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Physcial Sciences Division
Research Highlights

June 2009

Where in the Water Are Hydroxide Ions?

Chemists use new computer models, calculations to determine ion's travel plans

Results: Just as parents want to know where their teens go, scientists want to know where hydroxide ions travel. And, as is true for parents and progeny, this ion's choices have stumped researchers for years. Different studies yielded different answers. So theoretical chemists at Pacific Northwest National Laboratory and Louisiana Tech University teamed up and used a new approach that takes into account aspects of ion behavior not considered previously. They showed that hydroxide ions travel plans do not involve congregating at the water's surface.

Why it matters: From forming clouds to growing crops, many of the reactions that sustain life on earth occur where water and air meet, known as the water-air interface. So researchers want to know what is happening at the water-air interface to predict ecosystem behavior, such as climate changes, and mitigate problems, such as acid rain and hazardous waste spills.

"Some of the most important chemistry on the planet occurs where air and water meet," said Dr. Liem Dang, a theoretical chemist at PNNL and a researcher on this study.

Further, the behavior of ions at the water-air interface is vital in current and future industrial processes. For example, understanding how ions react at the surface could aid researchers striving to produce automotive fuel from poplar trees and other vegetation.

Methods: The researchers used two different approaches to model the hydroxide ion's propensity for the water's surface. The first model is a classical model that focuses on the ion's polarizability. Just as with politics, polarizability refers to the tendency to move toward one extreme or another. In ions, it is the electrons tendency to move to one location or extreme on the ion, instead of moving smoothly around the whole ion. 

"Understanding a hydroxide ion's polarizability may be key to understanding how it behaves at interfaces," said Dang.

The second model is known as the multistate empirical valence bond model, or MS-EVB for short. This model, used for the first time on this type of study, includes proton sharing in its modeling of the hydroxide ion. Proton sharing is when the hydrogen on the hydroxide ion can move back and forth or form a bond between the oxygen atoms of the hydroxide ion and the water molecules.

After running both models on computers provided by the Department of Energy and the Louisiana Optical Network Initiative, the scientists found the hydroxide ion prefers to congregate at the water's surface.

Next steps: The scientists are continuing to study how hydroxide and hydronium ions behave at the water-air interface. They are conducting not only new studies of the ions, but also refining existing models and building new tools to get needed answers.

Acknowledgments: DOE's Office of Basic Energy Sciences and the Louisiana Board of Regents funded this research. The work was done by Collin Wick of Louisiana Tech University and Liem Dang of PNNL.

Reference: Wick CD and LX Dang. 2009. "Investigating Hydroxide Anion Interfacial Activity by Classical and Multistate Empirical Valance Bond Molecular Dynamics Simulations." Journal of Physical Chemistry A  113(22):6356-6364.

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Hydroxide ions

A water molecule, comprised of two hydrogen atoms and an oxygen atom, typically breaks apart into two bits. One is an excess proton, which combines with a water molecule to form the hydronium species, H3O+.  This ion makes solutions acidic. The other bit is a hydroxide ion, which contains a hydrogen and oxygen atom bound together with an extra electron. The negative ion makes solutions basic.