Effect of the direction of static electric fields on water transport through nanochannels

Highlights

• The water flux can be increased significantly by regulating the direction of electric fields.

• Water-molecule chains with oxygen atoms moving forward move faster through narrow nanotubes.

Abstract

The transport properties of water molecules through single-walled carbon nanotubes (SWCNT) under static electric fields (SEFs) with different directions were studied by molecular dynamics simulations. It is found that the net water flux in the SEF direction opposite to the hydrostatic pressure direction is much larger than that in the same direction under the electric field intensity $0.01<E_0\le 0.6$ V/nm. The reason is mainly that the SEF tunes the polarization direction of the water chain in the SWCNT, leading to different barrier height to be overcome for water molecules channeling across the SWCNT. Further, it also implies that water molecule chains moving forward with the oxygen atom can pass through the nanotube faster than that with the hydrogen atom forward. These findings may be instructive to regulate the water transport properties using the SEF in nanofluid devices and biological nano-channels.

Introduction

Understanding and manipulating the behavior of water transport through nanochannels is of great importance in many physical, chemical, biological, and technological applications. The unusual dynamical behaviors of water molecules inside the nanochannel have been fascinated by their potential applications in desalination of seawater [1], molecular sieves [2], on–off control [3], drug delivery [4], and so on. In the past, water transport properties have been studied extensively in nanometer scale. Many researches indicated that the dynamical properties of water molecules through single-walled carbon nanotubes (SWCNTs) can be remarkably affected by temperature gradients [5], mechanical deformation [6], charge modification [7], or collective mode vibrations [8]. It is worth noting that the transport velocity of water across nanochannels was found to be faster than the result of the no-slip hydrodynamic flow predicted from the Hagen–Poiseuille equation [9], [10]. These achievements with unique properties revealed in these studies may be assimilated into the application for the design of innovative nanofluidic devices and the exploration for complex biological channels [6], [11], [12], [13], [14], [15].

In recent years, numerous simulations of water confined in or permeating through SWCNTs have shown many interesting and unique dynamical properties. Especially, electro-osmosis technology  [16], [17], [18], [19], [20], which can be used to accomplish controllable flows across nanochannels, is recognized as great promising and convenient approaches to water filters, flow sensors, high-flux nanofluid devices, and so on. Many previous researches using molecular dynamics (MD) revealed that electric fields (EFs) can remarkably affect and manipulate the transport of water molecules in nanochannels  [21], [22], [23]. Nevertheless, there are also some disputes about whether static electric fields (SEFs) can be used as the motive force to continuously drive water molecules through the SWCNTs or not in previous studies [24], [25]. Some reports have revealed that the pumping capability of SEFs may be due to the defects of the program package [25], [26]. It is well recognized that the energy acquisition and spatial asymmetry are two essential conditions to produce molecular pumping, whereas the electroneutral water molecules in the uniform SEF cannot be completely satisfied with these requirements. As we all know, although the steady SEF cannot uninterruptedly drive a unidirectional flow, it can be markedly influence the water intermolecular structures and interactions at the nanoscale indeed [27], [28], [29]. Many reports have demonstrated that water transport properties are closely related with the intermolecular structures and interactions at the nanoscale  [30], [31], [32]. It is noteworthy that Li et al. systematically demonstrated that the transport velocity and dynamical behavior of water molecules through the SWCNT are highly sensitive to the change of the structure and number of in-tube hydrogen bonds [33]. Recently, we have also reported that polarity water molecules can be automatically arranged along the SEF direction in SWCNTs, which significantly enhances the flow rate of water under hydrostatic pressure [34]. This phenomenon is attributed to the enhancement of the freedom energy of water molecules inside SWCNTs by applying SEFs. However, the effect of the SEF direction on the dipole distribution and dynamical properties of water molecules in the nanochannel has not been well discussed until now. Hence, in this paper, we exerted an extensive research on the transport properties of water molecules through SWCNTs under different SEF directions. We have found that the water dipole orientations present asymmetric distributions in SEFs with appropriate intensity, which leads to the remarkable different flux of water across SWCNTs. It has also been found that the net water flux in the SEF direction opposite to the hydrostatic pressure direction is much larger than that in the same direction. These findings have practical meaning for the development of nanofluidic devices to readily achieve controllable water pump subjected to the SEF.