The behavior of ions in water drives everything from desalination to corrosion. But scientists didn’t have a clear view of how ions acted at the interface of liquid water and air, when only partially in contact, and not surrounded by water. The challenge was asking the right questions. A team did that and designed calculations and simulations to get the answers. They made two discoveries. First, water slows the separation of ions at the interface with air. Second, when surrounded by water, sodium and chloride ions rearrange water's bonding structure, changing how hydrogen bonds are formed and broken.
The study shows that the liquid-air interface affects how ions act. Specifically, the interface slows how salts (like table salt, or sodium chloride) dissociate. The team’s work offers insights into ion behavior to better control ions in corrosion, desalination, and material synthesis, to name a few examples.
Although working with ionic solutions is common in synthesis, separations, and subsurface science, how the ions change their environment and, in turn, how the environment influences the ions’ behavior was not well known. Researchers discovered how at the interface, liquid water suppresses ions’ ability to dissociate. On the solution’s surface, sodium and chloride ions stayed together longer than they did deep in the liquid (bulk water). In addition, the team found that the ions change the hydrogen-bonding structure of the surface water compared to the bulk water. Different hydrogen bond patterns form depending on the ions’ locations. The team’s findings came thanks to simulations and calculations that took months to set up and run on massive parallel computers. Unlike other simulations, these folded in both the thermodynamics and kinetics of ions in bulk water and at interfaces in calculating the dissociation rates.
The Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division funded this research. Calculations were carried out using computer resources provide by Department of Energy, Office of Science, Basic Energy Sciences.
L.X. Dang, G.K. Schenter, and C.D. Wick, “Rate theory of ion pairing at the water liquid-vapor interface.” Journal of Physical Chemistry C 121, 10018 (2017). [DOI: 10.1021/acs.jpcc.7b02223]