Water molecule discovery contradicts textbook models

Many vital reactions related to climate and environmental processes happen where water molecules interface with air. For instance, the evaporation of ocean water plays a crucial role in atmospheric chemistry and climate science. Understanding these reactions is crucial to efforts to mitigate the human effect on our planet.

The distribution of ions on the interface of air and water can affect atmospheric processes. Nonetheless, a precise understanding of the microscopic reactions at these vital interfaces has to this point been intensely debated.

In a paper published today within the journal Nature Chemistry, researchers from the University of Cambridge and the Max Planck Institute for Polymer Research in Germany show that ions and water molecules on the surface of most salt-water solutions, referred to as electrolyte solutions, are organised in a very different way than traditionally understood. This may lead to raised atmospheric chemistry models and other applications.

Technique

The researchers set out to check how water molecules are affected by the distribution of ions at the precise point where air and water meet. Traditionally, this has been done with a method called vibrational sum-frequency generation (VSFG). With this laser radiation technique, it is feasible to measure molecular vibrations directly at these key interfaces. Nonetheless, although the strength of the signals might be measured, the technique doesn’t measure whether the signals are positive or negative, which has made it difficult to interpret findings up to now. Moreover, using experimental data alone may give ambiguous results.

The team overcame these challenges by utilising a more sophisticated type of VSFG, called heterodyne-detected (HD)-VSFG, to check different electrolyte solutions. They then developed advanced computer models to simulate the interfaces in numerous scenarios.

The combined results showed that each positively charged ions, called cations, and negatively charged ions, called anions, are depleted from the water/air interface. The cations and anions of easy electrolytes orient water molecules in each up- and down-orientation. It is a reversal of textbook models, which teach that ions form an electrical double layer and orient water molecules in just one direction.

Co-first writer Dr Yair Litman, from the Yusuf Hamied Department of Chemistry, said: “Our work demonstrates that the surface of easy electrolyte solutions has a special ion distribution than previously thought and that the ion-enriched subsurface determines how the interface is organised: on the very top there are a number of layers of pure water, then an ion-rich layer, then finally the majority salt solution.”

Co-first writer Dr Kuo-Yang Chiang of the Max Planck Institute said: “This paper shows that combining high-level HD-VSFG with simulations is a useful tool that can contribute to the molecular-level understanding of liquid interfaces.”

Professor Mischa Bonn, who heads the Molecular Spectroscopy department of the Max Planck Institute, added: “A majority of these interfaces occur in every single place on the planet, so studying them not only helps our fundamental understanding but also can lead to raised devices and technologies. We’re applying these same methods to check solid/liquid interfaces, which could have potential applications in batteries and energy storage.”