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Research Highlights

May 2009

Ions Disturb Water's Structure in Unexpected Ways

Research team finds symmetrical ions perturb bulk water asymmetrically

Results: When a symmetrical ion settles onto the asymmetrical surface of water, it does not disturb the surface evenly. But according to scientists at Pacific Northwest National Laboratory and Louisiana Tech University, the same occurs when a symmetrical ion settles in a symmetrical water environment. The ion pushes unevenly on the surrounding water molecules.

"The ion is symmetrical. The environment is symmetrical. The expectation was a symmetrical disruption—but it wasn't," said Dr. Sotiris Xantheas, the PNNL researcher on the paper. "This is important for what drives these ions to the interface. They form a small bubble on one side of them, due to their asymmetric environment, and we all know what happens to bubbles in water, they go to the surface," added Dr. Collin Wick, the Louisiana Tech professor on the paper.

These results graced the cover of the April 2009 special issue of The Journal of Physical Chemistry B. The issue was devoted to the results of the Aqueous Solutions and Their Interfaces Workshop, co-organized by Xantheas and Professor Gregory Voth, University of Utah. The workshop was the outgrowth of long, contentious debates in the scientific community about the structure of water in the bulk and at the air-water interface.

Why It Matters: Understanding the structure of a volume of water or bulk water and the way it is perturbed by accommodating various ions and solutes is of paramount importance to understanding key chemical and biological processes. For example, ions take part in acid-base reactions that determine the formation and transport of environmental pollutants. Also, ions are pumped across cell membranes by proteins creating necessary energy reservoirs for the cell.

Methods: To study the complex interactions that occur at the molecular level, the researchers modeled the interactions between 550 water molecules and 2 different types of negatively charged ions. The ions selected were chloride and iodide.

The models were run using home-grown software that takes into account the complex interactions between water molecules as well as between ions and water. Because of the computational demands of this software, the simulations used resources at the National Energy Research Scientific Computing Center and the Louisiana Optical Network Initiative.

The researchers analyzed the resulting simulations and determined that when chloride or iodide ions were added into bulk water, the molecules formed a cage or solvation shell around the ion. The solvation shell was uneven as a small cavity formed on one side between the ion and the water molecules.  Water generally will expel cavities to its surface, which is found to be the case for these ions.  When at the water surface, these ions are already in an asymmetric environment with water on one side and air on the other, so they can be found at the surface more than in the middle or bulk of water.

What's Next? Further studies are needed, specifically with more accurate descriptions of the underlying intermolecular interactions, to validate this study. Xantheas and his colleagues are conducting such research, examining the structural parameters of liquid water, and plan to publish their results shortly. 

Reference: Wick CD, and SS Xantheas. 2009. "Computational Investigation of the First Solvation Shell Structure of Interfacial and Bulk Aqueous Chloride and Iodide Ions." The Journal of Physical Chemistry B 113(13):4141-4146.

Acknowledgments: The research was funded by the Office of Basic Energy Sciences, U.S. Department of Energy, as well as the Louisiana Board of Regents Research Competitiveness Subprogram.

This work was done by Collin Wick at Louisiana Tech University and Sotiris Xantheas at PNNL. The work was done using resources at the National Energy Research Scientific Computing Center and the Louisiana Optical Network Initiative.

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