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Cygan, Randall Timothy

Publications and source records attributed to Cygan, Randall Timothy.

17 records
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DOE OSTI2015

Methane Hydrate Formation on Clay Mineral Surfaces: Thermodynamic Stability and Heterogeneous Nucleation Mechanisms

Heterogeneous nucleation of methane hydrates has been examined using molecular simulation, experimental bulk synthesis, and scanning probe microscopy. Theoretical nucleation rates were determined using molecular dynamics simulations as a function of clay surface represented by hydrophobic and hydrophilic systems. Methane hydrates were synthesized with and without Na-montmorillonite in a bulk reactor pressure assembly. X-ray diffraction and Raman spectroscopy confirm the nucleation and growth of the synthesized hydrates. Various kinetic pathways were explored to produce methane or isobutene clathrates in an ultra-high vacuum apparatus at very low temperatures but scanning probe microscopy only indicates the formation of ice.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Swelling properties of montmorillonite and beidellite clay minerals from molecular simulation: Comparison of temperature interlayer cation, and charge location effects

In this study, the swelling properties of smectite clay minerals are relevant to many engineering applications including environmental remediation, repository design for nuclear waste disposal, borehole stability in drilling operations, and additives for numerous industrial processes and commercial products. We used molecular dynamics and grand canonical Monte Carlo simulations to study the effects of layer charge location, interlayer cation, and temperature on intracrystalline swelling of montmorillonite and beidellite clay minerals. For a beidellite model with layer charge exclusively in the tetrahedral sheet, strong ion–surface interactions shift the onset of the two-layer hydrate to higher water contents. In contrast, for a montmorillonite model with layer charge exclusively in the octahedral sheet, weaker ion–surface interactions result in the formation of fully hydrated ions (two-layer hydrate) at much lower water contents. Clay hydration enthalpies and interlayer atomic density profiles are consistent with the swelling results. Water adsorption isotherms from grand canonical Monte Carlo simulations are used to relate interlayer hydration states to relative humidity, in good agreement with experimental findings.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Molecular simulation of structure and diffusion at smectite-water interfaces: Using expanded clay interlayers as model nanopores

In geologic settings relevant to a number of extraction and potential sequestration processes, nanopores bounded by clay mineral surfaces play a critical role in the transport of aqueous species. Solution structure and dynamics at clay–water interfaces are quite different from their bulk values, and the spatial extent of this disruption remains a topic of current interest. We have used molecular dynamics simulations to investigate the structure and diffusion of aqueous solutions in clay nanopores approximately 6 nm thick, comparing the effect of clay composition with model Na-hectorite and Na-montmorillonite surfaces. In addition to structural properties at the interface, water and ion diffusion coefficients were calculated within each aqueous layer at the interface, as well as in the central bulk-like region of the nanopore. The results show similar solution structure and diffusion properties at each surface, with subtle differences in sodium adsorption complexes and water structure in the first adsorbed layer due to different arrangements of layer hydroxyl groups in the two clay models. Interestingly, the extent of surface disruption on bulk-like solution structure and diffusion extends to only a few water layers. Additionally, a comparison of sodium ion residence times confirms similar behavior of inner-sphere and outer-sphere surface complexes at each clay surface, but ~1% of sodium ions adsorb in ditrigonal cavities on the hectorite surface. Thus, the presence of these anhydrous ions is consistent with highly immobile anhydrous ions seen in previous nuclear magnetic resonance spectroscopic measurements of hectorite pastes.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Surface Assisted Formation of methane Hydrates on Ice and Na Montmorillonite Clay

Methane hydrates are extremely important naturally-occurring crystalline materials that impact climate change, energy resources, geological hazards, and other major environmental issues. Whereas significant experimental effort has been completed to understanding the bulk thermodynamics of methane hydrate assemblies, little is understood on heterogeneous nucleation and growth of methane hydrates in clay-rich environments. Controlled synthesis experiments were completed at 265-285 K and 6.89 MPa to examine the impact of montmorillonite surfaces in clay-ice mixtures to nucleate and form methane hydrate. The results suggest that the hydrophilic and methane adsorbing properties of Namontmorillonite reduce the nucleation period of methane hydrate formation in pure ice systems.

clathrate hydrate↗