Surface levelled: chemists reverse understanding of salt water

An interesting new investigation has revealed that salt water is not what it seems – to chemists.

A team of UK and German researchers has found that water molecules at the surface of a salty solution have a different arrangement to what was previously thought.

The discovery has implications for climate and environmental science, as it generates information on how water may interact with air – particularly when it’s evaporating.

“These types of interfaces occur everywhere on the planet, so studying them not only helps our fundamental understanding but can also lead to better devices and technologies,” says Professor Mischa Bonn, from the molecular spectroscopy department of the Max Planck Institute, Germany.

The study is published in Nature Chemistry.

When a salt dissolves in water, it turns into positively and negatively charged atoms called ions.

Previously, a laser-based technique called vibrational sum frequency generation (VSFG) had suggested to scientists that in salty solutions, salt ions coaxed water molecules into orienting themselves one way.

But these researchers, who hail from the University of Cambridge and the Max Planck Institute for Polymer Research, used a more advanced version of VSFG: heterodyne-detected VSFG, or HD-VSFG.

And their technique, in combination with computer modelling, allowed them to build a clearer picture of the surface of electrolytes (water-based salt solutions).

They found that, contrary to previous belief, water molecules were oriented in both an up- and down- direction.

Salt ions were also distributed differently at the water surface to previous beliefs.

“At the very top there are a few layers of pure water, then an ion-rich layer, then finally the bulk salt solution,” says Dr Yair Litman, from Cambridge’s Yusuf Hamied Department of Chemistry.

The researchers are aiming to use their new HD-VSFG technique to examine more liquid surfaces.

“We are applying these same methods to study solid/liquid interfaces, which could have potential applications in batteries and energy storage,” says Bonn.

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