Linking microscopic guest properties to macroscopic observables in clathrate hydrates: guest-host hydrogen bonding

Molecular dynamics simulations are used to compare microscopic structures and guest dynamics to macroscopic properties in structure II clathrate hydrates with cyclopentane, tetrahydrofuran (THF), 1,3-dioxolane, tetrahydropyran (THP), and p-dioxane as guests. Significant differences are observed between structural parameters and rotational dynamics for the different guests. The simulations show the formation of guest-host hydrogen bonds between the ether oxygen atoms of THF and THP and the cage water hydrogen atoms of the clathrate but the absence of similar hydrogen bonds in the clathrate hydrates of the other guests on the time scale of the calculations. This guest-host hydrogen bonding leads to the formation of Bjerrum L-defects in the clathrate water lattice where two adjacent water molecules have no covalently bonded hydrogen atom between them. Unlike Bjerrum defects of ice lattices, these guest-induced L-defects are not accompanied by the formation of a D-defect at an adjacent site in the water lattice. At the simulation temperature of 200 K, the guest-water hydrogen bonds in the THF clathrate are short lived (lifetime less than 1 ps) but in the THP they are longer lived (a minimum of 100 ps). A van't Hoff plot for the probability of defect formation in THF as a function of temperature gives an activation barrier of approximately 8.3 kJ/mol for guest-host defect formation in the THF clathrate. The consequences of the defect formation on the thermal expansivity, isothermal compressibility, dipole-dipole correlation function, and mechanical stability of the clathrate are discussed.