Effect of Guest Size on the Mechanical Properties and Molecular Structure of Gas Hydrates from First-Principles

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.