Anomalously Soft Dynamics of Water in a Nanotube: A Revelation of Nanoscale Confinement

Quasi-one-dimensional water encapsulated inside single-walled carbon nanotubes, here referred to as nanotube water, was studied by neutron scattering. The results reveal an anomalously soft dynamics characterized by pliable hydrogen bonds, anharmonic intermolecular potentials, and large-amplitude motions in nanotube water. Molecular dynamics simulations consistently describe the observed phenomena and propose the structure of nanotube water, which comprises a square-ice sheet wrapped into a cylinder inside the carbon nanotube and interior molecules in a chainlike configuration.

Figure 1

(a) Proposed structure of nanotube water. The interior “chain” water molecules have been colored yellow to distinguish them from the exterior “shell” water molecules (colored red). (b) Low-$Q$ ND profiles around $0.4{\AA }^{-1}$ for the following: 1a, dry SWNT (red curve with solid circles) and SWNT with the encapsulated water of different isotopic compositions; 2a, ${\mathrm{H}}_2\mathrm{O}$ (pink dotted curve with solid triangles); 3a, ${\mathrm{H}}_2\mathrm{O}:{\mathrm{D}}_2\mathrm{O}=1:1$ mixture (green short-dashed curve with open triangles); 4a, ${\mathrm{D}}_2\mathrm{O}$ (blue solid curve with open circles). The calculated form factors for 1a, 2a, 3a, and 4a are shown by the curves labeled 1b, 2b, 3b, and 4b, respectively.

Figure 2

Experimental generalized vibrational DOS, $G\left(E\right)$, of nanotube ice and ice $\mathrm{I}h$ at 9 K for (a) intramolecular bending (H-O-H) and stretching (O-H) modes and (b) intermolecular vibrations. A part of the observed density around 120 meV is due to multiphonon neutron scattering. The excess density below 5 meV in nanotube ice can be seen more clearly in the inset (c). (d) The $G\left(E\right)$ obtained from MD simulations at 50 K for nanotube ice, chain molecules in nanotube ice, shell molecules separately, and for ice $\mathrm{I}h$.

Figure 3

(a) Free energies across the nanotube walls for water molecules in nanotube water/ice at 100 (red) and 300 K (blue). The LJ potential (black) provides the minima for location of individual water molecules initially entering the SWNT. (b) Temperature dependence of $\langle u_{\mathrm{H}}{}^2\rangle$ in nanotube water (red circles) and in ice $\mathrm{I}h$ (blue circles). The blue line shows the calculated $\langle u_{\mathrm{H}}{}^2\rangle$ for ice $\mathrm{I}h$ according to the measured one-phonon DOS with harmonic approximation. Adding a “delocalization” of $d\sim 0.2\AA$ for the hydrogen atoms to the calculated curve produced the red line that matches the $\langle u_{\mathrm{H}}{}^2\rangle$ for nanotube water up to about 120 K. This observation suggests schematically a flattened-bottom potential and a local minimum on the side for the water molecules in the SWNT (c). (d) $\langle u_{\mathrm{H}}{}^2\rangle$ calculated from MD simulations of nanotube water, chain molecules in the nanotube water, shell molecules separately (see text), and ice $\mathrm{I}h$.