Pure water is an almost perfect insulator.
Yes, water found in nature conducts electricity, but that’s because of the impurities it contains, which dissolve into free ions that allow electric current to flow. Pure water only becomes “metallic” (electronically conductive) at extremely high pressures, beyond our current capabilities to produce in a laboratory.
But, as researchers first showed in 2021, it’s not just high pressures that can induce this metallicity in pure water.
By putting pure water in contact with an alkali metal that shares electrons, in this case an alloy of sodium and potassium, free-moving charged particles can be added, making the water metallic.
The resulting conductivity only lasts a few seconds, but it is an important step in being able to understand this phase of water by studying it directly.
“You can see the phase transition to metallic water with the naked eye!” physicist Robert Seidel of Helmholtz-Zentrum Berlin für Materialien und Energie in Germany explained last year when the article was published.
“The silver drop of sodium and potassium is covered with a golden glow, which is very impressive.”
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Under high enough pressures, virtually any material could theoretically become conductive.
The idea is that if you squeeze the atoms together enough, the orbitals of the outer electrons would start to overlap, allowing them to move. For water, this pressure is around 48 megabars, just under 48 million times the Earth’s atmospheric pressure at sea level.
While pressures beyond this have been generated in a laboratory environment, such experiments would not be suitable for studying metallic water. So a team of researchers led by organic chemist Pavel Jungwirth of the Czech Academy of Sciences in Czechia focused on the alkali metals.
These substances release their outer electrons very easily, meaning they could induce the electron-exchange properties of highly pressurized pure water without the high pressures.
There’s just one problem: alkali metals are very reactive with liquid water, sometimes even to the point of exploding (there’s a really cool video below).
Drop the metal into the water and you will get a kaboom.
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The research team found a very clever way to solve this problem. What if, instead of adding the metal to the water, you added water to the metal?
In a vacuum chamber, the team began by extruding a small droplet of sodium-potassium alloy, which is liquid at room temperature, from a nozzle, and carefully added a thin film of pure water via vapor deposition.
Upon contact, electrons and metal cations (positively charged ions) flowed into the water from the alloy.
This not only gave the water a golden sheen, but made it conductive, just as we should see in high-pressure metallic pure water.
This was confirmed using optical reflection spectroscopy and synchrotron X-ray photoelectron spectroscopy.
The two properties, the golden glow and the conductive band, occupied two different frequency ranges, allowing them to be clearly identified.
In addition to giving us a better understanding of this phase transition here on Earth, the research could also allow detailed study of extreme high-pressure conditions within large planets.
On the ice planets of the Solar System, Neptune and Uranus, for example, liquid metallic hydrogen is thought to swirl. and it’s just Jupiter in which pressures are believed to be high enough to metallize pure water.
The prospect of being able to replicate the conditions inside the planetary colossus of our Solar System is truly exciting.
“Our study not only shows that metallic water can be produced on Earth, but also characterizes the spectroscopic properties associated with its beautiful golden metallic sheen.” seidel said.
The research was published in Nature.
A version of this article was first published in July 2021.
