A recent study has discovered new properties of water that previously went 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 and life itself would not be possible if water did not possess this characteristic.
Water is known to consist of a complicated network of weak, directional interactions known as hydrogen bonds. For nearly a century, it was thought that the mechanisms by which the 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 hard to pin down 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 post-doctoral 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.
It is common knowledge that ice floats on water and that lakes freeze from the top. 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 four degrees Celsius, the so-called temperature of maximum density, this difference in density dictates that liquid is always situated below ice.
By cooling water down to this temperature, the team employed nuclear magnetic resonance methods to show that the difference in lifetimes of the two ions reaches a maximum value by accentuating the asymmetry becoming glaringly clear.
As noted previously, water consists of one oxygen atom and two hydrogen atoms, but the hydrogen atoms are relatively mobile and can hop from one molecule to another, and it is this hoping that renders the two ionic species so mobile in water.
The full findings are present in the journal- Physical Review Letters....