Water as a metal
Peer-Reviewed PublicationIMAGE: IN THE SAMPLE CHAMBER, THE NAK ALLOY DRIPS FROM A NOZZLE. AS THE DROPLET GROWS, WATER VAPOUR FLOWS INTO THE SAMPLE CHAMBER AND FORMS A THIN SKIN ON THE DROP'S SURFACE. view more
CREDIT: HZB
Every child knows that water conducts electricity - but this refers to "normal" everyday water that contains salts. Pure, distilled water, on the other hand, is an almost perfect insulator. It consists of H2O molecules that are loosely linked to one another via hydrogen bonds. The valence electrons remain bound and are not mobile. To create a conduction band with freely moving electrons, water would have to be pressurised to such an extent that the orbitals of the outer electrons overlap. However, a calculation shows that this pressure is only present in the core of large planets such as Jupiter.
Providing electrons
An international collaboration of 15 scientists from eleven research institutions has now used a completely different approach to produce a aqueous solution with metallic properties for the first time and documented this phase transition at BESSY II. To do this, they experimented with alkali metals, which release their outer electron very easily.
Avoiding explosion
However, the chemistry between alkali metals and water is known to be explosive. Sodium or other alkali metals immediately start to burn in water. But the team found a way to keep this violent chemistry in check: They did not throw a piece of alkali metal into water, but they did it the other way round: they put a tiny bit of water on a drop of alkali metal, a sodium-potassium (Na-K) alloy, which is liquid at room temperature.
Experiment at BESSY II
At BESSY II, they set up the experiment in the SOL³PES high vacuum sample chamber at the U49/2 beamline. The sample chamber contains a fine nozzle from which the liquid Na-K alloy drips. The silver droplet grows for about 10 seconds until it detaches from the nozzle. As the droplet grows, some water vapour flows into the sample chamber and forms an extremely thin skin on the surface of the droplet, only a few layers of water molecules. This almost immediately causes the electrons as well as the metal cations to dissolve from the alkali alloy into the water. The released electrons in the water behave like free electrons in a conduction band.
Golden water skin
"You can see the phase transition to metallic water with the naked eye! The silvery sodium-potassium droplet covers itself with a golden glow, which is very impressive," reports Dr. Robert Seidel, who supervised the experiments at BESSY II. The thin layer of gold-coloured metallic water remains visible for a few seconds. This enabled the team led by Prof. Pavel Jungwirth, Czech Academy of Sciences, Prague, to prove with spectroscopic analyses at BESSY II and at the IOCB in Prague that it is indeed water in a metallic state.
Fingerprints of the metallic phase
The two decisive fingerprints of a metallic phase are the plasmon frequency and the conduction band. The groups were able to determine these two quantities using optical reflection spectroscopy and synchrotron X-ray photoelectron spectroscopy: While the plasmon frequency of the gold-coloured, metallic "water skin" is about 2.7 eV (i.e. in the blue range of visible light), the conduction band has a width of about 1.1 eV with a sharp Fermi edge. "Our study not only shows that metallic water can indeed be produced on Earth, but also characterises the spectroscopic properties associated with its beautiful golden metallic luster," says Seidel.
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JOURNAL
Nature
DOI
10.1038/s41586-021-03646-5
METHOD OF RESEARCH
Experimental study
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Spectroscopic Evidence for a Gold-Coloured Metallic Water Solution
ARTICLE PUBLICATION DATE
27-Jul-2021
COI STATEMENT
none
Metallic water prepared for first time under terrestrial conditions
Peer-Reviewed PublicationIMAGE: 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. view more
CREDIT: PHOTO: PHIL MASON / IOCB PRAGUE
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.
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.
“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.”
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.”
Artistic rendering of a pure sodium-potassium alloy drop and a drop with a layer of water, in which electrons liberated from the metal dissolved (IMAGE)
CAPTION
On the left is a pure drop of sodium-potassium alloy, on the right is the drop with a layer of water, in which electrons liberated from the metal dissolved, giving it a golden metallic sheen.
CREDIT
Artistic rendering: Tomáš Belloň / IOCB Prague
The study of the researchers at IOCB Prague and their colleagues, now published in Nature, 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.
The original article:
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. Spectroscopic evidence for a gold-coloured metallic water solution. Nature 2021. DOI: 10.1038/s41586-021-03646-5
JOURNAL
Nature
DOI
10.1038/s41586-021-03646-5
METHOD OF RESEARCH
Experimental study
ARTICLE TITLE
Spectroscopic evidence for a gold-coloured metallic water solution
ARTICLE PUBLICATION DATE
27-Jul-2021
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