A golden shimmer indicates the effect: Using a sophisticated method, researchers have succeeded in adding metallic features to a layer of water without applying gigantic pressure. The scientists explain that a conduction band with moving electrons is created in a membrane of water on a drop of liquid alkali metal.
It is the famous trademark of metals: They have a high electrical conductivity, which is based on the loosely binding of their electrons. The charge carriers can move freely in the lattice of metal atoms – a so-called conduction band with mobile electrons is formed. This is usually not the case with pure water – this makes it a strong insulator. Because with the H2O molecules, which are loosely linked with each other via hydrogen bonds, the electrons remain firmly bound.
At least in theory, however, that can be changed: To do this, one would have to put water under so much pressure that the orbitals of the outer electrons overlap. This would then create a conduction band with freely moving electrons, which gives the water an electrical conductivity comparable to that of copper. According to calculations, however, gigantic pressure is required for this: 50 megabars – around 50 million times more than on the earth’s surface. Such conditions only exist in the interior of large planets like Jupiter – so far they cannot be established experimentally. But now an international team of researchers has apparently succeeded in generating metallic water without mega-pressure.
Alkali metal donates charge carriers
The basis for this is the transfer of electrons to a film of water. They come from alkali metals, which give off their external electron very easily. To realize this concept, however, the scientists had to overcome a “bombastic” problem that can arise with the combination of alkali metals and water: “Throwing sodium into water is a popular school experiment: it causes a violent explosion,” says senior author Pavel Jungwirth from the Czech Academy of Sciences in Prague. “In order to contain this intensive reaction, which is problematic for laboratory purposes, we went the opposite way: Instead of introducing the alkali metal into the water, we added the water to the metal,” says Jungwirth.
The metal was in the form of a drop: the researchers used a sodium-potassium alloy that is liquid at room temperature. The experiments were carried out in a high vacuum sample chamber at BESSY II in Berlin. Inside was a fine nozzle from which the liquid Na-K alloy dripped. The silver drop grew for about ten seconds until it separated from the nozzle. Meanwhile, the scientists conducted water vapor into the sample chamber. As a result, a thin layer of water formed on the surface of the drop, which consisted of only a few layers of molecules.
Success with a golden shimmer
As the researchers explain, metal ions and, above all, electrons from the alkali alloy migrated into the water. There they then created an effect like the free electrons in a conduction band of metals. “You can see the phase transition to metallic water with the naked eye! The silvery sodium-potassium drop has a golden sheen, which is very impressive, ”reports co-author Robert Seidel from the Helmholtz Center Berlin for Materials and Energy. The thin layer of gold-colored metallic water was then visible for a few seconds. This allowed it to be examined and the researchers were also able to confirm the metallic features.
As they explain, the two decisive signatures of a metallic phase are the so-called plasmon frequency and certain effects of the conduction band. The scientists were able to determine these two quantities using optical reflection spectroscopy and synchrotron X-ray photoelectron spectroscopy. In this way, they were able to prove that it is actually water in a metallic state. “Our study not only shows that metallic water can actually be produced on earth, but also characterizes the spectroscopic properties associated with its beautiful golden metallic sheen,” says Seidel in conclusion.
(Video Credit: Phil Mason / IOCB Prague)
Source: Helmholtz Center Berlin for Materials and Energy, Czech Academy of Sciences, specialist article: Nature, doi: 10.1038 / s41586-021-03646-5