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 H₂ molecules in the Q1D gas hydrates are contained within a 1D nanochannel in which the H₂ molecules form a molecule wire. In particular, we show that in the (15,0) zigzag SW-CNT, the hexagonal H₂ hydrate tends to form, with one H₂ molecule per hexagonal prism, while in the (16,0) zigzag SW-CNT, the heptagonal H₂ hydrate tends to form, with one H₂ molecule per heptagonal prism. In contrast, in the (17,0) zigzag SW-CNT, the octagonal H₂ hydrate can form, with either one H₂ or two H₂ molecules per pentagonal prism (single or double occupancy). Interestingly, in the hexagonal or heptagonal ice nanotube, the H₂ wire is solid-like as the axial diffusion constant is very low (<5 × 10^-10 cm²/s), whereas in the octagonal ice nanotube, the H₂ wire is liquid-like as its axial diffusion constant is comparable to 10^-5 cm²/s.