Fluid Dynamics and Transport PhenomenaMolecular dynamics simulation of water transport through graphene-based nanopores: Flow behavior and structure characteristics☆
Graphical abstract
Flow behavior of pressure-driven water through graphene-based subnanometer nanochannels is simulated, along with the microscopic structures of water confined in the graphene nanopores. The flow behavior can be related to the microscopic structures of confined water.
Introduction
Graphene-based materials, such as graphene and graphene oxide, have been considered as promising membrane materials [1], [2], [3], [4]. Graphene layers usually self-assemble into laminate paper-like structures with interlayer distance on nanometer scales [5], [6], [7]. This specific laminate structure allows water molecules to permeate through interconnected nanochannels between graphene nanosheets. Recently, Nair et al. [3] have reported submicrometer-thick graphene-oxide laminate membrane with a typical ~ 1 nm pore width. This kind of membrane can impede the permeation of various species, including helium, but the membrane allows unhampered permeation for water. As further extension of the pioneer work, Joshi et al. [8] evaluated the filtration and separation performance of laminate membrane for extensive solutes, including ions and organics. In addition, ultrathin graphene nanofiltration membrane with two-dimensional nanocapillary has been fabricated by packing reduced graphene oxide structures [4]. The graphene membrane performs excellently for retention of organic dyes with high water flux.
Although carbon nanotubes (CNTs) have been reported as potential nanofiltration membranes, the CNT-based membranes suffer from the difficulty in larger-scale fabrication and preparation. On the contrary, the laminate two-dimensional graphene membranes, with superior flexibility and chemical stability, can be facilely prepared by filtration-assisted assembly method. According to a previous model [3], the interconnected nanochannels in the layered graphene membrane have two regions: functional and pristine. The former acts as spacers to keep adjacent graphene sheets apart, whereas the pristine region provides a capillary network that allows high water flux. It is also suggested that the nanoscale spacing between graphene nanosheets can be adjusted through inserting spacers [9]. Therefore, a broad spectrum of graphene membrane could be prepared, which shows extensive potential applications.
In order to develop this new-typed graphene-based laminate membrane, it is necessary to understand the flow behavior and confined structure of fluid molecules in the two-dimensional nanochannels. Molecular simulation has been extensively applied to study the transport behavior and confined structures in the nanoscale CNTs [10], [11], [12]. However, to our best knowledge, no molecular simulation has been reported for the pressure-driven infiltration transport of water molecules from bulk phase going through the slit nanochannels formed by graphene sheets. In particular, graphene pores with subnanometer size show significant molecular sieve performance, which can be used to develop new-typed membrane separation materials [3], [9]. For this kind of pore size, the interfacial effect is expected to overwhelmingly control the flow behavior. It is highly required to explore the unique flow behavior through the subnanometer graphene slit pores. In this work, we use molecular dynamics (MD) simulation to study the infiltration behavior of water driven by external pressure through the graphene nanocapillary with typical pore widths (≤ 1 nm). The confined water structures are also investigated in order to understand the flow mechanisms.
Section snippets
Simulation Methods
In this work, water molecules are simulated using the SPC/E model [13]. Carbon atoms of graphene are held stationary and modeled as Lennard-Jones (LJ) spheres employing the parameters proposed by Chang and Steele [14]. The van der Waals interactions between different particles are calculated with the LJ potential using the Lorentz–Berthelot mixing rule. This water-carbon interaction has been successfully applied to represent the structural and dynamic properties of water molecules confined in
Results and Discussion
Fig. 2(a) shows that the volumetric flow rate (Q, nm3·ns− 1) of water through the slit nanopores increases with the applied pressure. Fig. 2(b) shows an approximate linear dependence of the number of permeating molecules on the time, from which the flow rate is obtained from slopes of these curves. Moreover, effective flow velocity is estimated to be in the range of (1.5–27.8) × 104 nm·s− 1 in the pore of 1 nm, which is higher than the experimentally observable flow velocity of water within the CNT
Conclusions
In this work, MD simulations are performed to study the infiltration behavior of pressure-driven water flow through two-dimensional graphene nanochannels with the widths from 0.7 to 1 nm. Simulated flow rates are close to the relevant experimental data. This demonstrates that the simulated system could reasonably reflect the flow characteristics of the graphene-based slit nanopores. The flow enhancement of the water through microscopic nanopores is smaller than those reported for CNTs. There
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