Monday, March 4, 2024
Monday, March 4, 2024
HomeScienceLayers in ionic solutions may be more complex than thought

Layers in ionic solutions may be more complex than thought

Researchers have long thought that a layer of positively or negatively charged ions builds up on the surface of an electrolyte solution, with a second layer of oppositely charged ions just below the surface. This electrical double layer was believed to play a critical role in what happens where air and liquid meet. But for most solutions, that’s not quite right, according to new research.

A recent study shows that most solutions form a thin layer of pure water on the surface, followed by a layer of concentrated ions. In other words, they are active beneath the surface, he says. Mischa Bonn from the Max Planck Institute for Polymer Research, who led the study. The result is stratification: the first two layers and then a less concentrated bulk saline solution (Nat. Chemistry. 2024, DOI: 10.1038/s41557-023-01416-6).

The research team also found new information about the orientation of water molecules on the surface. Existing models show them with either the hydrogen atoms or the oxygen atoms pointing up, depending on the surface activity. But in the absence of surface activity, there does not appear to be preferential orientation, according to the team.

The surface activity of electrolyte solutions can be difficult to determine because the energies involved are so small that it is difficult for simulations to provide unambiguous answers, Bonn says. His team developed a laser spectroscopy technique that is sensitive to the surface, measuring the vibrations of water molecules in the region, while suppressing signals from water molecules in the bulk solution. The team combined the resulting data with machine learning-based molecular dynamics simulations to arrive at the final results.

Understanding the reactivity of molecules on the water surface and of the ions themselves is important for creating better climate models. It is also vital to understand the photochemistry that occurs in aerosol droplets, which is determined by the organization of molecules and ions on the surface. “Our work aims to gain (these) insights at the molecular level and understand the physics and chemistry that occur on these surfaces, so we can improve those models,” Bonn says.

The researchers also believe their technique could give them unique insights into other interfaces. “We’re using them right now to analyze batteries,” including the interfaces between the batteries’ electrolytes and air and the electrodes, Bonn says. His team also wants to investigate more complex ion mixtures, the role of organic ions in these systems and how those factors affect the organization of water.

charlie wandresearcher at the University of Exeter, and Mirza Galib, assistant professor at Howard University, consider the paper an important addition to research in fundamental chemistry. Wand says the study exemplifies how experimental and computational methods can complement each other.

Galib adds that not only is it important to study the chemistry at the surface of aerosols and oceans, but observing the “distribution of ions at the air/water interface and at the solid/liquid interface is fundamental to much of the chemical development and industrial”.

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