Water dynamics inside single-wall carbon nanotubes: NMR observations

Abstract

The dynamics of water (${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{D}}_{2}\mathrm{O}$) molecules adsorbed inside single-wall carbon nanotubes (SWCNTs) of average diameter $13.5\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ were investigated by means of $^{2}\mathrm{H}$- and $^{1}\mathrm{H}$-NMR between 100 and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Above $220\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the NMR spectra were substantially narrowed, and indicated that the water is in a liquidlike state with translational and quasifree rotational motions. Below $220\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, where water exhibits long range order inside SWCNTs, the large amplitude molecular motions start to freeze within a time scale of ${10}^{\ensuremath{-}6}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$, while below around $120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, almost all of the protons become fixed around each atomic site. The results support the dynamic properties and ice-nanotube transition predicted by previous molecular dynamics calculations.