Short CommunicationEffect of nanotube diameter on the transport of water molecules in electric fields
Introduction
The effect of nanoscale confinement on the mass transport has attracted increasingly interest in recent years, providing a huge application perspective from mass and energy storage to novel desalination materials [1,2]. It is well known that narrow channels and pores play a prominent role in biological systems such as aquaporin water transportation [3,4], proton pathway [5,6] and transmembrane ion conduction [7]. Substantially differ from bulk behavior that forms the tightly hydrogen-bonded network, the strong interaction between confined water molecules with the specific dipole orientation exhibit exceptionally rapid water flow. Unfortunately, the complex structure accompanies with strict environmental requirements make it difficult to apply these bio-channels directly. Therefore, it is necessary to develop stable and structure less complex nanofluidic channels with the function of fast water transport, ionic and molecular filtration, drug delivery, etc.
As a typical quasi-one-dimensional channel, the dynamics and thermodynamics of confined water in carbon nanotubes (CNTs) have been extensively investigated. During the past decade, specific CNT-based nanofluidic devices and membranes have been widely used in many areas ranging from thermal conductivity to desalination [[8], [9], [10], [11]]. However, as a starting point for these applications, their underlying mechanism is urgently desired. To date, some basic questions have been answered, such as why water molecules can permeate into narrow CNTs with loss of some hydrogen bonds and why the CNTs are fast water transporters than other materials such as boron nitride nanotubes. Differs from macromolecules such as proteins or DNAs that will loss the freedom or entropy when they move into confined space, water molecules behave in an opposite way, namely they will gain some entropy and van der Waals interaction with CNTs for compensation. Previous calculation showed the filling mechanism of CNTs with water, where the confined water is more stable than bulk that depends on the CNT diameter [12]. They found an entropy (both rotational and translational) stabilized, vapor-like phase of water for small CNTs of 0.8–1.0 nm, an enthalpy stabilized, ice-like phase for CNTs with 1.1–1.2 nm, and a bulk-like liquid phase for diameters larger than 1.4 nm, stabilized by the increased translational entropy. These results greatly enhance our understanding of the CNT filling mechanism from the thermodynamic point of view.
The dynamics of water inside CNTs are even more complex. Although, the rapid conduction of water through narrow CNTs was found by both early experiments [13,14] and simulations [15], a recent experimental work well answered why the CNTs are fast water transporters [16]. The major reason is that the water inside CNTs has large slip length, whose values increase drastically with the decrease of CNT diameter, leading to the same behavior of water flow enhancement; while the boron nitride nanotube has low slip length with low flow rates. The dependence of water flow enhancement on the CNT diameter highly coincides with a former simulation [15]. Therefore, the rapid conduction of water inside CNTs should be attributed to their large slip length at the water-CNT interface.
A number of factors sensitively determine the process of water permeation through CNTs. For example, by applying an external pressure [17], suitably change the CNT length [18] and shape [19], the transport dynamics can be greatly affected. Although previous work [18,[20], [21], [22]] reveal that the external electric fields have a positive impact on the transport dynamics of water molecules inside the CNTs, they mainly focus on the narrow CNTs in which special single-file or ice-like water structures exist. It is still unclear if the rotation-translation coupling transport still holds for wilder CNTs in which the water structures are close to bulk-like arrangement.
To address this question, in this paper, we use molecular dynamic simulations to systematically investigate the transport dynamics of water permeation through CNTs with different diameters under electric fields. Our results confirm that the net water flux can be induced along the field direction even for large CNTs with bulk-like water structures, and the flux values increase drastically with the CNT diameter. This is because in large CNTs the water dipole orientation can be also tuned along the field direction, verifying the existence of rotation-translation coupling transport phenomenon in large CNTs beyond the single-file case [20]. Furthermore, we find a simple equation between the water flow, occupancy and the translocation time for different field strengths, which is helpful to predict the transport dynamics.
Section snippets
Model and simulation methods
A snapshot of the simulation system employed in present study is shown in Fig. 1, where the uncapped armchair single-walled carbon nanotube is embed along the z direction in two graphene sheets and divides the box into two equal reservoirs. Water molecules will pass through the CNT channel under the drive of electric fields in +z direction. To explore the influence of diameters, we considered three CNTs of (8,8), (10,10), and (20,20), corresponding to diameters of 1.08, 1.35 and 2.69 nm with
Results and discussion
This work is trying to explore the hidden relation between the CNT diameter and water transport behavior under electric fields, which may help us to find out the best choice of external fields in practical applications. In general, the experimental CNT diameter can be ranging from one to several tens of nanometers for the study of water transport, where the confined water changes from single-file to bulk-like structures [16,32]. Thus, it becomes necessary for simulations to consider the CNT
Conclusion
In summary, using molecular dynamics simulations we systematically investigated the effect of CNT diameter on the transport of water molecules in electric fields. The water flow decreases as a whole with the increase of field strength, while the water flux changes oppositely, leading to the increase of transport efficiency ƞ. A critical field strength of E = 0.1 V/nm can be identified for the two-stage increase rates for water flux and ƞ. For a given field strength, the large diameter has high
Author contributions
Xinke Zhang: Conceptualization, Formal analysis, Investigation, Software, Data curation, Writing-original draft, Writing-review & editing, Visualization. Jiaye Su: Conceptualization, Supervision, Methodology, Project administration, Visualization.
Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgments
This work is financially supported by the National Natural Science Foundation of China (21873049, 21574066) and the Fundamental Research Funds for Central Universities (30920021150). The allocated computer time at the National Supercomputing Center in Shenzhen is gratefully acknowledged.
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