PNNL

The behavior of ions in water drives everything from desalination to corrosion. But how the behavior of ions changes at the interface of liquid water and air, not surrounded by the water, wasn't well known. The challenge was in asking the right questions and in designing the calculations and simulations to get the answers. Led by Dr. Liem Dang at Pacific Northwest National Laboratory, researchers made two discoveries. First, water slows the separation of ions at the interface with air. Second, when surrounded by water, spherical sodium and chloride ions rearrange water's bonding structure, breaking and forming hydrogen bonds.

"This type of research fundamentally changes how we understand water," said Dang, the principal investigator on the project.

Why It Matters: The transport and behavior of ions affect chemical reactions at air-liquid interfaces. Ions at water's boundary are vital to synthesis and corrosion. The team's research shows that the liquid-air interface affects the fundamental behavior of ions. The interface slows dissociation, effectively altering ions' behavior.

Summary: Although working with ionic solutions is fairly 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 from Pacific Northwest National Laboratory and Louisiana Tech University discovered how at the interface, the liquid water suppresses ions ability to dissociate, or move away from each other. On the solution's surface, the sodium and chloride ions stayed together longer than they did deep in the liquid (bulk water). In addition, they found that the ions changed the hydrogen bonding structure of the bulk water. Different hydrogen bond patterns form depending on the location of the ions.

"In general, if the ions move into the bulk liquid, it distorts the whole system," Dang said. "The water molecules have to re-adjust and create different interactions."

The team's findings came thanks to complex 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 rate. "Nobody's ever accomplished this before," said Dang.

The team's next step is delving into reactive phenomena, such as the behaviors of the hydronium and hydroxide ions at interfaces with other counter ions. The team's upcoming efforts are more challenging due to the quantum nature of the ions. The work requires more sophisticated methods, such as ab initio molecular dynamics. "It's been a long journey to understand these interactions and how they change the behavior of water," said Dang. "The next step is going to be much harder."

Acknowledgments

Sponsor: Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division

Facility Use: Calculations were carried out using computer resources provide by Department of Energy, Office of Science, Basic Energy Sciences

Reference: Dang LX, GK Schenter, and CD Wick. 2017. "Rate Theory of Ion Pairing at the Water Liquid-Vapor Interface." Journal of Physical Chemistry C 121:10018-10026. DOI: 10.1021/acs.jpcc.7b02223