Stability of doubly occupied N-2 clathrate hydrates investigated by molecular dynamics simulations

Classical molecular dynamics calculations were performed for a structure II clathrate hydrate with N-2 guest molecules in order to investigate the pos sibility of double occupancy, i.e., two N-2 molecules inside one large cage . For all of the pressures, temperatures, and compositions at which the sim ulations have been performed, the doubly occupied clathrate remained stable . The structure of the host lattice is indistinguishable from that of a sin gly occupied clathrate hydrate. The volumes and energies are linearly depen dent 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-N-2 radial distribution function, and therefore the structure in th e vicinity of the N-2 molecule, is similar to that of the mixed fluid. An e xtensive investigation of the distances in the cages shows a large similari ty between singly and doubly occupied clathrates. All these results indicat e that, upon filling the large cages with pairs of molecules, the stresses on the host lattice are comparable to those in single occupation. Small cha nges in the intermolecular interactions do not affect the results. The clat hrate hydrate destabilizes upon increasing the size of the guest molecules by more than about 10%. We also find that the clathrate hydrate remains sta ble 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 as in experiments. (C) 2001 American Institute of Physics.