Water at nanoscale confined in single-walled carbon nanotubes studied by NMR

Proton NMR studies have been carried out as a function of temperature from 210 K to 300 K on water confined within single-walled carbon nanotubes. The NMR lineshape at and below the freezing point of bulk water is asymmetric and can be decomposed into a sum of two Lorentzians. The intensities of both the components decrease with the lowering of the temperature below 273 K, one component, L₁, vanishing below 242 K and the other component, L₂, below 217 K. Following the simulations of Koga et al. showing that the radial density profile of confined water in single-wall carbon nanotubes has a distribution peak at the center which disappears below the freezing temperature, the L₁-component is associated with the protons of the water molecules at the center and the L₂-component is associated with protons of water molecules at a distance of ∼ 3 Å from the walls of the nanotubes. In this scenario the complete freezing of the water at ∼ 212 K is preceded by the withdrawal of the water molecules from the center.