The interface between a solution and a surface has been investigated by researchers. Credit: RUB
Researchers from the Cluster of excellence RESOLV at the Ruhr-Universitt Bochum, its sister research network CALSOLV in Berkeley, and the University of evry in Paris have been studying how positively and negatively charged ion particles behave at the interface between a solid surface and an They were able to observe how the water shells around the salts are stripped away when the voltages are applied to the solution at the SOLEIL synchrotron. The results of their work were published in the journal PNAS on November 15, 2021.
There is a double layer between the electrolyte and solid interface.
There are chemical compounds that have separate ions. When NaCl is dissolved in water, the positively charged sodium ion and the negatively charged chloride ion can separate and move freely in the solution. A hydration shell is formed around the individual ion due to the electrical attraction between the ion and the water molecule. There is a layer of charge carriers in the vicinity of the electrical boundary layer. The positive and negative charge layers are opposite each other, which is why this layer is called an electrochemical double layer. When a voltage is applied, the attraction between the electrode and the ion strips off the water shell and a charge transfer occurs.
The picture explains how a battery works. The researchers from Berkeley and Paris looked into whether it is correct at the molecular level. They looked at whether the process was the same when there were either negative or positive voltages applied.
It is difficult to observe the process.
It's a special challenge to observe chemical processes on a level of complexity. This is what the scientists succeeded in doing in the current study, which they combined with simulations. The researchers investigated the double layer that forms in a NaCl solution in the vicinity of a gold surface at the SOLEIL synchrotron.
It is possible to follow the stripping of the hydration shell live. The water networks on the charged gold surface were shown for the first time by the researchers. Understanding how the total energy changes in the process is important. "It was amazing to see that the process runs differently for positive and negative charges," says Professor Havenith.
The hydration shell is asymmetric.
The hydration shells of both salt and chloride behaved differently in the double layer. The hydration shell of the positively charged ion was detached and the sodium ion was attracted to it. This happened at a higher applied voltage for the negatively charged chloride ion. The team was able to attribute the differences to the behavior of the water networks. The scientists confirmed the results using computer simulations.
The method and the results can now be used to investigate the crucial role of water in other interfacial processes. For technological applications such as solar cell or fuel cell technologies, the results are important.
Serena R. Alfarano and her team show how water networks matter by stripping away ion hydration shells in electrical double-layer formation. There is a book titled "10.1073/pnas.2108568118."
The National Academy of Sciences has a journal.
The news about how positively and negatively charged ion behave at interface was retrieved fromphys.org on November 23, 2011.
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