Water Filtration in Carbon Nanotubes Resulting From Electronic Friction at the Walls

In this talk, a mechanism for removal of salt from salt water is discussed, which results from friction due to Ohm’s law heating, resulting from motion of an electron charge induced in the tube walls by the water molecules’ dipoles and the ions’ charges. The filtration occurs because this friction is larger for salt ions than for water molecules. Friction due to Ohm’s law heating might also provide an explanation for the observation by Secchi, et. al., that the flow velocity of water in carbon nanotubes increases rapidly as the tube radius decreases from 50 to 15nm, which does not occur for boron nitride nanotubes which are insulators. This friction is large enough to produce the observed slip-lengths. One possibility is that the nanotubes in this experiment were metallic, whose conductivity becomes large as their radius decreases, due to ballistic conduction. Another possibility is that when the tube circumference drops below the electron mean free path, the wall switches from behaving as a two dimensional conductor to behaving as a one dimensional conductor for which the electrons are more strongly localized. For sufficiently small conductivity, small distortions of the localized states can provide the dominant contribution to the induced charge, rather than current flow.