Pure water is not a good conductor of electricity. It is, in fact, an
electrical insulator. In order to conduct electricity, water must contain
dissolved salts, for example, yet the conductivity of such an electrolyte is
relatively low, several orders lower than that of metals. Is it possible to
produce water that is as conductive as, say, copper wire?
Scientists have hypothesized that this may take place in the cores of large
planets, where high pressure compresses water molecules to the point that
their electron shells begin to overlap. At present, generating that kind of
pressure on Earth exceeds human capabilities, and it was therefore assumed
that preparing metallic water under terrestrial conditions would remain an
elusive goal for the foreseeable future. However, an international team of
researchers headed by Pavel Jungwirth of IOCB Prague has developed a new
method with which they succeeded in making metallic water under terrestrial
conditions that lasted for several seconds. Their paper was recently
published in Nature.
The idea of using immense pressure to make metal out of water is nothing
new. In principle, it should be possible to compress water molecules to the
point that their electron shells begin to overlap and form a so-called
conduction band similar to the one in metallic materials. The required
pressure of 50 Mbar (i.e. approximately 50 million times greater than on the
surface of Earth) can be found in the cores of large planets, but we are not
yet able to achieve it under terrestrial conditions.
Dissolution of electrons
In collaboration with researchers from the University of Southern
California, the Fritz Haber Institute, and other institutes, Jungwirth’s
team recently developed a method that has allowed them to prepare metallic
water while completely sidestepping the need for high pressure. The method
builds on earlier research of the Pavel Jungwirth Group focusing on the
behavior of alkali metals in water and liquid ammonia. Inspired by work with
alkali metal-liquid ammonia solutions, which at high concentrations behave
like a metal, the researchers decided to attempt creation of a conduction
band not by compressing water molecules but rather by way of massive
dissolution of the electrons released from the alkali metal. In doing so,
however, they had to overcome a fundamental obstacle: on introduction to
water, alkali metals immediately explode.
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| The first image shows a pure drop of sodium-potassium alloy; in the next images, we see the drop exposed to the action of the water vapor at 10-4 mbar. A layer of water forms on the drop, in which electrons liberated from the metal dissolve, giving it a golden metallic sheen. Credit: Phil Mason / IOCB Prague |
“Throwing sodium into water is one of the most popular school experiments
and the subject of many a YouTube video. As is well known, when you throw a
chunk of sodium in water, you don’t get metallic water but an immediate and
substantial explosion that takes out your apparatus,” says Jungwirth, who
heads a group at IOCB Prague specializing in molecular modeling. “In order
to contain this intense and, for laboratory purposes, rather
counterproductive chemistry, we approached it the other way around; instead
of adding the alkali metal to the water, we added the water to the metal.”
Golden drop of metallic water
Inside a vacuum chamber, the researchers exposed a drop of sodium-potassium
alloy to a small amount of water vapor, which began to condense on its
surface. The electrons liberated from the alkali metal dissolved in the
layer of water on the surface faster than the chemical reaction that results
in the explosion. There were a sufficient number of them to overcome the
critical limit for the formation of a conduction band and thus give rise to
a metallic water solution, which in addition to the electrons also contained
dissolved alkali cations and chemically formed hydroxide and hydrogen.
“Thanks to this, we were able to create a thin layer of gold-colored metallic
water solution that lasted for several seconds, and that was enough for us to
not only see it with our own eyes but also measure it with spectrometers,”
says Jungwirth, adding: “We more or less jury-rigged the necessary apparatus
in a small lab at our institute in Prague, which is also where the fist
experiments took place. We then obtained the key evidence for the presence of
metallic water using X-ray photoelectron spectroscopy on the synchrotron in
Berlin.”
The study of the researchers at IOCB Prague and their colleagues not only
shows that metallic water can be prepared under terrestrial conditions, but it
also provides a detailed characterization of the spectroscopic properties
connected to its beautiful golden metallic sheen.
Reference:
Spectroscopic evidence for a gold-coloured metallic water solution by Philip
E. Mason, H. Christian Schewe, Tillmann Buttersack, Vojtech Kostal, Marco
Vitek, Ryan S. McMullen, Hebatallah Ali, Florian Trinter, Chin Lee, Daniel
M. Neumark, Stephan Thürmer, Robert Seidel, Bernd Winter, Stephen E.
Bradforth and Pavel Jungwirth, 28 July 2021, Nature.
DOI: 10.1038/s41586-021-03646-5
Tags:
Chemistry