The Role of Interface Ions in the Control of Water Transport through a Carbon Nanotube

Abstract

Controlling the water transport toward a given direction is still challenging, particularly due to thermal fluctuations of water motion at the nanoscale. While most of the previous works focus on the symmetric hydrophobic membrane systems, the role of the membrane in affecting the water transport remains largely unexplored. In this work, by using extensive molecular dynamics simulations, we find an interesting electropumping phenomenon, that is, the flowing counterions on an asymmetric hydrophobic−hydrophilic membrane can significantly drive the single-file water transport through a carbon nanotube, suggesting a nanometer water pump in a highly controllable fashion. The ion−water coupling motion in electric fields on the charged surface provides an indirect driving force for this pumping phenomenon. The water dynamics and thermal dynamics demonstrate a unique behavior with the change in electric fields, surface charge density, and even charge species. Particularly, due to the ion flux bifurcation for the positive and negative surfaces, the water dynamics such as the water flow, flux, and translocation time also exhibit similar asymmetry. Surprisingly, the positive surface charge induces an abnormal three-peak dipole distribution for the confined water and subsequent high flipping frequency. This can be attributed to the competition between the surface charge and interface water orientation on it. Our results indicate a new strategy to pump water through a nanochannel, making use of the counterion flowing on an asymmetric charged membrane, which are promising for future studies.

Enhanced Ion Rejection in Carbon Nanotubes by a Lateral Electric Field

“…As presented in Figure 2a,b, the average flux of both Na + and Cl − exhibits an excellent linear decay with the increase of E x , which means that the transport of ions is impeded by the lateral electric field. The reason for this behavior lies in the fact that the E x uniformly prevents ions from entering the CNT inlet: the enhanced ion velocity in the x-direction driven by E x causes the ions to slip away from the pore region, 57 ultimately reducing the probability of ion entering. Additionally, the applied pressure deeply influences the ion flux and the slope of flux decay.…”

A computational study of water in UiO‐66 Zr‐MOFs: Diffusion, hydrogen bonding network, and confinement effect

“…Therefore, anomalous modes, such as the single‐file mode (SFD, MSD ~ t 0.5 ) could emerge 43 . For example, a number of experimental and simulation results have revealed the SFD mode when water molecules are confined in carbon nanotubes (CNTs) with diameters less than 1.5 nm 44 . Das and co‐workers observed the SFD mode for water confined in CNTs with a diameter of 1.4 nm through method of pulsed field‐gradient NMR 45 .…”

“…43 For example, a number of experimental and simulation results have revealed the SFD mode when water molecules are confined in carbon nanotubes (CNTs) with diameters less than 1.5 nm. 44 Das and coworkers observed the SFD mode for water confined in CNTs with a diameter of 1.4 nm through method of pulsed field-gradient NMR. 45 And fast diffusion of water through CNTs of diameter 0.81 nm was ascribed to the single-file diffusion mechanism by molecular simulation studies.…”

Liquid Pumping by Nanoscopic “Flexible Oars”

“…The slippery nature of the graphene paddle allows higher water flows, in analogy to carbon nanotubes. 21 For paddles with very short lengths of l ≈ 1 nm, the flow is very small since the paddle deformations are very small (reversibility − scallop theorem). For longer paddles, the flow increases and reaches a local maximum at l opt ≈ 2−3 nm where the paddles are irreversibly waving without much restriction.…”

“…Figure a shows the calculated flows of water and pentane separately driven by graphene paddles of different lengths and a fixed width ( w ≈ 1.4 nm), oscillated at ω = 0.045 ps –1 . The slippery nature of the graphene paddle allows higher water flows, in analogy to carbon nanotubes …”