Research paperEffect of the direction of static electric fields on water transport through nanochannels
Graphical abstract
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.
Section snippets
Computational methods
A snapshot of the SWCNT-membrane model built with the molecular graphics software [35] is shown in Fig. 1, which is taken from our previous studies [34], [36]. An uncapped 144-carbon (6,6) SWCNT with 1.34 nm in length and 0.81 nm in diameter is inserted in the central pore of two monolayer graphenes, which is fully infiltrated in a reservoir. The monolayer sheets at the inlet and outlet are fixed to prevent water molecules from permeating into the vacuum space between two sheets. The sizes of
Results and discussions
The net water fluxes, as a function of the SEF intensity(), are shown in Fig. 2. The curves with red stars and black squares represent the relations with the SEF along - axis and + axis directions, respectively. For concision, the SEF along - and + directions are denoted with the symbol and , respectively. The net flux ratio of to is shown in the curve with blue circle. As can be inferred from Fig. 2, the net flux at is about 5–6 , being equal to the zero-field
Conclusion
In this work, we systematically investigated the effects of SEFs with different directions on the transport properties of water channeling across SWCNTs. Compared to the situation with the SEFs in the same direction with the hydrostatic pressure, the net flux is significantly larger for the systems with the opposite SEFs direction when the electric field strength is in the range of 0.01–0.6 V/nm. The reason is that the energy barrier between two ends of the SWCNT for water molecules to overcome
CRediT authorship contribution statement
Qi-Lin Zhang: Conceptualization, Writing - review & editing. Ya-Xian Wu: Writing - original draft. Rong-Yao Yang: Methodology, Software. Jin-Lun Zhang: Supervision. Rui-Feng Wang: Visualization, Investigation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank Dr. Mao-Dong Zhu and Guang-Zhen Dai for the kind help and useful discussions. The work was supported in part by the National Natural Science Foundation of China under Grant (11604001), Overseas and domestic visiting research projects of outstanding young backbone talents in Anhui Universities, China (gxgnfx2020092), University Natural Science Foundation of Anhui Province of China under Grant (KJ2019A0159), Foundation of Anhui Polytechnic Universities,China (2015yyzr04), and the
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