The distribution of ions at the air-water interface plays a decisive role in many natural processes. Some studies suggest that large ions tend to exhibit interfacial activity, suggesting that the ions sit above the water surface, thereby inducing an electric field that determines the interfacial water structure. But new research by chemists at the University of Cambridge and the Max Planck Institute for Polymer Research casts doubt on this view. Their results show that ions in typical electrolyte solutions are actually located in subsurface regions and that such interfaces stratify into two characteristic aqueous layers.
Littman other. They show that the ions and water molecules at the surface of most aqueous salt solutions, known as electrolyte solutions, are organized in a completely different way than previously understood.Image credit: Littmann other., doi: 10.1038/s41557-023-01416-6.
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.
Dr. Yair Littman of the University of Cambridge and his colleagues set out to study how water molecules are affected by the distribution of ions at the very 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 key 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 authors 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.
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.
“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 composition of the interface. .At the top you have a few layers of pure water, then you have the ions, the “dense layer,'' and finally the bulk salt solution,'' Dr. Littman said.
“Our paper shows that combining high-level HD-VSFG with simulation is a valuable tool that contributes to the molecular-level understanding of liquid interfaces,” said Max Planck Institute for Polymer Research researchers said Dr. Kuo-Yang Chiang. .
“These kinds of interfaces exist everywhere on Earth, and studying them not only helps our fundamental understanding, but can also lead to the development of better devices and technologies.” said Professor Misha Bonn, also of the Max Planck Institute for Polymer Research.
“We are applying these same methods to study solid/liquid interfaces, which could have applications in batteries and energy storage.”
of study It was published in the magazine natural chemistry.
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Y. Littman other. Surface stratification determines the structure of interfacial water in simple electrolyte solutions. nut.chemistry, published online on January 15, 2024. doi: 10.1038/s41557-023-01416-6
Source: www.sci.news
