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 210K to 300K 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 273K, one component,$L_1$, vanishing below 242K and the other component, $L_2$, below 217K . 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 $L-1$-component is associated with the protons of the water molecules at the center and the $L_2$-component is associated with protons of water molecules at a distance of ~3A from the walls of the nanotubes. In this scenario the complete freezing of the water at ~212K is preceded by the withdrawal of the water molecules from the center.