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

Abstract

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.

Monte Carlo Calculations of the Free Energy of Binary SII Hydrogen Clathrate Hydrates for Identifying Efficient Promoter Molecules

“…The hydrogen bonds affect the clathrate hydrate structure and the dynamics of guest and host water molecules. It was observed in the study 9 that the oxygen atoms of 1,3-dioxolane (DIO) does not form hydrogen bonds with the large cage water molecules. As temperature increases, the probability of hydrogen bonds between the guest and the host water molecules can either decrease or increase depending on the guest 11 , whereas it is expected that the stability of clathrate decreases.…”

“…The relaxation contribution to the Helmholtz free energy difference has been derived in paper 12 (9) where the Hamiltonian α H of the double-size system of the relaxation stage depends of the translational constraint rˆ and the orientational constraints ηˆ and κˆ, Putting the replication and relaxation free energy contributions (6) and (9) together gives the Helmholtz free energy per particle of a general molecular crystal (10) have been found for all cases discussed here. …”

Water molecular reorientation in ice and tetrahydrofuran clathrate hydrate from lineshape analysis of 17O spin-echo NMR spectra

“…56 However, in the case of THF hydrate the MD simulations showed that these H-bonds are rather short lived (0.28 ps at 200 K), and thus no effects on the structure can be detected. 53 This injection of L-defects by the guest appears to be quite a different process from the propagation of Bjerrum defects, which governs the water reorientation process as observed by dielectric and NMR relaxation. 10,15 For instance, for THF hydrate the activation energy for L-defect injection was found to be ∼8.3 kJ/mol from MD simulation, 53 compared with an experimental value of ∼30 kJ/mol for defect propagation (see refs.…”

“…10,15 Recent molecular dynamics (MD) simulation studies on nanosecond timescales have in fact observed the existence of both transient and persistent guest-host hydrogen bonds for oxygencontaining guests [53][54][55] . In the simplest cases, this corresponds exactly to the injection of an L-defect as there is indeed a misdirected H-bond, leaving a hydrogen vacancy in the water lattice.…”

Ammonia clathrate hydrates as new solid phases for Titan, Enceladus, and other planetary systems

“…Recent structural analysis and molecular simulations have shown that some guest molecules which form strong hydrogen bonds with the water framework of the clathrate hydrate lattice may nonetheless produce stable phases (20)(21)(22)(23). It is therefore reasonable to consider that ammonia has potential as a clathrate guest molecule.…”