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A team of researchers discovered that water molecules on the top of salt water are organised differently than previously assumed.
Many key reactions linked to climatic and environmental processes occur when water molecules interact with air. For example, the evaporation of ocean water is significant in atmospheric chemistry and climate research. Understanding these responses is critical to attempts to reduce human impact on our world. The distribution of ions at the air-water interface can influence atmospheric processes. However, a clear understanding of the microscopic events at these critical surfaces has thus far been hotly contested. In a paper published in 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 at the surface of most salt-water solutions, known as electrolyte solutions, are organised in a completely different way than traditionally understood. This could lead to better atmospheric chemistry models and other applications. 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 has been done with a technique called vibrational sum-frequency generation (VSFG). With this laser radiation technique, it is possible to measure molecular vibrations directly at these key interfaces. However, although the strength of the signals can be measured, the technique does not measure whether the signals are positive or negative, which has made it difficult to interpret findings in the past. Additionally, using experimental data alone can give ambiguous results. The team overcame these challenges by utilising 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 both positively charged ions, called cations, and negatively charged ions, called anions, are depleted from the water/air interface. The cations and anions of simple electrolytes orient water molecules in both up- and down-orientation. This is a reversal of textbook models, which teach that ions form an electrical double layer and orient water molecules in only one 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 organised: at the very top there are a few layers of pure water, then an ion-rich layer, then finally the bulk salt 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 molecular-level understanding of liquid interfaces." Professor Mischa Bonn, who heads the Molecular Spectroscopy department of 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." (ANI)
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