Neutron Scattering Investigation of the Structure and Dynamics of Propylene Oxide Clathrate Hydrate

Camille Jones, NCNR

Clathrate hydrates (CHs) have attracted considerable interest recently, owing to their presence in vast "gas hydrate" deposits under the ocean floor, their detrimental formation in gas and oil pipelines, possible existence in extraterrestrial environments, but primarily because of their potential as an energy source and gas storage medium. The synthesis and measurement of gas hydrates require extreme conditions of pressure and temperature, however, CHs of water-soluble cyclic ethers form under considerably milder conditions and their study has contributed a great deal to what is known about the structure and dynamics of this class of inclusion compounds.

Neutron diffraction and inelastic scattering studies of CHs of cyclic ethers tetrahydrofuran and trimethylene oxide provided early details on the structural distortions accompanying thermal expansion, the presence of distinct regions of low-temperature and high-temperature guest molecule dynamics, and correlations of these behaviors (and their temperature dependence) with the size of the guest. In this seminar, results of our most recent neutron scattering studies are presented for the structure II (sII) CH containing propylene oxide (PO). For comparison to the experimental results, we employed classical MD with empirical force-fields to simulate a fully-occupied sII PO CH crystal comprising eight unit cells with initial configurations of atoms assigned from neutron diffraction data obtained at 10 K. This seminar presents an overview of the thermal expansivities, cage volumes as a function of temperature, and quasielastic neutron scattering results and their comparison with results obtained from the MD simulations.

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