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

Abstract

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 × 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.

Curvature-dependent adsorption of water inside and outside armchair carbon nanotubes

“…2). The adsorption energy curves for a water molecule outside (6,6) CNT are very similar to those for water outside (4, 4) and (5, 5) CNTs and show nearly no curvature effects. Note that the adsorption energies of water outside (n, n) CNTs, see Table 1 also for larger CNTs, are close to the results of a water molecule adsorbed on a graphene surface, including the energy differences between Ù-and Ú-configurations which are 2123 and 2108 meV, respectively.…”

“…As an alternative, we aim here at obtaining reliable force field parameters by fitting effective water-CNT and water-graphene potentials to DF-LCCSD(T) results. To this end, we use the approximation of pairwise additive LJ potential energy functions which are often used to model nonbonded interactions and which served as a basis for almost all of the previous MD simulations of water confined in CNTs, see, for example, [5,6,9,12,13,15,18,23,31,33,36,37,54] : (6,6) CNT. The PBE-D2/D3 are from periodic calculations using VASP.…”

“…In recent years, the study of water confined inside low-diameter carbon nanotubes (CNTs) has attracted a lot of attention because of many exotic properties that differ from those of the bulk phases. [1][2][3][4][5][6] Being confined in tubes with diameters not much larger than the size of the water molecules themselves, the topology of their hydrogen-bonding networks is dominated by unique structures such as water wires, different helix-like or prism-like ice nanotubes, both single-or multi-walled. [7][8][9] Consequently, there is also a variety of different phase transitions of water inside small CNTs not seen in bulk water, and in some cases it was predicted that ice nanotubes can even exist at room temperature.…”

Formation of bilayer clathrate hydrates

“…[10][11][12][13][14][15][16][17][18][19][20][21][22][23] However, microscopic mechanism of the nucleation and growth of bulk clathrate hydrate is still not fully understood. [24][25][26][27][28][29][30][31][32][33] Moreover, the host cages formed in low-dimensional clathrate hydrates differ from those in bulk clathrate hydrates due to geometric constraints. 21 Recently, several simulation studies have shown that low-dimensional clathrate gas hydrates, as well as guest-free hydrates, can be formed spontaneously on the time scale of nanoseconds in highly confined environment.…”

Comment on “Calculations for the one-dimensional soft Coulomb problem and the hard Coulomb limit”

“…Some of the issues they bring about have already been extensively discussed and ultimately solved. Both the supposed negative infinite energy ground state and the parity eigenstates for the problem have been shown to be nonexistent on mathematical grounds using techniques that are very important for both ID systems and their applications [3][4][5][6][7][8][9][10][11][12],z lx | is not self-adjoint [13], but GW do not seem to pay enough attention to this crucial fact and its consequences [10]. Let us recall that conservation of probability is a direct consequence of the self-adjointness of the Hamiltonian according to Stone's theorem [14].…”