Water diffusion inside carbon nanotubes: mutual effects of surface and confinement

Abstract

The mutual effects of two crucial features of carbon nanotubes (CNTs) (surface and confinement) on the temperature-dependent water diffusion are studied through molecular dynamics simulations. A two-stage diffusion mechanism is detected in the CNTs of diameter smaller than 12.2 Å, which becomes obscure as the temperature increases. This peculiar phenomenon can be ascribed to the cooperation of the small confinement and the periodic surface. The diffusion coefficient of the confined water exhibits a nonmonotonic dependence on the confinement size and an unexpected increase inside the large CNTs (compared to that of bulk water). These anomalous behaviors can be attributed to the competition of the smooth surface and the small confinement. Considering the mutual effects, an empirical formula is proposed on the basis of two groups of numerical examples, whose results indicate that the confinement effect will dominate over the surface effect until the CNT diameter increases up to ~16 Å, whereas thereafter the surface effect becomes dominant and finally both of them vanish gradually.

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.…”

Diffusion enhancement in core-softened fluid confined in nanotubes

“…For the a > a c case, D decreases for decreasing a because of the confinement. This is not hard to understand since decreasing a allows less space for particles to move 38,[47][48][49] .…”

“…Some simulations show a monotonical decrease of D with decreasing nanotube radius [54][55][56] while other simulations indicate the presence of a minimum 38,[47][48][49] . 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 .…”

“…Similarly the self-diffusion coefficient, D, obtained through molecular dynamic simulations for atomistic models, below a certain radius increases with with decreasing radius 38,[47][48][49] .…”

Structure and Dynamics of Water at Carbon-Based Interfaces

“…This increase of longitudinal diffusion for confining distance < 1 nm has been discussed in other works [57,[66][67][68][69], finding relation with the water density under confinement [70], with controversial experimental and numerical results [59,71] and implications for the filtration of ions in water through sub-nm channels [48]. Table 2.…”