The distribution of ions at the air/water interface plays a decisive role in many natural processes. Several studies have reported that larger ions tend to be surface-active, implying ions are located on top of the water surface, thereby inducing electric fields that determine the interfacial water structure. However, new research by chemists from the University of Cambridge and the Max Planck Institute for Polymer Research challenges this view. Their results show that ions in typical electrolyte solutions are, in fact, located in a subsurface region, leading to a stratification of such interfaces into two distinctive water layers.
Litman et al. show that ions and water molecules at the surface of most salt-water solutions, known as electrolyte solutions, are organized in a completely different way than traditionally understood. Image credit: Litman et al., doi: 10.1038/s41557-023-01416-6.
Many important reactions related to climate and environmental processes take place where water molecules interface 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 the human effect on our planet.
The distribution of ions at the interface of air and water can affect atmospheric processes. However, a precise understanding of the microscopic reactions at these important interfaces has so far been intensely debated.
University of Cambridge’s Dr. Yair Litman and colleagues 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 authors overcame these challenges by utilizing 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.
“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 there are a few layers of pure water, then an ion-rich layer, then finally the bulk salt solution,” Dr. Litman said.
“Our 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,” said Dr. Kuo-Yang Chiang, a researcher at the Max Planck Institute for Polymer Research.
“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,” said Professor Mischa Bonn, also from the Max Planck Institute for Polymer Research.
“We are applying these same methods to study solid/liquid interfaces, which could have potential applications in batteries and energy storage.”
The study was published in the journal Nature Chemistry.
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Y. Litman et al. Surface stratification determines the interfacial water structure of simple electrolyte solutions. Nat. Chem, published online January 15, 2024; doi: 10.1038/s41557-023-01416-6