Structure of the Ice-Clathrate Interface

Clathrate hydrates are crystals in which water molecules form hydrogen-bonded cages that enclose small nonpolar molecules, such as methane. In the laboratory, clathrates are customarily synthesized from ice and gas guest under conditions for which homogeneous nucleation of hydrates is not possible. It is not known how ice assists in the nucleation of clathrate hydrates or how ice forms on clathrate hydrate in the case of self-preservation. There is no lattice matching between any plane of ice and clathrate hydrates; therefore, an interfacial transition layer has to form to connect the two crystals. Here, we use molecular dynamic simulations to study the structure of ice-clathrate interfaces produced by alignment and equilibration of the crystals, competitive growth of ice and clathrate from a common solution, nucleation of hydrate in the presence of a growing ice front, and nucleation of ice in the presence of clathrate hydrates. We find that the interfacial transition layer between ice and clathrate is always disordered and has a typical width of two to three water layers. Water in the interfacial transition layer has tetrahedral order lower than either ice or clathrate and higher than liquid water under the same thermodynamic conditions. The potential energy of the water in the interfacial transition layer is between those in liquid water and the crystals. Our results suggest that the disordered interfacial transition layer could assist in the heterogeneous nucleation of clathrates from ice and ice from clathrates by providing a lower surface free energy than the iceliquid and clathrateliquid interfaces.