Promoting Electroosmotic Water Flow through a Carbon Nanotube by Weakening the Competition between Cations and Anions in a Lateral Electric Field

Abstract

Understanding the electroosmotic flow through a nanochannel is essential to the design of novel nanofluidic devices, ranging from desalination to nanometer water pumps. Nonetheless, the competition between cation and anion in electric fields inevitably leads to a limited pumping of water, and thus weakening their competition could be a new avenue for the fundamental control of water transport. In this work, through a series of molecular dynamics simulations, we find a surprising phenomenon in which under the drive of a traditional longitudinal electric field, an additional lateral electric field can significantly weaken the competitive transport of a cation and anion through a carbon nanotube, which spontaneously leads to a massive increase in electroosmotic water flux. Specifically, with the increase in the lateral electric field, the anion flux exhibits an almost linear reduction, and the cation flux is stable and can even be enhanced. As a result, the net water flux along the cation direction increases significantly. The key to this unexpected phenomenon lies in the size and mobility difference between the cation and anion. The anion is larger and has greater mobility and is thus more susceptible to the lateral electric field, which ultimately leads to the reduction of its flux. For different ion types and CNT lengths, we can observe similar electropumping phenomenon, where the friction force induced by the lateral electric field becomes nontrivial for long CNTs. Our results provide a new route to tune the competitive transport of cations and anions and should be useful for the design of novel electroosmotic pumps.

Enhanced Ion Rejection in Carbon Nanotubes by a Lateral Electric Field

“…The longitudinal pressure drives water and ions upward in one direction, forming a stable z-directional flux, while the lateral electric field serves as an electrical gating to modulate the ionic transport. Consistent with previous work, 34,50,56 the flux represents the average net number of water and ions passing through the CNT per nanosecond from bottom to top where F i is the sum of Na + and Cl − flux; F w is the water flux; and N i and N w are the number of ions and water molecules in the solution, which are equal to 120 and 3136, respectively. Part A: Effect of Lateral Electric Field.…”

“…This phenomenon is caused by the extra friction of Cl − against its motion. 50 For the larger P z = 108.8 and 181.3 MPa, the translocation time of ions becomes smaller and is insensitive to the E x , where the values of Na + and Cl − are close to each other. This is because, under a large driving force, the friction in the CNT surface becomes trivial.…”

“…This also partially explains why the slopes of Na + are slightly larger than those of Cl − under the same pressure, whereas the size and mobility differences between Na + and Cl − are some decisive factors of the ion flux. 50,59 Moreover, the flux of Na + and Cl − are close to each other under the large E x , which is necessary to keep the electroneutrality in the CNT interior. The water flux in Figure 2c also has a linear decay with respect to E x because of the disruption in the hydrogen-bond connection between the reservoir and confined water.…”

“…The ionic solution contains 3136 water molecules and 60 NaCl with a typical concentration of 1.08 M. A hydrostatic pressure P z is applied along the CNT axis to drive the transport of water and ions, while an additional lateral electric field E x ranging from 0.1 to 1.0 V/nm is set along the x-direction to control the ion transport. 50 Furthermore, to illustrate the role of CNT size, we also consider the same ionic solution with different CNTs of (8,8), (9,9), and (12,12), corresponding to the diameters of D = 1.08, 1.21, and 1.62 nm, under P z = 181.3 MPa and different E x .…”

“…In contrast, the effect of the lateral electric field has been rarely discussed, which can actually hinder the water flow by disrupting the hydrogen-bond connection 40 and control the electroosmotic water flow by tuning the competition between cations and anions. 50 Thus, the lateral electric field could also manipulate the desalination performance in CNTs, which is still unexplored. Herein, systematical molecular dynamics simulations manifest that a lateral electric field can significantly promote the ion rejection in CNTs driven by a longitude pressure, while the water flux remains at a high level.…”