Discovery of a water molecule contradicts textbook models

Hand models will need to be redesigned after a team of researchers discovered that water molecules on the surface of salt water are organized differently than previously thought.

Many important reactions related to climatic and environmental processes occur where water molecules interact with air. For example, the evaporation of ocean water plays an important role in atmospheric chemistry and climate science. Understanding these reactions is crucial to efforts to mitigate human impact on our planet.

The distribution of ions at the interface of air and water can affect atmospheric processes. However, the precise understanding of the microscopic reactions at these important interfaces has until now been intensely debated.

In an article published in the journal Natural chemistry, researchers from the University of Cambridge and the Max Planck Institute for Polymer Research in Germany show that water ions and molecules on the surface of most salt water solutions, called electrolyte solutions, are organized in a completely different way than traditionally understood. This could lead to better atmospheric chemistry models and other applications.

A more sophisticated technique

The researchers set out to study how water molecules are affected by the distribution of ions at the exact point where air and water meet. Traditionally, this was done using a technique called vibrational sum-frequency generation (VSFG). Using this laser radiation technique, it is possible to measure molecular vibrations directly at these key interfaces.

However, although the strength of signals can be measured, the technique does not measure whether signals are positive or negative, which has made interpreting results difficult in the past. Additionally, using experimental data alone can yield ambiguous results.

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

The combined results showed that positively charged ions, called cations, and negatively charged ions, called anions, are depleted at the water/air interface. The cations and anions of simple electrolytes orient water molecules up and down. This is a reversal of classical models, which teach that ions form an electrical double layer and orient water molecules in a single direction.

Co-first author Dr Yair Litman, from the Yusuf Hamied Department of Chemistry, said: “Our work demonstrates that the surface of simple electrolyte solutions has a different ion distribution than previously thought and that the Ion-enriched subsurface determines how the interface is organized: at the very top are a few layers of pure water, then an ion-rich layer, then finally the bulk saline solution.

Co-first author Dr Kuo-Yang Chiang of the Max Planck Institute said: “This paper shows that combining high-level HD-VSFG with simulations is an invaluable tool that will contribute to the understanding of interfaces liquids at the molecular level. »

Professor Mischa Bonn, who heads the department of molecular spectroscopy at the Max Planck Institute, added: “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. We are applying these same methods to study solid/liquid interfaces, which could have potential applications in batteries and energy storage.