Kalinichev and Kirkpatrick's research is the horse that has to go in front of the cart. Before undertakings like sequestration can be fully understood, the physical and chemical properties of water and carbon dioxide solutions and how they interact with their surroundings have to be brought into relief.

To do that, the team focuses closely on the hydrogen bonding between the molecules in their simulated solutions. When a hydrogen atom bonds to another atom that strongly attracts electrons, the resulting molecule is very polar, with one end strongly positive and one strongly negative. Hydrogen bonds form between the opposing ends of these polar molecules.

In the sorts of environments where carbon dioxide sequestration would be most common—under thousands of feet of earth or ocean—temperature and pressure vary widely, from near freezing to 400 F and with pressures of up to 1,000 times the atmospheric pressure at sea level. Accordingly, the team uses these two factors as their two most common thermodynamic variables.

Hydrogen-bonded clusters of water molecules (blue) formed in supecritical carbon dioxide (gray and red). Hydrogen bonds are shown as dashed yellow lines.

"The dissolved species are dynamic objects. Hydrogen bonding is constantly changing. The models allow us to estimate lifetimes of different bonds under different conditions and states," says Kalinichev.

Already they have discovered that hydrogen bonding is reduced at high temperatures, while pressure has little impact. The reduced bonding makes carbon dioxide—which does not readily bond, and thus dissolve, because it is not very polar—more soluble under what would be common sequestration conditions. And by understanding the hydrogen bonds, the team can also predict properties such as density, viscosity, diffusion rates, and heat capacity under changing conditions.

The models previously included only pure water, but they now consider carbon dioxide, carbonates like limestone that might make up a mineral wall that the water carbon dioxide mixture would interact with, and salts like sodium chloride, as well. Adding these compounds—and basing the models on first principle calculations—brings the simulations much closer to the real world. The amount of time required to complete these models is substantial, and there's still a great deal of work to be done. One picosecond of the molecular dynamics simulation requires about an hour on five Origin2000 processors, and a typical run tracks the behavior of only several thousand atoms over the course of several hundred picoseconds.

"We're still simulating a relatively small number of molecules, therefore we are applying so called periodic boundary conditions to simulate bulk aqueous solutions and their interactions with mineral surfaces," says Kalinichev. "But you have to begin with these mechanical descriptions of each molecule to extract the information that you want."

Kirkpatrick adds, "Today's science requires—absolutely requires—thinking on the molecular scale to understand what takes place on the macroscopic scale."

This research is supported by the Center for Advanced Cement Based Materials, the National Science Foundation, and the Department of Energy Basic Energy Sciences Carbon Management Program, Geosciences Division.