Anisotropic lattice expansion of structure H clathrate hydrates induced by help guest: Experiments and molecular dynamics simulations

The structure H (sH) clathrate hydrates of neo-hexane with argon, krypton, and methane help gases are synthesized to study the effect of the help gas on the crystal lattice structure. Powder X-ray diffraction (PXRD) measurements on these hydrates were performed for temperatures in the range of 93-183 K, and the a-axis and c-axis lattice constants and the small and medium sH cage occupancies were determined. The PXRD results show that the a-axis lattice constants of the three clathrate hydrates are close in magnitude, but at each temperature, for the c-axis lattice constants c(CH₄) > c(Kr) > c(Ar). Parrinello-Rahman molecular dynamics (MD) simulations were performed on the three sH clathrate hydrate phases. The PXRD observed trends in the a-axis and c-axis lattice constants at different temperatures were reproduced by the simulations. The dynamics of the small cage guests are characterized by the velocity autocorrelation functions. The experiments and computations show the complex interplay of the molecular size and interaction energies in determining the lattice structure and stability of even relatively simple clathrate hydrates of nonpolar, hydrophobic molecules.