Mechanical Mechanism of Ion and Water Molecular Transport through Angstrom-Scale Graphene Derivatives Channels: From Atomic Model to Solid-Liquid Interaction

Ion and water transport at the Angstrom/Nano scale has always been one of the focuses of experimental and theoretical research. In particular, the surface properties of the angstrom channel and the solid-liquid interface interaction will play a decisive role in ion and water transport when the channel size is small to molecular or angstrom level. In this paper, the chemical structure and theoretical model of graphene oxide (GO) are reviewed. Moreover, the mechanical mechanism of water molecules and ions transport through the angstrom channel of GO are discussed, including the mechanism of intermolecular force at a solid/liquid/ion interface, the charge asymmetry effect and the dehydration effect. Angstrom channels, which are precisely constructed by two-dimensional (2D) materials such as GO, provide a new platform and idea for angstrom-scale transport. It provides an important reference for the understanding and cognition of fluid transport mechanism at angstrom-scale and its application in filtration, screening, seawater desalination, gas separation and so on.

Grafting Thin Layered Graphene Oxide onto the Surface of Nonwoven/PVDF-PAA Composite Membrane for Efficient Dye and Macromolecule Separations

This study investigates the permeance and rejection efficiencies of different dyes (Rhodamine B and methyl orange), folic acid and a protein (bovine serum albumin) using graphene oxide composite membrane. The ultrathin separation layer of graphene oxide (thickness of 380 nm) was successfully deposited onto porous polyvinylidene fluoride-polyacrylic acid intermediate layer on nonwoven support layer using vacuum filtration. The graphene oxide addition in the composite membrane caused an increased hydrophilicity and negative surface charge than those of the membrane without graphene oxide. In the filtration process using a graphene oxide composite membrane, the permeance values of pure water, dyes, folic acid and bovine serum albumin molecules were more severely decreased (by two orders of magnitude) than those of the nonwoven/polyvinylidene fluoride-polyacrylic acid composite membrane. However, the rejection efficiency of the graphene oxide composite was significantly improved in cationic Rhodamine B (from 9% to 80.3%) and anionic methyl orange (from 28.3% to 86.6%) feed solutions. The folic acid and bovine serum albumin were nearly completely rejected from solutions using either nonwoven/polyvinylidene fluoride-polyacrylic acid or nonwoven/polyvinylidene fluoride-polyacrylic acid/graphene oxide composite membrane, but the latter possessed anti-fouling property against the protein molecules. The separation mechanism in nonwoven/polyvinylidene fluoride-polyacrylic acid membrane includes the Donnan exclusion effect (for smaller-than-pore-size solutes) and sieving mechanism (for larger solutes). The sieving mechanism governs the filtration behavior in the nonwoven/polyvinylidene fluoride-polyacrylic acid/graphene oxide composite membrane.

Dynamics of water confined in a graphene nanochannel: dependence of friction on graphene chirality

Numerous recent studies reveal that water transport through graphene-based nanochannels exhibits unconventional behavior. Theoretical and experimental works have reported that the dynamic behavior of the constrained water could be affected by the atomic structure, surface curvature, confinement height, pressure, and mechanical strain of the confining channel. However, few studies concern the role of graphene chirality in the dynamics of confined water. In the present study, using equilibrium molecular dynamics (EMD) simulations, we simulated the water flow through different graphene-based channels to investigate the influence of graphene chirality on the dynamic behavior of the confined water. The friction coefficient at the water/graphene interface was found to be dependent on the crystallographic orientation of the graphene wall. The results also indicated that the chirality-dependent friction behavior was affected by the confinement height of the channel. Detailed analyses on the physical origin of such a chirality effect suggested that the chirality-dependent friction was ascribed to the water-graphene interaction energy barrier variation when the water flow was driven along the armchair edge and zigzag edge. These findings are beneficial to the design of graphene-based nanofluidic devices.

Molecular Dynamics of Water Embedded Carbon Nanocones: Surface Waves Observation

We employed molecular dynamics simulations on the water solvation of conically shaped carbon nanoparticles. We explored the hydrophobic behaviour of the nanoparticles and investigated microscopically the cavitation of water in a conical confinement with different angles. We performed additional molecular dynamics simulations in which the carbon structures do not interact with water as if they were in vacuum. We detected a waving on the surface of the cones that resembles the shape agitations of artificial water channels and biological porins. The surface waves were induced by the pentagonal carbon rings (in an otherwise hexagonal network of carbon rings) concentrated near the apex of the cones. The waves were affected by the curvature gradients on the surface. They were almost undetected for the case of an armchair nanotube. Understanding such nanoscale phenomena is the key to better designed molecular models for membrane systems and nanodevices for energy applications and separation.

Effect of graphene oxide (GO) nanosheet sizes, pinhole defects and non-ideal lamellar stacking on the performance of layered GO membranes: an atomistic investigation

The effect of non-idealities, namely pinhole defects and non-ideal lamellar stacking of nanosheets, on the performance of size-differentiated graphene oxide (GO) laminates is investigated using equilibrium molecular dynamics (MD) simulations. With the increase in sizes of the constituent GO nanosheets the water permeability of the layered GO membranes decreases and salt rejection increases. But with the inclusion of non-idealities the difference in water permeability between these membranes substantially reduced. The pinholes on the GO nanosheets provide shorter routes for trans-sheet flow, thereby increasing the water permeability of the membranes. The non-ideal stacking of the nanosheets without pinhole defects results in slight reduction in water permeability because of blockage of permeation pathways inside the membranes. However, with pinhole defects non-ideal stacking becomes favorable for water permeation through the layered GO membranes; as this time the non-ideal stacking leads to formation of voids inside the membranes, which act as routes for shorter permeation pathways. The effect of these non-idealities is more significant for layered GO membranes composed of large GO nanosheets. Although the water permeability through the layered GO membrane is greatly enhanced because of these non-idealities (about 10 times), the corresponding variation in the salt rejection is very low (<2%).

Monte Carlo Simulations of Framework Defects in Layered Two-Dimensional Nanomaterial Desalination Membranes: Implications for Permeability and Selectivity

Two-dimensional nanomaterial (2-D NM) frameworks, especially those comprising graphene oxide, have received extensive research interest for membrane-based separation processes and desalination. However, the impact of horizontal defects in 2-D NM frameworks, which stem from nonuniform deposition of 2-D NM flakes during layer build-up, has been almost entirely overlooked. In this work, we apply Monte Carlo simulations, under idealized conditions wherein the vertical interlayer spacing allows for water permeation while perfectly excluding salt, on both the formation of the laminate structure and molecular transport through the laminate. Our simulations show that 2-D NM frameworks are extremely tortuous (tortuosity ≈10³), with water permeability decreasing from 20 to <1 L m^-2 h^-1 bar^-1 as thickness increased from 8 to 167 nm. Additionally, we find that framework defects allow salt to percolate through the framework, hindering water-salt selectivity. 2-D NM frameworks with a packing density of 75%, representative of most 2-D NM membranes, are projected to achieve <92% NaCl rejection at a water permeability of <1 L m^-2 h^-1 bar^-1, even with ideal interlayer spacing. A high packing density of 90%, which to our knowledge has yet to be achieved, could yield comparable performance to current desalination membranes. Maximizing packing density is therefore a critical technical challenge, in addition to the already daunting challenge of optimizing interlayer spacing, for the development of 2-D NM membranes.

Effects of Edge Functional Groups on Water Transport in Graphene Oxide Membranes

Graphene oxide (GO) membranes assembled by GO nanosheets exhibit high water flux because of the unique water channels formed by their functionalized layer-by-layer structure. Although water transport in the GO membrane is in principle influenced by the functional groups at the edges of GO nanosheets, this is yet to be fully understood. To fill this knowledge gap, molecular dynamics simulation was employed in this work to gain insights into the influences of three typical edge functional groups of GO nanosheets: carboxyl (COOH), hydroxyl (OH), and hydrogen (H). A well-controlled numerical analysis with complete isolation of the functional groups at the edges was undertaken. The results reveal that the COOH group has a negative impact on water transport because of its relatively large steric geometric structure, which resists water flow. By contrast, the OH group promotes water transport by uniquely "pulling" water molecules across the nanosheet layer because of its relatively stronger interaction with water. The H atom promotes water transport as well, mainly because of its low-resistance steric structure. Moreover, the size of the inter-edge hub has an apparent impact on the influence of these functional groups on water transport. The results suggest that in the design of high water flux GO membranes, it would be strategic to remove COOH edge functional groups while maintaining a mixture of OH and H edge functional groups.

Water flow modeling through a graphene-based nanochannel: theory and simulation

Understanding the behavior of water molecule transport through artificial nano-channels is essential in designing novel nanofluidic devices that could be used especially in nanofiltration processes. In this study, using nonequilibrium molecular dynamics (MD) simulations, we simulated the water flow through different graphene-based channels to investigate the influences of some key factors such as the channel thickness and applied pressure on the water flow. It was demonstrated that the water flow was enhanced by increasing the applied pressure and channel thickness. Our results indicated that a third order polynomial curve could describe the variation of the water flow as a function of the channel thickness and the applied pressure. In addition, we improved the hydrodynamics equation used to consider the water flow through nano-channels, by adding two terms to describe the slip effect and the entrance/exit effect, in which the first term increased the water flow rate, while the second term reduced it. This study may be helpful in designing high-performance graphene-based membranes with some practical applications such as desalination.