Chemists invoke bizarre Maxwell’s demon on the largest scale yet

A 19th-century thought experiment that was once thought to defy the laws of thermodynamics has now been realised to make molecules accumulate on one side of a U-bend

Chemists invoke bizarre Maxwell’s demon on the largest scale yet
Physicist James Clerk Maxwell proposed his demonic thought experiment in 1867
 (Credit: SSPL / Science Museum/Getty Images)

A chemical pump based on a 19th-century thought experiment involving an invisible “demon” could be used to help separate chemicals in drug manufacturing.

Maxwell’s demon, first proposed by physicist James Clerk Maxwell in 1867, involves two boxes of gas separated by a weightless door that is controlled by a tiny demon. The demon only lets faster-moving particles pass through in one direction and slower particles pass in the other direction, which makes one box hotter and the other cooler. But this seems to violate the second law of thermodynamics, which says heat must flow from hot to cold unless energy is used to make things go the other way.

Physicists later solved this paradox by realising that a real-life demon would have to expend energy to measure the speed of each particle. In fact, scientists have spotted many real-world examples of Maxwell’s demon, albeit working at very small scales.

Now, Jonathan Nitschke at the University of Cambridge and his colleagues have designed a chemical pump that operates in the same way as Maxwell’s original thought experiment. It can separate molecules over several centimetres, the largest distance so far.

The set-up features a U-shaped tube filled with a light-sensitive chemical called fluoroazobenzene (FAB). The bend is filled with water and pyramid-shaped molecules called coordination cages, which can contain certain molecules and carry them from one side of the bend to the other. These cages are the equivalent of the demon opening or closing the gate.

When Nitschke and his team shine light on one side, the FAB changes its configuration so that it can more easily fit in the cage, which transports it to the other side. As a result, FAB accumulates on one side of the tube, which goes against what would happen if the system were left to its own devices. “We’re providing light energy to the system,” says Nitschke. “That’s what feeds the demon ultimately here.”

They then added the hydrocarbon naphthalene to one of the tubes and found that the movement of FAB from one side to the other helped push naphthalene in the opposite direction. If this can be replicated for other kinds of molecules, it could be a useful chemical separator for industries like drug manufacturing, says Nitschke.

Journal reference:

Nature Chemistry DOI: 10.1038/s41557-024-01549-2

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