Over the years the analysis methods have been perfected. And they regularly urge scientists to check their models. It just happened again. Researchers have discovered that salt water doesn't behave quite the way they thought.
This will also interest you
At first glance, it might only be of interest to physicists – or chemists – to know how ions and molecules are distributed at the interface of air and water. But upon closer inspection, this question of basic science could well have an impact on our daily lives. Let's use the recent discovery on this topic by researchers, including from the University of Cambridge (UK), to understand together why and how.
First, remember that many important natural reactions occur where water molecules interact with air. This is the example of evaporation of sea water. The process plays a crucial role in atmospheric chemistry and climate science. And it is determined by the molecular structure of the interface. Understanding this structure and its dynamics therefore appears to be crucial to efforts to mitigate our impact on our planet.
Technology to better understand water
Until now, physicists who wanted to study how water molecules are affected by the distribution of ions when air and water meet have relied on a very specific spectroscopy technique. In fact, vibrational sum frequency spectroscopy (VSFG) allows the study of the vibrational properties of molecules at interfaces. But she has a weak point. It is not possible to distinguish positive signals from negative signals. This makes it difficult to interpret the results.
To study their electrolytic solutions – solutions made up of water molecules and various ions – researchers at the University of Cambridge report in the journal Nature Chemistry that they have opted for a more sophisticated form of the technique based on what they call heterodyne detection . And they developed advanced computer models to simulate the interfaces in different scenarios.
A discovery that upends established theories about the interfaces between water and air
Physicists show that there are no ions at the water-air interface, whether positive – we call them cations – or negative – we call them anions. “Our work shows that the surface of simple electrolyte solutions has a different ion distribution than previously thought,” explains Yair Litman, a chemist, in a press release. Gone is the idea of the double ionic layer aligning water molecules in a single direction. The researchers' new scenario is completely different. “At the top of the solution there are a few layers of pure water, then an ion-rich layer, and finally the large salt solution. »
We gave the example of seawater evaporation above. But many other areas are also affected by this discovery. And even more generally, the study of solid-liquid interfaces can have applications in the development of more efficient batteries and energy storage systems. So yes, this discovery should interest much more than theoretical physicists – or chemists…