.

The experimental approach involved cooling water down to the TMD, where the asymmetry is expected to be most strongly manifest, thereby allowing it to be carefully detected.

“It is common knowledge that ice floats on water and that lakes freeze from the top,” the researchers said.

“This is because water molecules pack into a structure with lower density than that of liquid water — a manifestation of the unusual properties of water: the density of liquid water increases just above the freezing point and reaches a maximum at the TMD; this difference in density dictates that liquid is always situated below ice.”

“By cooling water down to this temperature, we employed nuclear magnetic resonance methods to show that the difference in lifetimes of the two ions reaches a maximum value (the greater the lifetime, the slower the transport).”

“By accentuating the difference in lifetimes, the asymmetry became glaringly clear.”

Hydrogen atoms in water are relatively mobile and can hop from one molecule to another, and it is this hopping that renders the two ionic species so mobile in water. In seeking explanations for the temperature-dependent characteristics, the team focused on the speed with which such hops can occur.

Prior research had indicated that two main geometrical arrangements of hydrogen bonds (one associated with each ion) facilitate the hops.

The researchers found that one of the arrangements led to significantly slower hops for OH- than for H+ at 4 degrees Celsius. Being that this is also the TMD, they felt that the two phenomena had to be linked. In addition, their results showed that molecules’ hopping behavior changed abruptly at this temperature.

“The study of water’s molecular properties is of intense interest due to its central role in enabling physiological processes and its ubiquitous nature,” Professor Jerschow said.

“The new finding is quite surprising and may enable deeper understanding of water’s properties as well as its role as a fluid in many of nature’s phenomena.”

“It is gratifying to have this clear piece of experimental evidence confirm our earlier predictions,” Professor Tuckerman said.

“We are currently seeking new ways to exploit the asymmetry between H+ and OH- transport to design new materials for clean energy applications, and knowing that we are starting with a correct model is central to our continued progress.”

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Emilia V. Silletta et al. 2018. Unusual Proton Transfer Kinetics in Water at the Temperature of Maximum Density. Phys. Rev. Lett 121 (7); doi: 10.1103/PhysRevLett.121.076001