In the feature image, On the right is the drop with a layer of water, and on the left is a pure drop of sodium-potassium alloy, in which electrons released from the metal dissolved, giving it a golden metallic sheen. Source: Artistic rendering by Tomáš Belloň / IOCB Prague
Everyone knows that water conducts electricity — but this applies to “normal” everyday water that contains salts. On the other hand, pure, distilled water is an almost ideal insulator. It consists of H2O particles that are loosely connected to one another via hydrogen bonds. The valence electrons remain bound and are not mobile. To form a conduction band with freely moving electrons, water would have to be pressurized to such an amount that the orbitals of the outer electrons overlap. Though, estimation reveals that this pressure is only existing in the core of large planets such as Jupiter.
A worldwide collaboration of 15 experts from eleven research organizations has now used an entirely distinct way to produce an aqueous solution with metallic characteristics for the first time and documented this phase transition at BESSY II. To do this, they experimented with alkali metals, which lose their outer electron very easily.
In the sample chamber, the NaK alloy drips from a nozzle. As the droplet grows, water vapor flows into the sample chamber and forms a thin skin on the drop’s surface. Source: HZB
Though, the chemistry between water and alkali metals is known to be bursting. Sodium or other alkali metals quickly start to burn in water. But the team found a process to retain 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 test in the SOL³PES high vacuum sample chamber at the U49/2 beamline. The sample chamber incorporates 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 expands, some water vapor flows into the sample chamber and forms a very thin skin on the surface of the droplet, only a few layers of water molecules. This instantaneously 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.
“You can observe the phase transformation to metallic water with the naked eye! The silvery sodium-potassium droplet covers itself with a golden glow, which is very exciting,” reports Dr. Robert Seidel, who conducted the experiments at BESSY II. The thin layer of gold-colored metallic water remains noticeable for a few seconds. This allowed the team headed by Prof. Pavel Jungwirth, Czech Academy of Sciences, Prague, to verify with spectroscopic analyses at BESSY II and the IOCB in Prague that it is indeed water in a metallic state.
Fingerprints of the metallic phase
The two crucial 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-colored, metallic “waterskin” 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 characterizes the spectroscopic properties associated with its beautiful golden metallic luster,” says Seidel.
“Spectroscopic evidence for a gold-colored metallic water solution” by Vojtech Kostal, Marco Vitek, Ryan S. McMullen, Hebatallah Ali, Florian Trinter, Philip E. Mason, H. Christian Schewe, Stephen E. Bradforth Tillmann Buttersack, Chin Lee, Daniel M. Neumark, Stephan Thürmer, Robert Seidel, Bernd Winter and Pavel Jungwirth, 28 July 2021, Nature.