Exploration of Low-Dimensional Gas Clathrate Hydrates

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Water molecules consist of two hydrogen atoms bound to a central oxygen in a boomerang shape. When many water molecules condense into liquid water, or solid ice, this simple placement of atoms results in a unique molecular-level structure and complex properties that depend only on the arrangement of atoms in the water molecule, and not the container in which its placed. At least this is true for large containers. As the size of the container is reduced to nanometers (about 100,000 times thinner than sheet of paper), the structure of the liquid is also affected by the interaction of the individual molecules with the sides of the container. Nanoscale confinement can result in structures and properties that differ dramatically from bulk water. In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) Program of the Chemistry Division, Professors Joseph S. Francisco, Xiao Cheng Zeng, Chin Li Cheung, and Jaeil Bai of the University of Nebraska-Lincoln (UNL) are using computer simulations combined with laboratory experiments to explore the physical properties of water and water/methane mixtures (known as clathrate hydrates) when they are confined to nano-slits and isolated carbon nanotubes. The fundamental insights gained from this work contribute to our understanding of a broad range of societal problems, including frost heaving in soil (an upwards swelling of soil during freezing conditions) and the extraction of natural gas from shale rock. In addition, the project is training students and postdoctoral associates in computer-aided chemical research and in the design of advanced experimental devices. Measurements are carried out in a Department of Energy National Laboratory. The project focuses on molecular insights into physical properties and phase transitions of highly confined water and ice, low-dimensional polymorphous and polyamorphous transitions, as well as the prediction of new phases of low-dimensional ice and clathrate hydrates. More specifically, the UNL team is pursuing three objectives: seek new polymorphous and polyamorphous phases of water and gas hydrate in nano-slits with widths of 8 - 12 Angstroms (or the bilayer to trilayer forms of ice); investigate the mechanism of low-dimensional carbon dioxide (CO2) and carbon monoxide (CO) hydrate formation; study the molecular hydrogen hydrate formation mechanism in nanoscale confinement focusing on hydrogen storage. Molecular dynamics (MD) simulations, including the state-of-the-art, replica-exchange MD simulations for computing the free-energy surface of confined fluids, is used.