Spontaneous Formation of One-Dimensional Hydrogen Gas Hydrate in Carbon Nanotubes

We present molecular dynamics simulation evidence of spontaneous formation of quasi-one-dimensional (Q1D) hydrogen gas hydrates within single-walled carbon nanotubes (SW-CNTs) of nanometer-sized diameter (1-1.3 nm) near ambient temperature. Contrary to conventional 3D gas hydrates in which the guest molecules are typically contained in individual and isolated cages in the host lattice, the guest H-2 molecules in the Q1D gas hydrates are contained within a 1D nanochannel in which the H-2 molecules form a molecule wire. In particular, we show that in the (15,0) zigzag SW-CNT, the hexagonal H-2 hydrate tends to form, with one H-2 molecule per hexagonal prism, while in the (16,0) zigzag SW-CNT, the heptagonal H-2 hydrate tends to form, with one H-2 molecule per heptagonal prism. In contrast, in the (17,0) zigzag SW-CNT, the octagonal H-2 hydrate can form, with either one H-2 or two H-2 molecules per pentagonal prism (single or double occupancy). Interestingly, in the hexagonal or heptagonal ice nanotube, the H-2 wire is solid-like as the axial diffusion constant is very low (<5 x 10(-10) cm(2)/s), whereas in the octagonal ice nanotube, the H-2 wire is liquid-like as its axial diffusion constant is comparable to 10(-5) cm(2)/s.