Structure and dynamics of water confined in single-wall carbon nanotubes

The structure and dynamics of water confined in open-ended single-wall carbon nanotubes (SWNTs), here referred to as nanotube-water, were investigated by a combined neutron-scattering and molecular-dynamics-simulation study. A 'shell+chain' configuration of nanotube-water that is consistent with both experimental observations and simulation results was identified at low temperatures. The shell consists of a square-ice sheet rolled into a hollow cylinder inside an SWNT in a tube-in-tube configuration. The chain along the centreline of the shell comprises a single file of water molecules. Large fluctuations via hydrogen-bond breaking/formation, including those associated with molecules between the shell and the chain, prevail even at very low temperatures and the resulting overall hydrogen-bond network of nanotube-water is weakened significantly. Hydrogen bonds associated with the chain are especially pliable. The fluctuations increase drastically with temperature, leading to the disappearance of the shell-chain structure at ∼210K and the realization of confined supercooled water. A comparison of the hydrogen-bond energetics and relaxation processes of nanotube-water with those of confined supercooled water in porous silica MCM-41-S is discussed.