Stability of doubly occupied N_2 clathrate hydrates investigated by molecular dynamics simulations

摘要

Classical molecular dynamics calculations were performed for a structure II clathrate hydrate with N2' guest molecules in order to inves;tigate the possibility of double occupancy, i.e., two N2 molecules inside one large cage. For all of the pressures, temperatures, and compositions at which the simulations have been performed, the doubly occupied clathrate remained stable. The structure of the host lattice is indistinguishable from that of a singly 'occupied clathrate hydrate. The volumes and energies are linearly dependent on the filling fraction. The range of values are the same for both the singly as well as doubly occupied clathrates. In the doubly occupied cages, the O-N2 radial distribution function, and .therefQre the s~cture in the vicinity of the N2 molecule, is similar to that of the mixed fluid. An extensive investigation of the distances in the cages shows a large similarity between singly and doubly occupied clathrates.All these results indicate that, upon filling the large cages with pairs of molecules, the stresses on the host lattice are comparable to those in single occupation. Small changes in the intermolecular interactions d() not affect the results. The clathrate hydrate destabilizes upon increasing .the size of the guest molecules by more than about 10%. We also find that the clathrate hydrate remains stable when experimental conditions are closely followed, i.e., for partially doubly occupied clathrates with a pressure dependent filling fraction. The calculated compressibility is the same a~ in experiments.