High Interfacial Barriers at Narrow Carbon Nanotube–Water Interfaces

Abstract

Water displays anomalous fast diffusion in narrow carbon nanotubes (CNTs), a behavior that has been reproduced in both experimental and simulation studies. However, little is reported on the effect of bulk water-CNT interfaces, which is critical to exploiting the fast transport of water across narrow carbon nanotubes in actual applications. Using molecular dynamics simulations, we investigate here the effect of such interfaces on the transport of water across arm-chair CNTs of different diameters. Our results demonstrate that diffusion of water is significantly retarded in narrow CNTs due to bulk regions near the pore entrance. The slowdown of dynamics can be attributed to the presence of large energy barriers at bulk water-CNT interfaces. The presence of such intense barriers at the bulk-CNT interface arises due to the entropy contrast between the bulk and confined regions, with water molecules undergoing high translational and rotational entropy gain on entering from the bulk to the CNT interior. The intensity of such energy barriers decreases with increase in CNT diameter. These results are very important for emerging technological applications of CNTs and other nanoscale materials, such as in nanofluidics, water purification, nanofiltration, and desalination, as well as for biological transport processes.

Interfacial structure of polymers near a surface: a molecular dynamics study

“…79,80 Internal interfacial barriers: Fluid molecules must overcome the thermodynamic adsorption/desorption barriers due to the strong entropic and enthalpic changes near the phase boundary to enter into the pore network. [80][81][82] Such internal interfacial barriers exist on the solid side of the crystal and are coupled with effects of the asymmetric potential experienced by the fluid molecules inside the crystal but near the phase boundary, as shown in Figure 2-10. Further source of these barriers includes grain boundaries and internal defects, due to which the gas molecules must detour to reach the outer surface.…”

“…EMD simulations: The contributions of external and internal interfacial barriers to gas transport in nanoporous membranes have been investigated by employing EMD simulations. 82 The internal interfacial barriers ( internal R ) in a membrane can be determined, considering a finite system of length L and surface area Ac, as depicted in Figure 2-12. The intra-crystalline and internal interfacial resistances for this system, follow a resistance in series model, leading to 89 :…”

“…77,113,130 Nevertheless, surface pore blockage or constrictions as well as terminal functional groups can contribute to these barriers significantly, as fluid molecules experience difficulties to locate an open pore on the surface. 76 On the other hand, the interfacial barriers to water transport in CNT membranes are found to be significant, 82,103 and the physical mechanisms involved in water transport are quite different from gas transport due to presence of hydrogen bonding in water, both outside and inside of CNTs 82,88 . It is reported that the energy barrier present at the pore entrance plays an important role in water transport through CNTs and intensity of these energy barriers can be assessed through potential of mean force (PMF) analysis 82,[132][133][134] , as shown in Figure 2-14 (a).…”

“…76 On the other hand, the interfacial barriers to water transport in CNT membranes are found to be significant, 82,103 and the physical mechanisms involved in water transport are quite different from gas transport due to presence of hydrogen bonding in water, both outside and inside of CNTs 82,88 . It is reported that the energy barrier present at the pore entrance plays an important role in water transport through CNTs and intensity of these energy barriers can be assessed through potential of mean force (PMF) analysis 82,[132][133][134] , as shown in Figure 2-14 (a). Further, the influence of nanotube diameter on the entrance and exit effects is explained in terms of entropy contrast between the bulk and confined regions, where water molecules undergo large increase in translational and rotational entropy on entering from the bulk to the CNT interior 82 .…”

“…It is reported that the energy barrier present at the pore entrance plays an important role in water transport through CNTs and intensity of these energy barriers can be assessed through potential of mean force (PMF) analysis 82,[132][133][134] , as shown in Figure 2-14 (a). Further, the influence of nanotube diameter on the entrance and exit effects is explained in terms of entropy contrast between the bulk and confined regions, where water molecules undergo large increase in translational and rotational entropy on entering from the bulk to the CNT interior 82 . In addition, the intensity of such energy barriers is found to decrease with increase in CNT diameter 82 .…”