At one time, it wathought that the mechanisms by which water transports the H+ and OH− ions were mirror images of each other, though in recent years, asymmetry was glimpsed:
A team of scientists has uncovered new molecular properties of water—a discovery of a phenomenon that had previously gone unnoticed.
Liquid water is known to be an excellent transporter of its own autoionization products; that is, the charged species obtained when a water molecule (H2O) is split into protons (H+) 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 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 state-of-the-art 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. Different experiments have only provided glimpses of the predicted asymmetry.
A team of scientists at New York University, led by Professor Alexej Jerschow and including Emilia Silletta, an NYU postdoctoral fellow, and Mark Tuckerman, a professor of chemistry and mathematics at NYU, devised a novel experiment for nailing down this asymmetry. The experimental approach involved cooling water down to its so-called temperature of maximum density, where the asymmetry is expected to be most strongly manifest, thereby allowing it to be carefully detected.
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By cooling water down to this temperature, the team employed nuclear magnetic resonance methods (the same type of approach is medically in magnetic resonance imaging) 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.
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In addition, their results showed that molecules’ hopping behavior changed abruptly at this temperature. James DeVitt, “The Behavior of Water: Scientists Find New Properties of H2O” at New York University
Water is pretty unusual.
See also: Water can exist in two different liquid phases
A new piece found in the puzzle of water’s strange, life-enabling behavior
and
Michael Denton: Does water’s remarkable fitness for life point to design?