The researchers found that the hydration shells of sodium and chloride ions behaved differently in the electrochemical double layer. “It was astonishing for us to see that the process runs differently for positive and negative charges,” sums up Professor Martina Havenith, spokesperson of RESOLV.Īsymmetric detachment of the hydration shell This is essential to understand how the total energy changes in the process. The researchers also showed for the first time how the water networks on the charged gold surface change. Terahertz spectroscopy makes it possible to follow the stripping of the hydration shell live. For this purpose, the researchers investigated the electrochemical double layer that forms in a NaCl solution in the immediate vicinity of a gold surface at the SOLEIL synchrotron in Paris. This is exactly what the scientists succeeded in doing in the current study with terahertz spectroscopy, which they combined with simulations. Observing chemical processes on a molecular level while a voltage is applied is a special experimental challenge. Observation during the process is difficult They also checked whether the process is identical when negative or positive voltages are alternately applied. In the present work, the researchers from Bochum, Berkeley and Paris investigated whether it is also correct at the molecular level. This simple picture explains how a battery works. According to chemistry textbooks, the following happens when a voltage is applied: the attraction between the electrode and the ions strips off the water shell and a charge transfer, a current, occurs. A positive and a negative charge layer are opposite each other, which is why this layer is also called an electrochemical double layer. A layer of charge carriers forms in the immediate vicinity of the electrical boundary layer between the electrode and water. Due to the electrical attraction between the ions and the water molecules, a shell of water molecules forms around the individual ions – a so-called hydration shell that is stable in the solution. For example, when sodium chloride (NaCl) is dissolved in water, the positively charged sodium ions and the negatively charged chloride ions separate and can move freely in the solution. ![]() They describe their results in the journal Proceedings of the National Academy of Sciences of the United States of America, or PNAS, published online on 15 November 2021.Įlectrochemical double layer between electrolyte and solid interfaceĮlectrolytes are chemical compounds in which separate ions occur. At the SOLEIL synchrotron, they were able to use terahertz spectroscopy to observe exactly when and how the water shells around sodium and chloride ions are stripped away when voltages are applied in an electrolyte solution. ![]() How positively and negatively charged ions behave at the interface between a solid surface and an aqueous solution has been investigated by researchers from the Cluster of Excellence RESOLV at the Ruhr-Universität Bochum, its sister research network CALSOLV in Berkeley, and the University of Evry in Paris.
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