Geochemists develop new modeling techniques

An article published in Nature Materials features a new theory to explain how minerals and glass interact with water. The paper was co-authored by James Rustad, a UC Davis Geology Department emeritus professor, and William Casey, a professor in the UC Davis Chemistry Department.

Existing models “could explain anything and predict nothing,” Casey said.

“Environmental chemists and geochemists inherited these models from gas chemistry and then tried to apply them to complicated materials in water,” Casey said.

“To test the application, Jim [Rustad] and I looked at the isotope-exchange reactions in a structure where one or two atoms could be changed at will,” Casey said. “It turned out we couldn’t predict anything using those old theories.”

The topic of how minerals interact with water is important because it informs many developing applications, such as generating electricity in an automobile by using waste heat created by the motor, finding a way to capture and store carbon dioxide produced by power plants to reduce greenhouse gas levels in the atmosphere or obtaining oxygen for use from water molecules.

Rustad, as a computational chemist, handled the computer-simulation aspects of the research project. He explained how he was able to create computer models based on the actual substances that Casey, an experimental chemist, and his research group created.

“What Bill’s research group did — they actually could make a real ‘chunk’ [of a mineral] that was similar in size to what a modeler can calculate, and then measure very precisely the lifetimes of each oxygen atom in that chunk,” Rustad said.

Previous methods involved guesswork which hindered making effective findings, but under the new method the “different ways of predicting how rapidly the oxygen atoms exchange with the surrounding water” could be directly tested, Rustad said.

“That’s really the first time that’s happened… in geochemistry,” Rustad said.

The created substances, or “molecular models,” are made up of polyoxometalate ions, or POMs, which are negatively charged substances that contain metals linked together with oxygen atoms. By experimentally modifying clusters of POMs, Rustad and Casey were able to test predictions about oxygen-exchange rates in the material.

Existing models, prior to Casey and Rustad’s research, attempted to explain oxygen-exchange rates based on the rupture of just one or two chemical bonds. The structures Casey and Rustad examined were more complex and formed temporary configurations of atoms called “metastable” states, meaning higher-energy, less stable states.

Metastable states can be described using the analogy of a ball resting in a recessed area on the slope of a hill, Rustad explained. When the ball is pushed out of the recessed area, it loses the extra energy and returns to a ground state.

“We used the computer simulations to identify the metastable states,” Rustad said. These metastable states cannot yet be observed experimentally, due to the liquid environment. Tools such as the electron microscope can only be used to view substances in a vacuum, Casey said.

“These metastable forms could be detectable, but only via methods suitable for wet samples,” Casey said.

“These results are important to a wide range of fields, including materials engineering, nanotechnology and geochemistry,” said Andy Ohlin, a Queen Elizabeth II Fellow at Monash University in Melbourne, Australia. The research team also included Eric Villa, a post-doc at the University of Notre Dame.

BRIAN RILEY can be reached at science@theaggie.org.