Groundbreaking research shows that water molecules on saltwater surfaces behave differently than previously thought, providing a new perspective for environmental science and technology.
Textbook models may need to be redrawn after a team of researchers discovered that the structure of water molecules on the surface of salt water is different from what was previously thought.
Many important reactions related to climate and environmental processes occur where water molecules come into contact with air. For example, ocean water evaporation plays an important role in atmospheric chemistry and climate science. Understanding these responses is critical to efforts to reduce human impact on the planet.
The distribution of ions at the air-water interface can influence atmospheric processes. However, accurately understanding the microscopic reactions at these important interfaces has been hotly debated.
innovative research methods
In a paper published today (January 15) in the journal natural chemistry, researchers at the University of Cambridge and Germany’s Max Planck Institute for Polymer Research have discovered that ions and water molecules on the surface of most salt aqueous solutions, known as electrolyte solutions, work in a completely different way than previously understood. They showed they were organized. 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 precise point where air and water meet. Traditionally, this has been done with a technique called oscillatory sum frequency generation (VSFG). Using this laser irradiation technique, it is possible to directly measure molecular vibrations at these critical interfaces. However, while the strength of the signal can be measured, this technique does not measure whether the signal is positive or negative, which has previously made the results difficult to interpret. Furthermore, using only experimental data can lead to ambiguous results.
The research team overcame these challenges by utilizing a more sophisticated form of VSFG, called heterodyne detection (HD)-VSFG, to study different electrolyte solutions. We then developed sophisticated computer models to simulate the interface in various scenarios.
Revolutionize the traditional model
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 both upward and downward. This is a reversal of the textbook model that teaches that ions form an electric double layer, orienting water molecules in only one direction.
Co-lead author Yair Littman, from the Yusuf Hameed Department of Chemistry, said: “Our study shows that the surface of a simple electrolyte solution has a different ion distribution than previously thought, and that the ion-rich subsurface determines the organization of the interface. ”: On top there are several layers of pure water, then an ion-rich layer, and finally a bulk salt solution. ”
Co-lead author Dr. Kuo-Yang Chiang of the Max Planck Institute said, “This paper shows that combining high-level HD-VSFG with simulation is a valuable tool that contributes to the molecular-level understanding of liquid interfaces. It shows that.”
Professor Mischa Bonn, Head of Molecular Spectroscopy at the Max Planck Institute, added: “These types of interfaces exist everywhere on Earth, so studying them not only helps our fundamental understanding, but can also lead to the development of better devices and technologies. ” We apply these same methods to the study of solid/liquid interfaces, with potential applications in batteries and energy storage. ”
Reference: “Surface stratification determines the interfacial water structure of simple electrolyte solutions” January 15, 2024 natural chemistry.
DOI: 10.1038/s41557-023-01416-6