Water as metal
Under normal conditions, pure water is almost an electrical insulator. Water would only become metallic under extreme pressures, such as found in the interior of very large planets or stars. Now an international collaboration has succeeded to create a metallic state of water using a completely different approach.
Water conducts electricity, that's how you learn it at school - but this usually refers to tap water containing dissolved salts, which as ions are the actual charge carriers. Pure, deionized water, on the other hand, comes very close to a perfect insulator. It only consists of neutral H2O molecules that interconnect via a hydrogen bond network. There, the electrons remain bound in the molecular orbitals and cannot move freely in the water. In order to bring liquid water into a metallic state, it would have to be pressurized to such an extent that the outermost molecular orbitals begin to overlap, which then create a so-called metallic conduction band where electrons can move freely. However, calculations show that the pressures required for this can practically not be generated mechanically and which are possibly be present inside very large planets such as Jupiter.
An international collaboration of 15 scientists at eleven research institutions have pusued a completely different approach to generate water with metallic properties. They experimented with alkali metals, which give off their outermost electron very easily and which themselves also easily dissolve as ions in water.
The chemistry between alkali metals such as sodium or potassium and water is known to be highly vigorous - many will remember the school experiment in which sodium immediately starts to burn as soon as it comes into contact with water. Here the research team chose the opposite approach in order to avoid the explosive chemical reaction: not alkali metal was added to the water, but the other way around: water was evaporated onto a drop of alkali metal.
For this purpose, drops of the so-called sodium-potassium alloy - a liquid metal - were created through a ~ 100 µm thin capillary in a vacuum chamber. The silver drops grow for about 10 seconds until they loosen and fall into a beaker. If additionally water vapour is leaked into the vacuum chamber, a very thin layer of water molecules adsorb on the surface of the drop. Electrons as well as sodium and potassium cations dissolve almost instantly into the water.
“One can see the dynamics with the naked eye! The silvery, shiny sodium-potassium drop changes its colour to a golden sheen for a few seconds, ”report the three first authors, Dr. Phil Mason, Dr. Christian Schewe and Dr. Tillmann Buttersack. That was enough time for the team around Pavel Jungwirth from the Czech Academy of Sciences in Prague, who is also visiting the FHI as part within a research award of the Alexander von Humboldt Foundation, to use spectroscopic methods to investigate the nature of the water on the drop’s surface more closely.
Using UV-Vis reflection spectroscopy, it was possible to determine an absorption band in the blue part of the optical spectrum, which explains the golden color of the metal drop. With the help of synchrotron X-ray photo-electron spectroscopy, even deeper insights into the electronic structure could be determined at BESSY II in Berlin Adlershof: “In the photo-electron spectra, we were able to detect both a peak at around 2.7 eV as a plasmon and a conduction band with a width of approximately 1.1 eV with a sharp Fermi edge. These spectral features clearly show that the aqueous, golden solution has metallic properties, ”explain Dr. Bernd Winter, Dr. Hebatallah Ali and Dr. Florian Trinter from FHI.
These Scientists have observed for a brief moment that a metallic water solution is formed when well-dosed amounts of water vapor are adsorbed on the alkali metal before a hydroxide solution is formed. In principle, the metallization of water can be imagined as doping with alkali metal, similar to how in solid-state physics semiconductors are put into a metallic state by doping with other atomic species.