Structure and stability of water chain in a carbon nanotube

Water molecules form a single-file chain structure in a (6, 6) carbon nanotube (CNT), and this stability is different from that of water molecules confined in CNTs with larger diameters, let alone the bulk. Using the molecular dynamics (MD) method and quantum mechanical (QM) calculations, we investigate the characteristics in the context of density dependence of the collective structure and hydrogen bond behavior. The results obtained from MD show that high water density leads to substantially longer hydrogen bond lifetimes. On the other hand, the hydrogen bond lifetime does not noticeably decrease with decreasing density but remains roughly the same when the density is lower than a certain critical value. The mean molecular orientation angle of the water molecule, defined by the angle that comprises the water dipole moment and the CNT axis, is smaller for higher densities, and asymptotically approaches 33 degrees on the low density side. Such an asymptotic nature of the structure and stability stems from non-uniform distribution of water molecules. The mean orientation angle obtained from QM calculations using density functional theory coincides with the MD result. QM analysis also suggests that the charge distribution of water in the CNT originates from the molecular configuration due to spatial confinement rather than strong electronic interaction between water and the CNT.