Neutrons Reveal Secrets of How Water Forms Cages around Gas Molecules

Gas hydrates, ice-like materials composed of water and natural gases such as methane, have grabbed lots of headlines in the last few years as a potentially vast store of energy.

Getting fewer headlines, but holding as much promise to solve other energy-related problems, are carbon-dioxide hydrates. Argonne’s Intense Pulsed Neutron Source (IPNS) has provided the first detailed look at carbon-dioxide hydrates as they form.

Using one of the world’s most productive sources of neutrons for materials research, scientists from Argonne’s IPNS and Energy Systems Divisions have unlocked some of the secrets of how water spontaneously forms molecular “cages” around gas molecules at low temperatures and under high pressures. This research is of interest to the oil and gas industries because gas hydrates sometimes form and cause blockages in natural gas and oil pipelines and equipment.

GLOBAL WARMING
This research may also have implications for global warming studies. Researchers in that field are investigating the possibility of sealing some of the atmosphere’s excess carbon dioxide in hydrates and storing them deep in the ocean where high pressures and low temperatures will keep them solid.

The crystalline structures of hydrates have been determined with X-ray, neutron and nuclear magnetic resonance techniques. However, using neutrons at IPNS, Argonne researchers were able to observe the rates, or kinetics, of the formation of carbon-dioxide hydrates from ice. By performing this measurement at several different temperatures, the Argonne scientists were able to determine the amount of energy needed to activate this process.

Scientists used liquid nitrogen to freeze deuterated water, water made with an isotope of hydrogen containing one proton and one neutron in its nucleus, instead of regular hydrogen which contains only a proton. Plain hydrogen causes interference in the instrument they used.

Then the ice was ground to a fine powder, placed in a cooled aluminum cell and lowered into the High-Intensity Powder Diffractometer at the IPNS. Under a barrage of neutrons, carbon dioxide gas was piped into the sample cell under high pressure.

Neutrons are uncharged particles found in nearly all matter. When used to penetrate materials, neutrons reveal structural and dynamic properties that can provide scientists with a better understanding of the physics that control the material’s behavior. This understanding can help scientists develop new materials with improved properties.

NEUTRON SNAPSHOTS
Unlike X-rays and other forms of light, neutrons penetrate both the aluminum cell and the ice crystals, providing a series of snapshots as the ice restructured itself around the carbon dioxide molecules.

The reaction proceeded according to the well-known “shrinking core model,” which describes many reactions, including that of concrete as it sets. The data hint at the mechanism by which the water molecules arrange themselves around the carbon dioxide molecules. It seems to occur, Argonne scientist Art Schultz said, at a quasi-liquid, “pre-melt” surface layer of water molecules. This layer is “slightly organized,” Schultz said, but doesn’t have the random movement of water or the crystal lattice of ice.

Future projects include varying the ice grain size, pressures and type of gas molecules. Preliminary tests are underway with argon.

This gas hydrate research is one example of the work being performed at the IPNS, the U.S. Department of Energy’s most reliable source of neutrons for studying the atomic arrangements and motions in liquids and solids. It has been used for more than 5,000 experiments since opening in 1981, and scientists request more than double the time the IPNS has available for experiments.

The IPNS has more users, more instruments, more experienced staff members and is the site of more experiments than any other pulsed neutron source in the nation. With its expertise in accelerator-based neutron source technology, Argonne is leading the design and construction of research instruments for what will be the most powerful spallation source in the world for neutron scattering research, the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory.

“Argonne was chosen because of its 20 years of experience in this game,” said Intense Pulsed Neutron Source Director Bruce Brown. “The first spallation source for neutron scattering was built here by Jack Carpenter and others in the mid-seventies.”

Scientists will use the SNS to study problems in chemistry, engineering, magnetism, superconductivity, crystalline materials, structural biology, polymers, disordered materials and complex fluids.

For more information please contact Dave Jacque at 630-252-5582

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