The Mechanism of Water Diffusion in Narrow Carbon Nanotubes

Abstract

Carbon nanotubes show exceptional physical properties that render them promising candidates as building blocks for nanostructured materials. Many ambitious applications, ranging from gene therapy to membrane separations, require the delivery of fluids, in particular aqueous solutions, through the interior of carbon nanotubes. To foster these and other applications, it is necessary to understand the thermodynamic and transport properties of water confined within long narrow carbon nanotubes. Previous theoretical work considered either short carbon nanotubes or short periods of time. By conducting molecular dynamics simulations in the microcanonical ensemble for water confined in infinitely long carbon nanotubes of diameter 1.08 nm, we show here that confined water molecules diffuse through a fast ballistic motion mechanism for up to 500 ps at room temperature. By comparing the results obtained for the diffusion of water to those obtained for the diffusion of a reference Lennard-Jones fluid, we prove here that long-lasting hydrogen bonds are responsible for the ballistic diffusion of water clusters in narrow carbon nanotubes, as opposed to spatial mismatches between pore-fluid and fluid-fluid attractive interactions which, as shown previously by others, are responsible for the concerted motion of simple fluids in molecular sieves. Additionally we prove here for the first time that, despite the narrow diameter of the carbon nanotubes considered which may suggest the existence of single-file diffusion, when the trajectories of confined water are studied at time scales in excess of 500 ps, a Fickian-type diffusion mechanism prevails. Our results are important for designing nano fluidic apparatuses to develop, for example, novel drug-delivery devices.

Structural and dynamical properties of water confined in carbon nanotubes

“…It can be seen that the water density is strongly dependent on the CNT diameter in narrower nanotubes, what can be attributed to the different arrangements observed for water molecules inside these nanotubes. In (6,6) CNT water forms a single chain of molecules and, as diameter increases, tubular structures are observed, which is in agreement with previous reports [14][15][16][17][18][19]. These differences on the structural ordering is a direct consequence of the changing in the effective volume of nanotube, in the sense that not all the inner volume is available to water due to the repulsion (scaling to r À12 ) between these molecules and CNT wall.…”

Strong correlations and Fickian water diffusion in narrow carbon nanotubes

“…Our work also brings out that the normal mode diffusion found recently by Striolo [8] for water molecules in narrow carbon nanotubes with periodic boundary conditions persists in the more realistic situation of open-ended nanotubes immersed in water.…”

“…For these reasons, we believe that the simulations mentioned above do not provide conclusive evidence for subdiffusive behavior of molecules inside a carbon nanotube. Recently, Striolo [8] has performed MD simulations of the diffusion of water molecules inside a (8,8) nanotube using periodic boundary conditions in the direction of the axis of the nanotube. This artificial boundary condition prevents the water molecules from escaping the nanotube and hence it is possible 3 to obtain MSD data at long times with good statistics.…”

Diffusion enhancement in core-softened fluid confined in nanotubes

“…It has been suggested that the length of the nanotube and the length of the simulation would be responsible for the different results 3,57,58 and that friction should play also a relevant role 59 .…”