newton cradle

When charge is transferred through water, the oxygen atoms barely move at all. Instead, only the end atoms move while the charge is transferred in between, similar to how Newton’s cradle works. Credit: Pixabay

Water, in its pure form, is one of the worst electrical conductors out there. It makes up for it, however, by being a great ionic solvent and it is these dissolved ions, like salts and metals, that conduct electricity well. But despite its crucial importance in biology and chemistry mechanisms, the way water conducts electricity has never been truly understood at a molecular level. Well that’s not the case anymore after a team led by Yale chemistry professor Mark Johnson has finally cracked the case.

The researchers used novel tools and techniques to trace the structural changes that occur in the murky web of interconnected water molecules as one extra proton gets transferred from one oxygen atom to the next. In order to study this proton relay race, scientists had previously relied on infrared colour changes but results end up looking like a blurred photograph — useful, but not very revealing.

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In 1806, Chemist Theodor Grotthuss first described how charge moves in water suggesting protons pass from molecule to molecule by hopping from oxygen atom to oxygen atom. We now know this process as the Grotthuss mechanism — we haven’t learned an awful lot more in the past two centuries, though.

The approach Johnson and colleagues used involved fast-freezing the chemical process, resulting in snapshots of the proton movements akin to still frames from a movie. To make the snapshots as clear as possible, charge was transferred between only a couple of ‘heavy water’ molecules —  water made of the deuterium isotope of hydrogen. The molecules were chilled close to absolute zero then studied with spectroscopy.

“The oxygen atoms don’t need to move much at all,” Johnson said. “It is kind of like Newton’s cradle, the child’s toy with a line of steel balls, each one suspended by a string. If you lift one ball so that it strikes the line, only the end ball moves away, leaving the others unperturbed.”

Armed with this knowledge, it’s possible to optimize applications ranging from alternative energy technologies to the development of pharmaceuticals. Concerning fundamental research, the new findings could help us understand the chemical processes that take place at the surface of water. The same method could, for instance, tell us if the water’s surface is more or less acidic than the bulk of water. Right now, there’s no way to measure the surface pH of water.

“In essence, we uncovered a kind of Rosetta Stone that reveals the structural information encoded in color,” Johnson said. “We were able to reveal a sequence of concerted deformations, like the frames of a movie.”

Journal Reference: C. T. Wolke, J. A. Fournier, L. C. Dzugan, M. R. Fagiani, T. T. Odbadrakh, H. Knorke, K. D. Jordan, A. B. McCoy, K. R. Asmis, M. A. Johnson. Spectroscopic snapshots of the proton-transfer mechanism in water. Science, 2016; 354 (6316): 1131 DOI: 10.1126/science.aaf8425