The elastic and acoustic properties of several structure II gas hydrates with hydrocarbon guests (methane, ethane, propane, and isobutane) were investigated and quantified using density functional theory. The shear modulus of ethane−methane hydrates was found to be the highest among all investigated hydrates. Simple (single-guest) hydrates were found to be less resistant to shear stresses than mixed (double-guest) hydrates. In fact, the shear properties (i.e., shear modulus and shear wave velocity) were shown to be closely related to the level of anisotropy in the hydrate crystal lattice, which itself was a function of guest size. A linearly decreasing relationship between the compressional wave velocity and the molecular weight of the guest was also presented. The hydrate crystal structure was analyzed at the atomistic level during triaxial compression and extension. The main findings were that the ultimate tensile strength decreases with guest size, the large cages are more compressible than the small cages, and the bond lengths (H-bonds and O−H bonds) exhibit opposite behavior (i.e., when one lengthens the other shortens), as observed in other hydrogen-bonded systems. The reported properties, structure−property relations, and molecular understanding provide a foundation for the evolving fundamental understanding and technological advances of these materials.

This work uses density functional theory (DFT) to investigate the poorly characterized structure II gas hydrates, for various guests (empty, propane, butane, ethane-methane, propane-methane), at the atomistic scale to determine key structure and mechanical properties such as equilibrium lattice volume and bulk modulus. Several equations of state (EOS) for solids (Murnaghan, Birch-Murnaghan, Vinet, Liu) were fitted to energy-volume curves resulting from structure optimization simulations. These EOS, which can be used to characterize the compressional behaviour of gas hydrates, were evaluated in terms of their robustness. The three-parameter Vinet EOS was found to perform just as well if not better than the four-parameter Liu EOS, over the pressure range in this study. As expected, the Murnaghan EOS proved to be the least robust. Furthermore, the equilibrium lattice volumes were found to increase with guest size, with double-guest hydrates showing a larger increase than single-guest hydrates, which has significant implications for the widely used van der Waals and Platteeuw thermodynamic model for gas hydrates. Also, hydrogen bonds prove to be the most likely factor contributing to the resistance of gas hydrates to compression; bulk modulus was found to increase linearly with hydrogen bond density, resulting in a relationship that could be used predictively to determine the bulk modulus of various structure II gas hydrates. Taken together, these results fill a long existing gap in the material chemical physics of these important clathrates.

The infrared spectra of sII gas hydrates have been computed using density functional theory for the first time, at equilibrium, and under pressure. It is also the first account of a full vibrational analysis (both guest and host vibrations) for gas hydrates with hydrocarbon guest molecules. Five hydrate structures were investigated: empty, propane, isobutane, ethane–methane, and propane–methane sII hydrates. The computed IR spectra are in good agreement with available experimental and theoretical results. The OH stretching frequencies were found to increase, while the H-bond stretching and H2O libration frequencies decreased with an increase in guest size and cage occupancy and with a decrease in pressure. The H2O bending vibrations are relatively independent of guest size, cage occupancy, pressure, temperature, and crystal structure. The guest vibrational modes, especially the bending modes, also have minimal pressure dependence. We have also provided more quantitative evidence that gas hydrate material properties are defined by their hydrogen bond properties, by linking H-bond strength to Young’s modulus. The results and ensuing vibrational analysis presented in this paper are a valuable contribution to the ongoing efforts into developing more accurate gas hydrate identification and characterization methods in the laboratory, in industry/nature, and even in outer space.

Tetrahydrofuran (THF) hydrates are often used as analogues for natural gas hydrates in experimental research because they can form at atmospheric pressure, despite the fundamental differences between THF and hydrocarbon guest molecules. In this work, we provide new and significant insights regarding the accuracy of this substitution, which has been a point of contention for many years, by investigating the elastic properties, crystal anisotropy, atomic structure, and vibrational properties of THF and THF−xenon hydrates using density functional theory. We found that our computed THF hydrate properties fall within the range of literature values for hydrocarbon hydrates, suggesting that THF hydrates are suitable substitutes for research into the mechanical properties of natural gas hydrates. Furthermore, we found that THF hydrates follow the same structure−property relationships as hydrocarbon hydrates: compressibility is governed primarily by hydrogen bond density, compressional wave velocity is a function of the average guest molecular mass, and the Young's modulus can be approximated from hydrogen bond properties. Taken together, these results have important implications for the development of THF and natural gas hydrate technologies.