Water Alignment and Proton Conduction inside Carbon Nanotubes

First-principles molecular dynamics simulations have been carried out to investigate the structure, electronic properties, and proton conductivity of water confined inside single-walled carbon nanotubes. The simulations predict the formation of a strongly connected one-dimensional hydrogen-bonded water wire resulting in a net electric dipole moment directed along the nanotube axis. An excess proton injected into the water wire is found to be significantly stabilized, relative to the gas phase, due to the high polarizability of the carbon nanotube.

Figure 1

(a) Snapshot from the simulation at 298 K illustrating the preferential alignment of the encapsulated water wire. (b) Plots of the average dipolar angle versus simulation time at 298 and 598 K for 5 ps of simulation.

Figure 2

Vibrational spectrum of bulk water (dashed line) and the water filled (6,6) SWCNT (solid line). (b) First peak in the oxygen-hydrogen radial distribution function, $g_{\mathrm{O}\mathrm{H}}(r)$, corresponding to the covalent O-H bonds.

Figure 3

Free energy along the reaction coordinate $q$ for the translocation of an excess proton through the interior of the (6,6) SWCNT at 298 K. The excess proton is located on one of the end waters when $q\approx \pm 3.0$. Shown in the inset is the stabilized eigen complex formed along the encapsulated water wire.

Figure 4

Time evolution of the proton reaction coordinate at 298 K for proton conduction through the unsolvated water wire (solid line) and the water wire capped with a single hydroxide ion (dashed line). The excess proton is located on one of the end waters when $q\approx \pm 3.0$