Formation of methane clathrates in carbon nanotubes: a molecular dynamics study†
In this study, we investigated the possible formation of methane clathrates in CNTs with different sizes. Our results show that tetragonal, pentagonal, hexagonal, heptagonal, and octagonal ice nanotubes are formed in the (13,0), (14,0), (15,0), (16,0), and (17,0) CNTs, respectively, when only water molecules are present in the CNTs. When methane molecules are added to the CNTs, the water molecules in the ice nanotubes are replaced by the methane molecules, and the replaced water molecules occupy the middle of the CNTs in the hollow spaces. This is because the methane–CNT interaction is stronger than the water–CNT interaction. Another reason for this result can be due to the tendency of water molecules to form hydrogen bonds (HBs) as a row in the center of the CNTs. The results also show that the formation of methane clathrate in the CNTs is not spontaneous and needs application of hydrostatic pressure. We also investigated the thermodynamic, structural, and dynamic properties of the systems by employing the total energy, radial distribution function (RDF), radial and axial density profiles, and self-diffusion coefficients.