Charged molecules do not form a boundary layer between saltwater and air as previously thought, new research reveals. Indeed, they are depleted there relative to their abundance in the liquid as a whole. Instead, at a depth of a few molecule’s diameters an ion-enrichment layer lurks, like some mythical beast waiting to surprise. The discovery upends perceptions of these boundaries that were viewed with such confidence they were written into scientific textbooks.
Life is most abundant where
“Our work demonstrates that the surface of simple electrolyte solutions has a different ion distribution than previously thought and that the ion-enriched subsurface determines how the interface is organized: going from air into the bulk salt solution, one first encounters a few layers of pure water, then comes a layer enriched in ions, before reaching the bulk,” Dr Yair Litman, of the Max Planck Institute for Polymer Research and the University of Cambridge, said in a
Besides having layers of water above them, Litman and colleagues found the
The long-standing error occurred because studies of the molecules at the boundary were done using lasers to measure the surface molecules’ vibrations, a method known as vibrational sum-frequency generation (VSFG). This reveals changes in vibration intensity at specific wavelengths when salt is added to water, which was thought to indicate a build-up of ions there.
Although VSFG is effective at measuring the strength of vibrations, it can’t detect their orientation – specifically whether the hydrogen atoms in the
Using a more advanced version, known as heterodyne detected-VSFG, the team examined the boundary layers in 11 types of electrolyte solutions at varying concentrations, and created computer models to make sense of what they saw.
The old models were not entirely wrong, however. Two common electrolytes, HCl and NaClO4, did indeed congregate at the surface.
Co-author Professor Mischa Bonn said, “These types of interfaces occur everywhere on the planet, so studying them not only helps our fundamental understanding but can also lead to better devices and technologies. We are applying these same methods to study solid/liquid interfaces, which could have potential applications in batteries and energy storage.”
The study is published open access in