Formation of bilayer clathrate hydrates

We report molecular dynamics (MD) simulation evidence for a new family of two-dimensional (2D) clathrate hydrates. Particular attention is placed on the effect of the size and hydrophilicity of guest-molecules on the formation of 2D clathrate hydrates. Among the MD simulations undertaken, the spontaneous formation of bilayer (BL) clathrate hydrates in nanoslits are found with five different hydrophobic guest molecules, namely, ethane (C₂H₆), ethene (C₂H₄), allene (C₃H₄), carbon dioxide (CO₂) and hydrogen (H₂) molecules. Our simulations suggest that the host cages in a water framework are likely BL-hexagonal cages with single occupancy for H₂ or BL-heptagonal cages for CO₂. With a further increase in guest size, the host cages for C₂H₆, C₂H₄, and C₃H₄ are BL-octagonal cages with single occupancy, and their long molecular axis tends to be normal to the surface of clathrate hydrates. In addition, for hydrophilic guest molecules, such as NH₃ and H₂S, which can form strong hydrogen bonds with water, we find that most guest molecules can preferentially displace water molecules from the lattice sites of the water framework, instead of being separately trapped within the water cages. Structural analogy between the 2D and 3D clathrates enlightens us to predict the stability of several bulk gas hydrates, namely, "ethane clathrate III", "CH₄ ice-i" and "H₂ ice-i". Our findings can not only enrich clathrate structures in the hydrate family but may also improve the understanding of hydrate formation in microporous media.