Water diffusion in carbon nanotubes under directional electric fields: Coupling between mobility and hydrogen bonding.

We have investigated the diffusion and structure of TIP4P/2005 water confined in carbon nanotubes subjected to external electric fields. A wide range of diameters has been used to show a highly size-dependent behavior of the water diffusion. We also found that the diffusion is extremely affected by the intensity of the applied field. However, is the relative direction between the field and the tube axis that causes the most intriguing behavior. Electric fields forming angles of $0^{\circ}$ and $45^{\circ}$ with the tube axis were found to slow down the water dynamics by increasing organization, while fields perpendicular to the tube axis can enhance water diffusion in some cases by decreasing the hydrogen bond formation. Remarkably, for the 1.2 nm diameter long (9,9) nanotube, the field along the tube axis melts the water structure increasing the water mobility. These results points out that the structure and dynamics of confined water are extremely sensitive to external fields and suggest the use of electric fields as a facilitator for filtration processes.