152
C. Sun et al.
RO systems, being comparable with the predicted water permeability at very high
pressures.
The desalination performance of multilayer NPG membranes was also investigated by MD simulations. The two-layer NPG membranes with a lower surface
tension were shown by Garnier et al. [60] to exhibit a lower water permeation flux
compared to the monolayer NPG membranes. Shahbabaei et al. [61] observed that
the multilayer NPG membranes with hourglass-shaped pores presented a high water
permeation flux with a dependence on the length of water flow path inside the pores
and especially that the hydrophilic graphene surfaces could double the water permeation flux owing to the strong hydrogen bond interactions. They [62] also indicated
that the water occupancy across the asymmetric pore increased and thus a higher water
permeation flux was yielded compared to the symmetric pore owing to the enhanced
hydrogen bond interactions. The effects of channel morphology on the water permeation through the bilayer graphene membranes were studied by Liu et al. [63] and
the results showed that the water permeation rate greatly depended on the curvature of graphene sheets. The same authors [64] found that the layered water structure
inside the bilayer graphene nanochannels was significantly important for the channel
size-dependent water transport rate. Based on the understanding of the mechanisms
of water transport through multilayer NPG membranes, Jiang et al. [65] established
an equivalent one-dimensional (1D) nanochannel model to describe the multilayer
graphene membranes. By using MD simulations and continuum fracture mechanics,
Cohen-Tanugi and Grossman [66] demonstrated that the NPG membranes can maintain the mechanical integrities under the high pressures in the RO systems with the
water flowing through the NPG membranes. The NPG membranes can endure a
pressure higher than 57 MPa on a porous substrate with pores of a diameter smaller
than 1 µm. They [67] further showed that the desalination performance of multilayer NPGs was as good as the monolayer NPG membranes and that the desalination
performance can be regulated by the configurational parameters.
The permeation of ions and water through graphene nanopores are very essential for the understanding of the mechanisms of NPG water purification membranes.
Here, the studies on the permeation of water and ions are summarized, although
some works have been conducted before the demonstration of NPG water purification membranes. Sint et al. [24] showed from MD simulations that the diverse
ions can selectively transport through the graphene nanopores with a high selectivity and a very high ion transport rate (see Fig. 4). Hu et al. [68] investigated the
transport of Na
+ and Cl
− ions through graphene nanopores and studied the effects
of the hydrodynamic transport, thermal fluctuations as well as electric pressure.
Zhao et al. [69] demonstrated that the selectivity of K
+ and Cl
− ions was greatly
dependent on the pore size and the partial charges on pore rim. Kang et al. [70]
showed that the selectivity of Na
+ and K
+ ions through graphene nanopores was
affected by the ionic size and the distance between the functional groups. Suk and
Aluru [71] concluded that the concentration and mobility of ions in the graphene
nanopores decreased as the pore radius was smaller than 0.9 nm because the layered
liquids appeared in the pores. They [25] also found that the graphene nanopores
with a bigger diameter can provide a higher water permeation flux comparing to
C. Sun et al.
RO systems, being comparable with the predicted water permeability at very high
pressures.
The desalination performance of multilayer NPG membranes was also investigated by MD simulations. The two-layer NPG membranes with a lower surface
tension were shown by Garnier et al. [60] to exhibit a lower water permeation flux
compared to the monolayer NPG membranes. Shahbabaei et al. [61] observed that
the multilayer NPG membranes with hourglass-shaped pores presented a high water
permeation flux with a dependence on the length of water flow path inside the pores
and especially that the hydrophilic graphene surfaces could double the water permeation flux owing to the strong hydrogen bond interactions. They [62] also indicated
that the water occupancy across the asymmetric pore increased and thus a higher water
permeation flux was yielded compared to the symmetric pore owing to the enhanced
hydrogen bond interactions. The effects of channel morphology on the water permeation through the bilayer graphene membranes were studied by Liu et al. [63] and
the results showed that the water permeation rate greatly depended on the curvature of graphene sheets. The same authors [64] found that the layered water structure
inside the bilayer graphene nanochannels was significantly important for the channel
size-dependent water transport rate. Based on the understanding of the mechanisms
of water transport through multilayer NPG membranes, Jiang et al. [65] established
an equivalent one-dimensional (1D) nanochannel model to describe the multilayer
graphene membranes. By using MD simulations and continuum fracture mechanics,
Cohen-Tanugi and Grossman [66] demonstrated that the NPG membranes can maintain the mechanical integrities under the high pressures in the RO systems with the
water flowing through the NPG membranes. The NPG membranes can endure a
pressure higher than 57 MPa on a porous substrate with pores of a diameter smaller
than 1 µm. They [67] further showed that the desalination performance of multilayer NPGs was as good as the monolayer NPG membranes and that the desalination
performance can be regulated by the configurational parameters.
The permeation of ions and water through graphene nanopores are very essential for the understanding of the mechanisms of NPG water purification membranes.
Here, the studies on the permeation of water and ions are summarized, although
some works have been conducted before the demonstration of NPG water purification membranes. Sint et al. [24] showed from MD simulations that the diverse
ions can selectively transport through the graphene nanopores with a high selectivity and a very high ion transport rate (see Fig. 4). Hu et al. [68] investigated the
transport of Na
+ and Cl
− ions through graphene nanopores and studied the effects
of the hydrodynamic transport, thermal fluctuations as well as electric pressure.
Zhao et al. [69] demonstrated that the selectivity of K
+ and Cl
− ions was greatly
dependent on the pore size and the partial charges on pore rim. Kang et al. [70]
showed that the selectivity of Na
+ and K
+ ions through graphene nanopores was
affected by the ionic size and the distance between the functional groups. Suk and
Aluru [71] concluded that the concentration and mobility of ions in the graphene
nanopores decreased as the pore radius was smaller than 0.9 nm because the layered
liquids appeared in the pores. They [25] also found that the graphene nanopores
with a bigger diameter can provide a higher water permeation flux comparing to