Water permeability of nanoporous graphene at realistic pressures for reverse osmosis desalination

A Reverse Nonequilibrium Molecular Dynamics Algorithm for Coupled Mass and Heat Transport in Mixtures

We present a new method for introducing stable nonequilibrium concentration gradients in molecular dynamics simulations of mixtures. This method extends earlier reverse nonequilibrium molecular dynamics (RNEMD) methods, which use kinetic energy scaling moves to create temperature or velocity gradients. In the new scaled particle flux (SPF-RNEMD) algorithm, energies and forces are computed simultaneously for a molecule existing in two nonadjacent regions of a simulation box, and the system evolves under a linear combination of these interactions. A continuously increasing particle scaling variable is responsible for the migration of the molecule between the regions as the simulation progresses, allowing for simulations under an applied particle flux. To test the method, we investigate diffusivity in mixtures of identical but distinguishable particles and in a simple mixture of multiple Lennard-Jones particles. The resulting concentration gradients provide Fick diffusion constants for mixtures. We also discuss using the new method to obtain coupled transport properties using simultaneous particle and thermal fluxes to compute the temperature dependence of the diffusion coefficient and activation energies for diffusion from a single simulation. Lastly, we demonstrate the use of this new method in interfacial systems by computing the diffusive permeability of a molecular fluid moving through a nanoporous graphene membrane.

Ion Selectivity in Multilayered Stacked Nanoporous Graphene

Nanoporous graphene is an ideal candidate for molecular filtration as it can potentially combine high permeability with high selectivity at molecular levels. To make use of graphene in filtration setups, the defects formed during its growth and during the transfer of graphene to the carrier support pose a challenge. These uncontrolled pores can be avoided by stacking graphene layers, and then, controlled pores can be initiated with oxygen plasma. Here, we show that two-layer stacks provide the best balance of defect coverage and high selectivity compared with other stacks. Using the electrical characterization of ionic solutions in the standard diffusion cell, we compare the ionic transport and ionic selectivity of up to three-layered stacks of graphene that have been plasma-treated. We find that there is a decrease in the ionic selectivity of a two-layered stack as one more layer of graphene is added. We provide a model for this occurrence. Our results will be helpful for making practical and high-performance filtration systems from two-dimensional materials.

Water purification modeling by functionalized hourglass-shape multilayer nano-channel

In this study, inspired by the overall structure and operation of the aquaporin channel, graphene-based nanochannels are proposed to be used as potential membranes for the water purification process. To this end, an hourglass-shaped channel has been designed using the three-layer porous graphene sheets and the effects of some main channel's elements, such as the channel bending angle and attached functional groups to it, on the filtration performance have been examined by using molecular dynamics simulations. We find that a suitable bending channel shape can improve the channel efficiency, i.e. both the water permeability and the ion rejection rate of the suitable bent channels were more than for the straight channels. In addition, regarding the different functionalized channels, the half-functionalized channels were more efficient than the completed functionalized ones. Furthermore, by monitoring the dynamics of water molecules as they pass through the narrowest part of the channels, it was found that water molecule rotation assists water transport.

Graphene Nanopores Enhance Water Evaporation from Salt Solutions: Exploring the Effects of Ions and Concentration

With increased water stress, the development of clean water technologies is an active area of research. Evaporation-based solutions offer the advantage of low energy consumption, and recently a 10-30 fold enhancement in water evaporation flux has been observed through Å-scale graphene nanopores (Lee, W.-C., et al., ACS Nano 2022, 16(9), 15382). Herein, using molecular dynamics simulations, we examine the suitability of Å-scale graphene nanopores in enhancing water evaporation from salt solutions (LiCl, NaCl, and KCl). Cation-π interactions between ions and the surface of nanoporous graphene are found to significantly influence ion populations in the nanopore vicinity, leading to varied water evaporation fluxes from different salt solutions. The highest water evaporation flux was observed for KCl solutions, followed by NaCl and LiCl solutions, with the differences reducing at lower concentrations. Relative to the bare liquid-vapor interface, 4.54 Å nanopores exhibit the highest evaporation flux enhancements ranging from 7 to 11, with an enhancement of 10.8 obtained for 0.6 M NaCl solution, which closely resembles seawater compositions. Functionalized nanopores induce short-lived water-water hydrogen bonds and reduce surface tension at the liquid-vapor interface, thereby lowering the free energy barrier for water evaporation with a negligible effect on the ion hydration dynamics. These findings can aid in developing green technologies for desalination and separation processes with low thermal energy input.

Recent advances on graphyne and its family members as membrane materials for water purification and desalination

Graphyne and its family members (GFMs) are allotropes of carbon (a class of 2D materials) having unique properties in form of structures, pores and atom hybridizations. Owing to their unique properties, GFMs have been widely utilized in various practical and theoretical applications. In the past decade, GFMs have received considerable attention in the area of water purification and desalination, especially in theoretical and computational aspects. More recently, GFMs have shown greater prospects in achieving optimal separation performance than the experimentally derived commercial polyamide membranes. In this review, recent theoretical and computational advances made in the GFMs research as it relates to water purification and desalination are summarized. Brief details on the properties of GFMs and the commonly used computational methods were described. More specifically, we systematically reviewed the various computational approaches employed with emphasis on the predicted permeability and selectivity of the GFM membranes. Finally, the current challenges limiting their large-scale practical applications coupled with the possible research directions for overcoming the challenges are proposed.

Graphene-Based Membranes for Water Desalination: A Literature Review and Content Analysis

Graphene-based membranes have unique nanochannels and can offer advantageous properties for the water desalination process. Although tremendous efforts have been devoted to heightening membrane performance and broadening their application, there is still lack of a systematic literature review on the development and future directions of graphene-based membranes for desalination. In this mini-review, literature published between 2011 and 2022 were analyzed by using the bibliometric method. We found that the major contributors to these publications and the highest citations were from China and the USA. Nearly 80% of author keywords in this analysis were used less than twice, showing the broad interest and great dispersion in this field. The recent advances, remaining gaps, and strategies for future research, were discussed. The development of new multifunctional nanocomposite materials, heat-driven/solar-driven seawater desalination, and large-scale industrial applications, will be important research directions in the future. This literature analysis summarized the recent development of the graphene-based membranes for desalination application, and will be useful for researchers in gaining new insights into this field.

Edges in bilayered h-BN: insights into the atomic structure

This study is devoted to the study of the edges of bilayered h-BN, whose atomic structure was previously generally unknown. It is shown that the edges tend to connect regardless of the edge cut. A defectless connection can be expected only in the case of a zigzag edge, while in other cases a series of tetragonal and octagonal defects will be formed. This result was obtained by carrying out an analogy between the edge of bilayered h-BN and the interface of monolayer h-BN. Information on the structure and energetics of closed edges allowed us to predict the shape of holes in h-BN, which agreed with the reference experimental data. Finally, it is shown that the closed edges do not create electronic states in the band gap, thus not changing the dielectricity of h-BN.

On the Performance of Vertically Aligned Graphene Array Membranes for Desalination

In this paper, we perform molecular dynamics simulations to investigate the performance of multilayer graphene slit membranes. Graphene slit membranes at a critical slit size have been found to be promising desalination membranes. In this contribution, it is shown that multilayer slit membranes have the potential to provide significantly better permeability while retaining outstanding salt rejection. Improved permeability of the membrane is achieved by using slits of widths larger than the critical slit size required to reject salt through size exclusion, and desalination of sea water is performed by increased resistance to salt passage through the multilayering. To facilitate the design process of future multilayer membranes, we analyze the flow resistance of the membrane as a combination of electrical resistors in series and show that this analogy works for membranes where the layers possess the same slit size, as well as membranes with layers of different slit sizes. Comparing with single layer graphene membranes, it was shown that it is possible to obtain 55% improvement in permeability without loss in salt rejection capabilities through multilayering. This opens up possibilities for membrane designers to be free from the restrictions of using a single layer graphene slit membrane with a fixed slit width.

Microscopic Insight into Water Desalination through Nanoporous Graphene: The Influence of the Dipole Moment

Nanoporous graphene membranes with controllable pore size and chemical functionality may be one of the most desirable materials for water desalination. Herein, we investigate desalination performance of hydrogen-functionalized nanoporous graphene membranes. The charge values on hydrogen atoms (q_H) and carbon atoms at the pore rim are systematically adjusted. For q_H > 0, the flow rate decreases as q_H increases, whereas for q_H < 0, the flow rate tends to increase first and then decrease with increasing q_H, yielding a peak at ∼ -0.2 e. Moreover, nanopores with large dipole moments at the rim have little effect on the salt rejection. The calculated oxygen and hydrogen density maps, the potential of mean force for water molecule and salt ion passage through the nanopores, and the coordination number unveil the mechanisms underlying water desalination in nanoporous graphene. This work may inspire the design and improvement of two-dimensional membranes for water desalination.

How Grain Boundaries and Interfacial Electrostatic Interactions Modulate Water Desalination via Nanoporous Hexagonal Boron Nitride

To fulfill the increasing demand for drinking water, researchers are currently exploring nanoporous two-dimensional materials, such as hexagonal boron nitride (hBN), as potential desalination membranes. A prominent, yet unsolved challenge is to understand how such membranes will perform in the presence of defects or surface charge in the membrane material. In this work, we study the effect of grain boundaries (GBs) and interfacial electrostatic interactions on the desalination performance of bicrystalline nanoporous hBN using classical molecular dynamics simulations supported by quantum-mechanical density functional theory (DFT) calculations. We investigate three different nanoporous bicrystalline hBN configurations, with symmetric tilt GBs having misorientation angles of 13.2, 21.8, and 32.2°. Using lattice dynamics calculations, we find that grain boundaries alter the areas and shapes of nanopores in bicrystalline hBN, as compared to the nanopores in monocrystalline hBN. We observe that, although bicrystalline nanoporous hBN with a misorientation angle of 13.2° shows an improved water flow rate by ∼30%, it demonstrates reduced Na⁺ ion rejection by ∼6%, as compared to monocrystalline hBN. We also uncover the role of the nanopore shape in water desalination, finding that more elongated pores with smaller sizes (in 21.8- and 32.2°-misoriented bicrystalline hBN) can match water permeation through less elongated pores of slightly larger sizes, with a concomitant ∼3-4% decrease in Na⁺ rejection. Simulations also predict that the water flow rate is significantly affected by interfacial electrostatic interactions. Indeed, the water flow rate is the highest when altered partial charges on B and N atoms were determined using DFT calculations, as compared to when no partial charges or bulk partial charges (i.e., charged hBN) were considered. Overall, our work on water/ion transport through nanopores in bicrystalline hBN indicates that the presence of GBs and surface charge can lead, respectively, to a decrease in the ion rejection and water permeation performance of hBN membranes.

Phonon-Fluid Coupling Enhanced Water Desalination in Flexible Two-Dimensional Porous Membranes

Water purification using 2D nanoporous membranes has been drawing significant attention for over a decade because of fast water transport in ultrathin membranes. We perform a comprehensive study using molecular dynamics (MD) simulations on water desalination using 2D flexible membranes where the coupling between the fluid dynamics and mechanics of the membrane plays an important role. We observe that a considerable deformation and fluctuation in the 2D membrane results in an enhanced water permeability (up to 122%) along with a slight decrease in the salt rejection rate (less than 11%). Simulations on harmonically vibrating membranes indicate that the vibrational match at the membrane-water interface can significantly increase the permeance. We conduct mechanical stability tests and discuss the maximum endurable pressure of 2D porous membranes for water desalination. These findings will contribute to advances in applications using ultrathin membranes, such as energy harvesting and molecular separation.

Tunable Electric and Magnetic Properties of Transition Metal@Nx Cy -Graphene Materials by Different Metal and Defect Types

Transition metal@N_x C_y -graphene (TM@N_x C_y -GR) materials have been widely used as redox reaction catalysts in the field of fuel cells due to their low cost and high performance. In the present work, we systematically investigate the effect of different metal and defect types on the electro-magnetic properties of TM@N_x C_y -GR materials using first principles calculations. Our simulations show that TM@N₃ -GR (the minimum defect size) and TM@N₇ -GR (the maximum defect size) materials always possess metallic property regardless the metal type. However, doping different TM can regulate the medium defects (TM@N₂ C₂ -GR-I and TM@N₂ C₂ -GR-II) among metallicity, half-metallicity and semi-conductivity. In addition, we found that different TM and defect type largely affects the magnetic moment. The spin density and projected density of state calculations show that the net charges of the defect structure are mainly located near the hole, and the magnetic regulation comes from the coupling of TM-d orbital with carbon (nitrogen)-s(p) orbitals. The present study provides abundant valuable information for the TM@N_x C_y -GR materials designs and applicants in the future.

Emerging graphitic carbon nitride-based membranes for water purification

Membrane separation is a promising technology that can effectively remove various existing contaminants from water with low energy consumption and small carbon footprint. The critical issue of membrane technology development is to obtain a low-cost, stable, tunable and multifunctional material for membrane fabrication. Graphitic carbon nitride (g-C₃N₄) has emerged as a promising membrane material, owing to the unique structure characteristics and outstanding catalytic activity. This review paper outlined the advanced material strategies used to regulate the molecule structure of g-C₃N₄ for membrane separation. The presentative progresses on the applications of g-C₃N₄-based membranes for water purification have been elaborated. Essentially, we highlighted the innovation integration of physical separation, catalysis and energy conversion during water purification, which was of great importance for the sustainability of water treatment techniques. Finally, the continuing challenges of g-C₃N₄-based membranes and the possible breakthrough directions in the future research was prospected.

Aqueous Affinity and Interfacial Dynamics of Anisotropic Buckled Black Phosphorous

The structure of black phosphorous (BP) is similar to the honeycomb arrangement of graphene, but the layered BP is found to be buckled and highly anisotropic. The buckled surface structure affects interfacial molecule mobility and plays a vital role in various nanomaterial applications. The BP is also known for wettability, droplet formation, stability, and hydrophobicity in the aqueous environment. However, there is a gap concerning the structural and dynamical behavior of water molecules, which is available in abundance for other monoatomic and polyatomic two-dimensional (2D) materials. Motivated by the technological importance, we try to bridge the gap by explaining the surface anisotropy-facilitated behavior of water molecules on bilayer BP using classical and first principles molecular dynamics (MD) simulations. From our classical MD study, we find three distinct layers of water molecules. The water layer closest to the interface is L1, followed by L2 and L3/bulk perpendicular to the BP surface. Water molecules in the L1 layer experience some structural disintegration in hydrogen bond (HB) phenomena compared to the bulk. There is a loss of HB donor-acceptor count per water molecule. The average HB count decreases because of an elevated rate of HB formation and deformation; this would affect the dynamic properties in terms of HB lifetime. Therefore, we observe the reduced lifetime of HB in the layer in close contact with BP, which again complements our finding on the diffusion coefficient of water molecules in distinct layers. Water diffuses relatively faster with diffusion coefficient 3.25 × 10^-9 m² s^-1 in L1, followed by L2 and L3. The BP layer shows moderate hydrophobic nature. Our results also indicate the anisotropic behavior as the diffusion along the x-direction is faster than that along the y-direction. The gap in the slope of the x and y components of mean-squared displacement (MSD) complements the pinning effect in an aqueous environment. We observe blue-shifted and red-shifted libration and O-H stretching modes from the calculated power spectra for the L1 water molecules compared to the L2 and L3 molecules from first principles MD simulations. Our analysis may help understand the physical phenomena that occur during the surface wetting of the predroplet formation process observed experimentally.

Strain-Tunable Carbon Nanothread-Derived Membranes for Water Desalination

Carbon nanothread-derived nanomeshes are highly flexible two-dimensional (2D) structures with tunable pore size and shape, which allows fine control of their transport properties when applied as membranes. In this work, we use molecular dynamics simulations to investigate the performance of several nanomesh structures as membranes for water desalination through reverse osmosis. Results show that these membranes can operate in a wide range of water flow rate, with an optimal point that yields 100% NaCl rejection and water permeability as high as 106 L·cm^-2·day^-1·MPa^-1, higher than other nanoporous 2D materials reported in the literature. This promising performance is partially due to the elliptical pores of strained nanomeshes, which allow the passage of rotated water molecules while rejecting hydrated salt ions. Our results show that carbon nanothread-derived nanomeshes have great potential for application in water desalination processes and emphasize the importance of engineering pore shape in 2D materials when applied as reverse osmosis membranes.

Scale Effect on Simple Liquid Transport through a Nanoporous Graphene Membrane

The transport mechanism of a simple liquid through nanoporous graphene membranes (NPGMs) with pores of various diameters has been explored by utilizing nonequilibrium molecular dynamics (NEMD) simulation. The flow is initiated using a pressure-driven flow mechanism that moves the specular reflection wall at a constant velocity. Both the local density peak near the membrane and the pressure drop are dependent on the pore diameter. For accurate calculation of the velocity profile inside the nanopore, we implemented three boundary approaches and local nanoscale variants to see the effect of these factors on the nature of the nanoscale flow. We found an optimized definition of the pore boundary, which minimizes the deviation between MD results and slip-viscosity-modified Sampson's prediction for nanopores of various diameters. Additionally, we observed that with decreasing pore size, the pore center velocity increases, as does the slip velocity, which we attributed to van der Waals interaction between the liquid and wall atoms inside the nanopore. However, the effects of slip velocity, interfacial viscosity, and pore boundary decay exponentially with increasing pore diameter because of the dominance of van der Waals repulsive forces at the molecular level.

Molecular fluid flow in MoS2 nanoporous membranes and hydrodynamics interactions

We study the impact of the induced pressure fields on the water flow and salt rejection in nanopores produced in MoS₂ membranes. We observe that the water permeability and the salt rejection are not impacted by the distance between the pores. This result contradicts the continuous fluid mechanics calculations in microfilters, which indicates the existence of hydrodynamic interactions between adjacent pores that increase the water mobility. Our results suggest that at this nanoscale, the hydrodynamic interactions do not affect the water mobility through nanopores.

Novel adjustable monolayer carbon nitride membranes for high-performance saline water desalination

In this study, via molecular dynamic simulations, we showed that the latest described graphene-like carbon nitride membranes, such as g-C₄N₃, g-C₆N₆, and g-C₃N₄ single-layers, can be used as high-performance membranes for water desalination. In addition to having inherent nanopores and extraordinary mechanical properties, the carbon nitride membranes have high water permeability and strong ion rejection (IR) capability. The important point about carbon nitride membranes is that the open or closed state of the pores can be changed by applying tensile stress and creating a positive strain on the membrane. The effect of the imposed pressure, the tensile strain, the ion concentration, and the effective pore size of the membranes are reported. It is demonstrated that, with the applied tensile strain of 12%, the g-C₆N₆ membrane is the best purification membrane, with a water permeability of 54.16 l cm^-2 d^-1 MPa^-1 and the IR of 100%. Its water permeability is one order of magnitude greater than other one-atom-thick membranes.

Mechanical strength of a nanoporous bicrystalline h-BN nanomembrane in a water submerged state

Due to superior water permeability, structural stability, and adsorption capability, h-BN nanosheets are emerging as an efficient membrane for water desalination. In order to cater to the demand for potable water, large size membranes are required to maintain a high desalination rate from water purification systems. These large size membranes usually contain polycrystals with an offset in their mechanical properties from pristine h-BN nanosheets. In this article, molecular dynamics based simulations were performed in conjunction with a hybrid interatomic potential (reactive force field, TIP3P, and Lennard Jones) to simulate the mechanical strength of nanoporous single and bicrystalline h-BN nanosheets under water submerged conditions. The interaction between the atomic configuration of grain boundary atoms and nanopores in the presence of water molecules helps in investigating the viability of defective h-BN nanomembranes for underwater applications. Higher dislocation density enhances the mechanical strength of nanoporous bicrystalline h-BN nanosheets containing twin nanopores, which makes them a better substitute for water submerged applications as compared to the pristine nanosheets. The mechanical strength of nanoporous single crystalline h-BN nanosheets deteriorates with an increase in the number of nanopores, whereas a contrasting trend was observed with bicrystalline h-BN nanosheets.

Influence of substrate on ultrafast water transport property of multilayer graphene coatings

The ultrafast water transport in graphene nanoplatelets (GNPs) coating is attributed to the low friction passages formed by pristine graphene and the hydrophilic functional groups which provide a strong interaction force to the water molecules. Here, we examine the influence of the supporting substrate on the ultrafast water transport property of multilayer graphene coatings experimentally and by computational modelling. Thermally cured GNPs manifesting ultrafast water permeation are coated on different substrate materials, namely aluminium, copper, iron and glass. The physical and chemical structures of the GNPs coatings which are affected by the substrate materials are characterized using various spectroscopy techniques. Experimentally, the water permeation and absorption tests evidence the significant influence of the substrate on the rapid water permeation property of GNPs-coating. The water transport rates of the GNPs coatings correspond to the wettability and the free surface energy of their substrates where the most hydrophilic substrate induces the highest water transport rate. In addition, we conduct molecular dynamics (MD) simulations to investigate the transport rate of water molecules through multilayer GNPs adjacent to different substrate materials. The MD simulations results agree well with the experimental results inferring the strong influence of the substrate materials on the fast water transport of GNPs. Therefore, selection of substrate has to be taken into consideration when the GNPs-coating is placed into applications.

Toxic impacts and industrial potential of graphene

Advancement in the field of nanotechnology has increased the synthesis and exploitation of graphene-like nanomaterials. Graphene is a two-dimensional planar and hexagonal array of carbon atoms. Due to its flexible nature graphene and its derivatives have several significant prospects extending from electronics to life sciences and drug delivery systems. In this review, we enlist some of the toxic effects of graphene family nanomaterials (GFNs) in various aspects of biosystems viz., in vitro, in vivo, microbial, molecular and environmental. We also appreciate their extensive and promising applications though with some underlying challenges. This review also draws attention toward current and future prospect of global graphene market for wide-range commercialization.

Molecular transportation phenomena of simple liquids through a nanoporous graphene membrane

The mechanism of pressure-driven transport of simple liquid through a nanoporous graphene membrane has been analyzed using nonequilibrium molecular dynamics simulation. In this study, we investigate liquid dynamics properties such as local density, pressure variation, and local viscosity depending on the flow region. With movement of the specular reflection wall at the end of the front and back reservoirs, a pressure difference occurs mainly due to the change in the relative distance between the liquid molecules in the corresponding reservoir. The interfacial pressure difference strongly depends on the intermolecular force of the graphene membrane governed by the layered structure of the simple liquid and the applied flow velocity. The local viscosity was calculated for a nanochannel of simple liquid sheared by graphene walls. The liquid velocity adjacent to the pore edge was considered as the slip velocity, which provides updates in the Sampson flow equation. We observed that the entrance interfacial pressure and higher local viscosity in the vicinity of the graphene membrane, which are associated with the optimized definition of the wall-liquid boundary near the pore edge, play a critical role in the permeation of simple liquids through the nanoporous graphene membrane.

A Molecular Dynamics Study on Rotational Nanofluid and Its Application to Desalination

In this work, we systematically study a rotational nanofluidic device for reverse osmosis (RO) desalination by using large scale molecular dynamics modeling and simulation. Moreover, we have compared Molecular Dynamics simulation with fluid mechanics modeling. We have found that the pressure generated by the centrifugal motion of nanofluids can counterbalance the osmosis pressure developed from the concentration gradient, and hence provide a driving force to filtrate fresh water from salt water. Molecular Dynamics modeling of two different types of designs are performed and compared. Results indicate that this novel nanofluidic device is not only able to alleviate the fouling problem significantly, but it is also capable of maintaining high membrane permeability and energy efficiency. The angular velocity of the nanofluids within the device is investigated, and the critical angular velocity needed for the fluids to overcome the osmotic pressure is derived. Meanwhile, a maximal angular velocity value is also identified to avoid Taylor-Couette instability. The MD simulation results agree well with continuum modeling results obtained from fluid hydrodynamics theory, which provides a theoretical foundation for scaling up the proposed rotational osmosis device. Successful fabrication of such rotational RO membrane centrifuge may potentially revolutionize the membrane desalination technology by providing a fundamental solution to the water resource problem.

The effect of temperature on water desalination through two-dimensional nanopores

Two-dimensional (2D) materials such as graphene, molybdenum sulfide, and hexagonal boron nitride are widely studied for separation applications such as water desalination. Desalination across such 2D nanoporous membranes is largely influenced by the bulk transport properties of water, which are, in turn, sensitive to the operating temperature. However, there have been no studies on the effect of temperature on desalination through 2D nanopores. We investigated water desalination through hydrogen functionalized graphene nanopores of varying pore areas at temperatures 275.0 K, 300.0 K, 325.0 K, and 350.0 K. The water flux showed a direct relation with the diffusion coefficient and an inverse relation with the hydrogen-bond lifetime. As a direct consequence, the water flux was found to be related to the temperature as per the Arrhenius equation, similar to an activated process. The results from the present study improve the understanding on water and ion permeation across nanoporous 2D materials at different temperatures. Furthermore, the present investigation suggests a kinetic model, which can predict the water and ion permeation based on the characteristics of the nanopore.

Exploring fast water permeation through aquaporin-mimicking membranes

Using molecular dynamics simulations, herein, we illustrate that a bending structure shows different behaviors for fast water transport through aquaporin-mimicking membranes in multilayer graphene and tubular structures.

Dynamic Properties of Water Confined in Graphene-Based Membrane: A Classical Molecular Dynamics Simulation Study

We performed molecular dynamics simulations of water molecules inside a hydrophobic membrane composed of stacked graphene sheets. By decreasing the density of water molecules inside the membrane, we observed that water molecules form a droplet through a hydrogen bond with each other in the hydrophobic environment that stacked graphene sheets create. We found that the water droplet translates as a whole body rather than a dissipate. The translational diffusion coefficient along the graphene surface increases as the number of water molecules in the droplet decreases, because the bigger water droplet has a stronger van der Waals interaction with the graphene surface that hampers the translational motion. We also observed a longer hydrogen bond lifetime as the density of water decreased, because the hydrophobic environment limits the libration motion of the water molecules. We also calculated the reorientational correlation time of the water molecules, and we found that the rotational motion of confined water inside the membrane is anisotropic and the reorientational correlation time of confined water is slower than that of bulk water. In addition, we employed steered molecular dynamics simulations for guiding the target molecule, and measured the free energy profile of water and ion penetration through the interstice between graphene sheets. The free energy profile of penetration revealed that the optimum interlayer distance for desalination is ~10 Å, where the minimum distance for water penetration is 7 Å. With a 7 Å interlayer distance between the graphene sheets, water molecules are stabilized inside the interlayer space because of the van der Waals interaction with the graphene sheets where sodium and chloride ions suffer from a 3–8 kcal/mol energy barrier for penetration. We believe that our simulation results would be a significant contribution for designing a new graphene-based membrane for desalination.

Macroscopic Salt Rejection through Electrostatically Gated Nanoporous Graphene

Atomically thin porous graphene is emerging as one of the most promising candidates for next-generation membrane material owing to the ultrahigh permeation. However, the transport selectivity relies on the precise control over pore size and shape which considerably compromises the scalability. Here, we study electrolyte permeation through a sheet of large-area, porous graphene, with relatively large pore sizes of 20 ± 10 nm. Counterintuitively, a high degree of salt rejection is observed by electrostatic gating, reducing the diffusive flux by up to 1 order of magnitude. We systematically investigate the effects of salt concentration and species, including developing a theory to model the electrolyte diffusion through a nanopore drilled in a sheet of gated graphene. The interplay between graphene quantum capacitance and the electrical double layer is found to selectively modulate the anionic and cationic transport paths, creating voltage-dependent electrochemical barriers when the pore size is comparable to the Debye length. Our findings reveal a new degree of freedom regulating electrolyte permeation through porous two-dimensional materials, complementary to the pore size design and engineering.

Theoretical Prediction of Mechanical Strength and Desalination Performance of One-Atom-Thick Hydrocarbon Polymer in Pressure-Driven Separation

One-atom-thick materials hold promise for the future of membrane-based gas purification and water filtration applications. However, there are a few investigations on the mechanical properties of these materials under pressure-driven condition. Here, by employing molecular simulation techniques and continuum mechanics simulation, we investigate the mechanical strength of two-dimensional hydrocarbon polymers containing sub-nanometer pores with various topologies. We demonstrate that the mechanical strengths of the membranes are correlated with their pore sizes and geometries. In addition, when the pore size of substrates is controlled within a reasonable range, all of the membrane candidates can withstand the practical hydraulic pressure of few megapascal. The studied materials also exhibit better seawater desalination performance as compared to the traditional polymeric reverse osmosis membrane. This work presents a new route to design new separation membrane, and also propose a simulation method to evaluate the mechanical strength and desalination performance.

Separation of water-alcohol mixtures using carbon nanotubes under an electric field

Carbon nanotubes (CNTs) are a promising candidate for separation membranes because of their ability to transport substances at very high flow rates. However, there is a tradeoff between achieving a high selectivity using small pore sizes and the reduction of water flux. Here, using molecular dynamics simulations, we report that CNTs can effectively separate water-methanol mixtures under an electric field. Without an electric field and under piston pressure, both water and methanol flow through a CNT, resulting in no separation effect. In contrast, under an electric field and high piston pressure, CNTs allow selective water permeation while rejecting the permeation of methanol molecules. This separation effect is caused by the ordered structures of water molecules in the CNT. A high filtering effect is observed under the conditions of high methanol concentration in the solution or even with large-diameter CNTs up to 3.39 nm. As long as the ordered structure of water in the CNTs can be maintained, the strong filtering effect can be maintained.

Assessing the Thickness-Permeation Paradigm in Nanoporous Membranes

Driven by the need of maximizing performance, membrane nanofabrication strives for ever thinner materials aiming to increase permeation while evoking inherent challenges stemming from mechanical stability and defects. We investigate this thickness rationale by studying viscous transport mechanisms across nanopores when transitioning the membrane thickness from infinitely thin to finite values. We synthesize double-layer graphene membranes containing pores with diameters from ∼6 to 1000 nm to investigate liquid permeation over a wide range of viscosities and pressures. Nanoporous membranes with thicknesses up to 90 nm realized by atomic layer deposition demonstrate dominance of the entrance resistance for aspect ratios up to one. Liquid permeation across these atomically thin pores is limited by viscous dissipation at the pore entrance. Independent of thickness and universal for porous materials, this entrance resistance sets an upper bound to the viscous transport. Our results imply that membranes with near-ultimate permeation should feature rationally selected thicknesses based on the target solute size for applications ranging from osmosis to microfiltration and introduce a proper perspective to the pursuit of ever thinner membranes.

Breakdown of continuum model for water transport and desalination through ultrathin graphene nanopores: insights from molecular dynamics simulations

In the quest for identifying a graphene membrane for efficient water desalination, molecular dynamics simulations were performed for the pressure-driven flow of salty water across a multilayer graphene membrane.

Charged nanoporous graphene membranes for water desalination

Water desalination using positively and negatively charged single-layer nanoporous graphene membranes.

What governs the nature of fouling in forward osmosis (FO) and reverse osmosis (RO)? A molecular dynamics study

Difference in the distribution of water molecules around the protein leads to different fouling structures in FO and RO.

An overview of graphene materials: Properties, applications and toxicity on aquatic environments

Due to unique chemical and physical properties, nanomaterials from the Graphene family are being increasingly introduced in all fields of science. The specific roles they can occupy within different applications are attracting increased attention by several industrial sectors. These carbon nanoparticles are released into the environment especially accumulating in aquatic systems. Since the discovery of graphene, a number of research actives are being conducted to find out the toxic potential of the Graphene family materials to different organism's models. Although their toxicity effects are well described for biomedical applications, few data were produced with the specific aim of assessing the toxic effects of these carbon nanomaterials in the aquatic environment. The purpose of this review is to compile up-to-date information on properties, applications and characterization methods of graphene family materials in aquatic environments and identified biological toxic impacts of these NMs, with special focus on graphene oxide based on the most recent literature.

Rational Design of Two-Dimensional Hydrocarbon Polymer as Ultrathin-Film Nanoporous Membranes for Water Desalination

Membrane-based water desalination has drawn considerable attention for its potential in addressing the increasingly limited water resources, but progress remains limited due to the inherent constraints of conventional membrane materials. In this work, by employing state-of-the-art molecular simulation techniques, we demonstrated that two-dimensional hydrocarbon polymer membranes, materials that possess intrinsic and tunable nanopores, can provide opportunities as molecular sieves for producing drinkable water from saline sources. Moreover, we identified a unique relationship between the permeation and selectivity for membranes with elliptical pores, which breaks the commonly known trade-off between the pore size and desalination performance. Specifically, increase in the area of elliptical pores with a controlled minor diameter can offer an improved water flux without compromising the ability to reject salts. Water distributions and water dynamics at atomic levels with the potential of mean force profiles for water and ions were also analyzed to understand the dependence of permeation and selectivity on the pore geometry. The outcomes of this work are instrumental to the future development of ultrathin-film reverse osmosis membranes and provide guidelines for the design of membranes with more effective and efficient pore structures.

Single- to Few-Layered, Graphene-Based Separation Membranes

Two-dimensional, graphene-based materials have attracted great attention as a new membrane building block, primarily owing to their potential to make the thinnest possible membranes and thus provide the highest permeance for effective sieving, assuming comparable porosity to conventional membranes and uniform molecular-sized pores. However, a great challenge exists to fabricate large-area, single-layered graphene or graphene oxide (GO) membranes that have negligible undesired transport pathways, such as grain boundaries, tears, and cracks. Therefore, model systems, such as a single flake or nanochannels between graphene or GO flakes, have been studied via both simulations and experiments to explore the transport mechanisms and separation potential of graphene-based membranes. This article critically reviews literature related to single- to few-layered graphene and GO membranes, from material synthesis and characteristics, fundamental membrane structures, and transport mechanisms to potential separation applications. Knowledge gaps between science and engineering in this new field and future opportunities for practical separation applications are also discussed.

Water treatment by new-generation graphene materials: hope for bright future

Water is the most important and essential component of earth's ecosystem playing a vital role in the proper functioning of flora and fauna. But, our water resources are contaminating continuously. The whole world may be in great water scarcity after few decades. Graphene, a single-atom thick carbon nanosheet, and graphene nanomaterials have bright future in water treatment technologies due to their extraordinary properties. Only few papers describe the use of these materials in water treatment by adsorption, filtration, and photodegradation methods. This article presents a critical evaluation of the contribution of graphene nanomaterials in water treatment. Attempts have been made to discuss the future perspectives of these materials in water treatment. Besides, the efforts are made to discuss the nanotoxicity and hazards of graphene-based materials. The suggestions are given to explore the full potential of these materials along with precautions of nanotoxicity and its hazards. It was concluded that the future of graphene-based materials is quite bright.