Atom-by-atom recordings track what happens after substances dissolve

In a world first, researchers have recorded exactly how solvents interact with dissolved materials, one atom at a time.

After a solute, such as salt, dissolves in a solvent, such as water, the two interact in a complicated way. Their interactions constitute solvation – or hydration if the solvent is water – but what exactly each solvent atom does at the start of the solvation process has eluded researchers for decades.

Now, Henrik Stapelfeldt at Aarhaus University in Denmark and his colleagues have recorded solvation atom by atom.

Instead of water, they used a nanoscopic droplet of liquid helium cooled to -273°C as the solvent – and instead of a grain of salt, they used an atom of sodium.

The researchers embedded a xenon atom at the centre of the helium droplet, while the sodium atom sat at its perimeter. They then hit the sodium atom with an ultrashort pulse from a laser to turn it into a positively charged ion, which kicked off solvation as the helium atoms from the nanodroplet started sticking to it – just as water molecules surround sodium ions from table salt once you mix it into water.

The researchers then used a second laser pulse to make the xenon atom into a positively charged ion as well. The two ions repelled each other strongly enough for the sodium ion and all the helium atoms attached to it to move out of the droplet and onto a detector. By waiting longer and longer to use the second pulse, the team was able to make “snapshots” of the solvation process over time, ultimately combining them into a “molecular movie” of helium atoms attaching to the sodium ion one by one.

Stapelfeldt says the helium droplet is an exceptionally well-controlled “nanosized laboratory” and that the snapshots would have been too difficult to time in a solvent like water where reactions happen more quickly and chaotically. The properties that his team measured – like the amount of energy released during solvation or how statistically likely a helium atom is to attach to the ion after some period of time – have never been measured before and may advance complex quantum mechanical models of solvation.

There may be other applications as well. “In outer space, there are exotic molecules and atoms at play, and understanding how they interact and combine is crucial for understanding the chemistry of the cosmos,” says Davide Galli at the University of Milan in Italy. “These tiny droplets provide the perfect conditions to mimic the cold and dense regions of the interstellar medium.”

Observing chemical reactions in space, like on cold grains of dust, is currently impractical, so helium nanodroplet experiments present a unique opportunity for discovery, he says.