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Physcial Sciences Division
Research Highlights

January 2008

Stopping at the Water's Edge

Scientists learn more about certain ions inclination to cross a boundary into water

Results: Dr. Liem Dang and Dr. Collin Wick at Pacific Northwest National Laboratory conducted a detailed computational study of how ions behave at the boundary or interface between water and 1,2-dichloroethane (DCE). The researchers discovered that the orientation of the DCE molecules can be used to control whether or not ions easily move into the water or stop before the water's edge.

Through exacting simulations, the researchers determined how molecules and ions moved every quadrillionth of a second. They found that positively charged cesium and sodium ions prefer to be near the boundary with water, while negatively charged chloride ions are repelled from the interface.

A snapshot of the water (right) and DCE interface with the large green cesium ion. This artwork was featured on the cover of the January 24, 2008, issue of The Journal of Physical Chemistry Part C.

While other studies have examined the interface between water and other liquids, this study addressed the structure of the interface and its effect on the distribution and transfer of polarizable ions. An ion is labeled "polarizable" if the particle's surrounding electron cloud can be distorted from its normal shape.

Why does it matter? Understanding how polarizable ions treat water, easily crossing into it or shyly staying in their solution, could provide insights for environmental remediation, energy storage, and other applications. Further, insights from the study could have implications for other liquid and ion combinations.

Methods: The researchers developed a model of the atoms in DCE and water. This model contained detailed information about the structure and behavior of DCE, water, and the boundary that forms between them as well as detailed information about the polarizable ions. Parameters were added so that the simulations mimicked the conditions found in laboratory experiments. With the models built, the scientists simulated different situations and analyzed the results.

Next steps: The researchers are continuing to explore how ions behave at the interface between water and other liquids. Their research has added new insights, but more questions remain that need to be answered.

Acknowledgments: The Office of Basic Energy Sciences in the U.S. Department of Energy funded this research, with support provided by the Chemical Sciences program and the Engineering and Geosciences Division.

Citation: Wick CD, and LX Dang. 2007. "Molecular Dynamics Study of Ion Transfer and Distribution at the Interface of Water and 1,2-Dichloroethane." Journal of Physical Chemistry C, 112(3):647-649.

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