Fast mass transport through carbon nanotube membranes

Use of Boundary-Driven Nonequilibrium Molecular Dynamics for Determining Transport Diffusivities of Multicomponent Mixtures in Nanoporous Materials

The boundary-driven molecular modeling strategy to evaluate mass transport coefficients of fluids in nanoconfined media is revisited and expanded to multicomponent mixtures. The method requires setting up a simulation with bulk fluid reservoirs upstream and downstream of a porous media. A fluid flow is induced by applying an external force at the periodic boundary between the upstream and downstream reservoirs. The relationship between the resulting flow and the density gradient of the adsorbed fluid at the entrance/exit of the porous media provides for a direct path for the calculation of the transport diffusivities. It is shown how the transport diffusivities found this way relate to the collective, Onsager, and self-diffusion coefficients, typically used in other contexts to describe fluid transport in porous media. Examples are provided by calculating the diffusion coefficients of a Lennard-Jones (LJ) fluid and mixtures of differently sized LJ particles in slit pores, a realistic model of methane in carbon-based slit pores, and binary mixtures of methane with hypothetical counterparts having different attractions to the solid. The method is seen to be robust and particularly suited for the study of study of transport of dense fluids and liquids in nanoconfined media.

Current Understanding of Water Properties inside Carbon Nanotubes

Confined water inside carbon nanotubes (CNTs) has attracted a lot of attention in recent years, amassing as a result a very large number of dedicated studies, both theoretical and experimental. This exceptional scientific interest can be understood in terms of the exotic properties of nanoconfined water, as well as the vast array of possible applications of CNTs in a wide range of fields stretching from geology to medicine and biology. This review presents an overreaching narrative of the properties of water in CNTs, based mostly on results from systematic nuclear magnetic resonance (NMR) and molecular dynamics (MD) studies, which together allow the untangling and explanation of many seemingly contradictory results present in the literature. Further, we identify still-debatable issues and open problems, as well as avenues for future studies, both theoretical and experimental.

Understanding Carbon Nanotube-Based Ionic Diodes: Design and Mechanism

The rectification of ion transport through biological ion channels has attracted much attention and inspired the thriving invention and applications of ionic diodes. However, the development of high-performance ionic diodes is still challenging, and the working mechanisms of ionic diodes constructed by 1D ionic nanochannels have not been fully understood. This work reports the systematic investigation of the design and mechanism of a new type of ionic diode constructed from horizontally aligned multi-walled carbon nanotubes (MWCNTs) with oppositely charged polyelectrolytes decorated at their two entrances. The major design and working parameters of the MWCNT-based ionic diode, including the ion channel size, the driven voltage, the properties of working fluids, and the quantity and length of charge modification, are extensively investigated through numerical simulations and/or experiments. An optimized ionic current rectification (ICR) ratio of 1481.5 is experimentally achieved on the MWCNT-based ionic diode. These results promise potential applications of the MWCNT-based ionic diode in biosensing and biocomputing. As a proof-of-concept, DNA detection and HIV-1 diagnosis is demonstrated on the ionic diode. This work provides a comprehensive understanding of the working principle of the MWCNT-based ionic diodes and will allow rational device design and optimization.

Interfacial Ions Sieving for Ultrafast and Complete Desalination through 2D Nanochannel Defined Graphene Composite Membranes

The layered graphene membrane has high potential for efficient desalination owing to its frictionless surface and hydrophobic nature. However, it has not been demonstrated so far due to the challenges related to controlling membrane microstructure. Herein, we develop a facile and simple thiol-ene click method to prepare a perfluoro-alkyl grafted graphene (fGraphene) membrane on porous ceramic, which features an ultrahigh antiwetting surface, oriented mesoporous surface entrances, and a well-defined interlamellar spacing of ∼1.1 nm. With vacuum membrane distillation, the fGraphene membranes post ∼100% rejections to the small ions of seawater, at least 1 order of magnitude higher water fluxes than those of commercial membranes and graphene-oxide-based membranes, as well as robust stability in the desalination. Fast NaCl desalinations on the fGraphene membrane were also confirmed by the reverse/forward osmosis tests. The complete rejection of ions and high flux are attributed to the interfacial sieving effect over the 2D nanochannels as well as the vapor-phase transport in the mesoscale channels, which is fundamentally different from the solution-diffusion mechanism of dense polymeric membranes and the size-sieving mechanism of microporous membranes. This work not only demonstrates a special separation effect for complete desalination over the layered graphene-based membrane but also offers a reliable method to functionalize and structure graphene membranes for other potential applications.

Flow of long chain hydrocarbons through carbon nanotubes (CNTs)

The pressure-driven flow of long-chain hydrocarbons in nanosized pores is important in energy, environmental, biological, and pharmaceutical applications. This paper examines the flow of hexane, heptane, and decane in carbon nanotubes (CNTs) of pore diameters 1-8 nm using molecular dynamic simulations. Enhancement of water flow in CNTs in comparison to rates predicted by continuum models has been well established in the literature. Our work was intended to observe if molecular dynamic simulations of hydrocarbon flow in CNTs produced similar enhancements. We used the OPLS-AA force field to simulate the hydrocarbons and the CNTs. Our simulations predicted the bulk densities of the hydrocarbons to be within 3% of the literature values. Molecular sizes and shapes of the hydrocarbon molecules compared to the pore size create interesting density patterns for smaller sized CNTs. We observed moderate flow enhancements for all the hydrocarbons (1-100) flowing through small-sized CNTs. For very small CNTs the larger hydrocarbons were forced to flow in a cork-screw fashion. As a result of this flow orientation, the larger molecules flowed as effectively (similar enhancements) as the smaller hydrocarbons.

Plasmodesmata and the problems with size: Interpreting the confusion

Plant tissues exhibit a symplasmic organization; the individual protoplasts are connected to their neighbors via cytoplasmic bridges that extend through pores in the cell walls. These bridges may have diameters of a micrometer or more, as in the sieve pores of the phloem, but in most cell types they are smaller. Historically, botanists referred to cytoplasmic bridges of all sizes as plasmodesmata. The meaning of the term began to shift when the transmission electron microscope (TEM) became the preferred tool for studying these structures. Today, a plasmodesma is widely understood to be a 'nano-scale' pore. Unfortunately, our understanding of these nanoscopic channels suffers from methodological limitations. This is exemplified by the fact that state-of-the-art EM techniques appear to reveal plasmodesmal pore structures that are much smaller than the tracer molecules known to diffuse through these pores. In general, transport processes in pores that have dimensions in the size range of the transported molecules are governed by different physical parameters than transport process in the macroscopic realm. This can lead to unexpected effects, as experience in nanofluidic technologies demonstrates. Our discussion of problems of size in plasmodesma research leads us to conclude that the field will benefit from technomimetic reasoning - the utilization of concepts developed in applied nanofluidics for the interpretation of biological systems.

Analytic description of anomalous diffusion in heterogeneous environments: Fokker-Planck equation without fractional derivatives

This article presents a one-dimensional model of diffusion in a heterogeneous environment, which qualitatively reflects the transport properties of a polymeric membrane with carbon nanotube areas. We derive the Fokker-Planck equation from a system of stochastic equations using a diffusion regime in polymers and a ballistic diffusion regime in nanotube areas. We demonstrate how the probability density function changes in the presence of nanotubes. The mean-square displacement of nonlinear time dependence is observed, indicating anomalous diffusion. This model explains the mechanism of anomalous diffusion in a ballistic-diffusion regime. Our approach does not suppose any type of distribution and does not use a fractional differentiate apparatus.

Nanofluidic Transport Theory with Enhancement Factors Approaching One

High performance water transport in nanopores has drawn a great deal of attention in a variety of applications, such as water desalination, power generation, and biosensing. High water transport enhancement factors in carbon-based nanopores have been reported over the classical Hagen-Poiseuille (HP) equation which does not account for the physics of transport at molecular scale. Instead, comparing the experimentally measured transport rates to that of a theory, that accounts for the microscopic physics of transport, would result in enhancement factors approaching unity. Such a theory is currently missing. Here, molecular corrections are introduced into the HP equation by considering the variation of key hydrodynamical properties (viscosity and friction) with thickness and diameter of pores in ultrathin graphene and finite-length carbon nanotubes (CNTs) using Green-Kubo relations and molecular dynamics (MD) simulations. The corrected HP (CHP) theory successfully predicts the permeation rates from nonequilibrium MD pressure driven flows. The previously reported enhancement factors over no-slip HP (of the order of 1000) approach unity when the permeations are normalized by the CHP flow rates. The results of our study will help better understand nanoscale flows in carbon-based pores and tubes.

Theory and simulation developments of confined mass transport through graphene-based separation membranes

The perspectives of graphene-based membranes based on confined mass transport from simulations and experiments for water desalination.

Electric charge of nanopatterned silica surfaces

The surface charge density of a nanopatterned silica decreased at the pits but increased at the tips of surface patterns. For a case of self-repeating surface structures, the average of local surface charges becomes lower than the theoretical predictions. Our phenomenological model developed as an extension to the existing flat surface theory predicts the average surface charge on a nanopatterned surface as a function of surface pattern size, ionic concentration and pH.

Size dependent surface charge properties of silica nano-channels: double layer overlap and inlet/outlet effects

Transport inside nano-channels and tubes is highly dependent on their surface charge properties. While previous studies assume that the charge density of a surface is a material property and independent of confinement size, this study properly characterized the surface charge of a nanochannel as a function of channel height and length under various solution conditions. By calculating the local surface charge based on local ionic concentrations, the surface charge of a nano-channel was studied by considering the effects of both overlapping electrical double layers (EDLs) and inlet/outlet regions. First, the surface charge of silica decreased with the increase in EDL overlap, which is characterized by the ratio of EDL thickness to channel height. Second, the local surface charge showed variation at the inlet/outlet regions where the channel's electrokinetics was in development. We defined a general entrance length as a function of EDL thickness for the electrokinetically developing part of different cases, after which the surface charge reached its equilibrium value and remained constant. Based on such length scales, we extended the existing theory to include nano-effects. A phenomenological model was developed, which can predict the average nano-channel surface charge as a function of EDL thickness, pH, channel length and channel height.

Combined effects of molecular geometry and nanoconfinement on liquid flows through carbon nanotubes

Molecular dynamics simulations are carried out to investigate the geometry effects of diatomic molecules on liquid flows in carbon nanotubes (CNTs). Oxygen molecules are considered as the fluid inside armchair (n,n) (n=6-20) CNTs. The simulated fluid temperature and bulk pressure for the liquid state are T=133 K and ρ_{b}=1346kg/m^{3}, respectively. In the agglomerated molecular cluster, nanoconfinement-induced structural changes are observed. As the CNT diameter decreases, it is confirmed that the flow rate significantly increases with irregular trends (discontinuity points in the profiles). From the discussion of the structure of the agglomerated fluid molecules, it is found that those trends are not simply caused by the structural changes. The main factor to induce the irregularity is confirmed to be the interlayer molecular movement affected by the combination of the molecular geometry and the arrangement of the multilayered structure.

High-flux water desalination with interfacial salt sieving effect in nanoporous carbon composite membranes

Freshwater flux and energy consumption are two important benchmarks for the membrane desalination process. Here, we show that nanoporous carbon composite membranes, which comprise a layer of porous carbon fibre structures grown on a porous ceramic substrate, can exhibit 100% desalination and a freshwater flux that is 3-20 times higher than existing polymeric membranes. Thermal accounting experiments demonstrated that the carbon composite membrane saved over 80% of the latent heat consumption. Theoretical calculations combined with molecular dynamics simulations revealed the unique microscopic process occurring in the membrane. When the salt solution is stopped at the openings to the nanoscale porous channels and forms a meniscus, the vapour can rapidly transport across the nanoscale gap to condense on the permeate side. This process is driven by the chemical potential gradient and aided by the unique smoothness of the carbon surface. The high thermal conductivity of the carbon composite membrane ensures that most of the latent heat is recovered.

Enhanced diffusion on oscillating surfaces through synchronization

The diffusion of molecules and clusters under nanoscale confinement or absorbed on surfaces is the key controlling factor in dynamical processes such as transport, chemical reaction, or filtration. Enhancing diffusion could benefit these processes by increasing their transport efficiency. Using a nonlinear Langevin equation with an extensive number of simulations, we find a large enhancement in diffusion through surface oscillation. For helium confined in a narrow carbon nanotube, the diffusion enhancement is estimated to be over three orders of magnitude. A synchronization mechanism between the kinetics of the particles and the oscillating surface is revealed. Interestingly, a highly nonlinear negative correlation between diffusion coefficient and temperature is predicted based on this mechanism, and further validated by simulations. Our results provide a general and efficient method for enhancing diffusion, especially at low temperatures.

Negative effect of nanoconfinement on water transport across nanotube membranes

Nanoconfinement environments are commonly considered advantageous for ultrafast water flow across nanotube membranes. This study illustrates that nanoconfinement has a negative effect on water transport across nanotube membranes based on molecular dynamics simulations. Although water viscosity and the friction coefficient evidently decrease because of nanoconfinement, water molecular flux and flow velocity across carbon nanotubes decrease sharply with the pore size of nanotubes. The enhancement of water flow across nanotubes induced by the decreased friction coefficient and water viscosity is markedly less prominent than the negative effect induced by the increased flow barrier as the nanotube size decreases. The decrease in water flow velocity with the pore size of nanotubes indicates that nanoconfinement is not essential for the ultrafast flow phenomenon. In addition, the relationship between flow velocity and water viscosity at different temperatures is investigated at different temperatures. The results indicate that flow velocity is inversely proportional to viscosity for nanotubes with a pore diameter above 1 nm, thereby indicating that viscosity is still an effective parameter for describing the effect of temperature on the fluid transport at the nanoscale.

Wettability effect on nanoconfined water flow

Understanding and controlling the flow of water confined in nanopores has tremendous implications in theoretical studies and industrial applications. Here, we propose a simple model for the confined water flow based on the concept of effective slip, which is a linear sum of true slip, depending on a contact angle, and apparent slip, caused by a spatial variation of the confined water viscosity as a function of wettability as well as the nanopore dimension. Results from this model show that the flow capacity of confined water is 10^-1∼10⁷ times that calculated by the no-slip Hagen-Poiseuille equation for nanopores with various contact angles and dimensions, in agreement with the majority of 53 different study cases from the literature. This work further sheds light on a controversy over an increase or decrease in flow capacity from molecular dynamics simulations and experiments.

Effect of vertically aligned carbon nanotube density on the water flux and salt rejection in desalination membranes

In this paper, vertically aligned carbon nanotube (VACNT) membranes of different densities are developed and their performances are investigated. VACNT arrays of densities 5 × 10⁹, 10¹⁰, 5 × 10¹⁰ and 10¹¹ tubes cm⁻², are initially grown on 1 cm × 1 cm silicon substrates using chemical vapour deposition. A VACNT membrane is realised by attaching a 300 μm-thick 1 cm × 1 cm VACNT array on silicon to a 4″ glass substrate, applying polydimethylsiloxane (PDMS) through spin coating to fill the gaps between the VACNTs, and using a microtome to slice the VACNT–PDMS composite into 25-μm-thick membranes. Experimental results show that the permeability of the developed VACNT membranes increases with the density of the VACNTs, while the salt rejection is almost independent of the VACNT density. The best measured permeance is attained with a VACNT membrane having a CNT density of 10¹¹ tubes cm⁻² is 1203 LMH at 1 bar.

Ultrabreathable and Protective Membranes with Sub-5 nm Carbon Nanotube Pores

Small-diameter carbon nanotubes (CNTs) are shown to enable exceptionally fast transport of water vapor under a concentration gradient driving force. Thanks to this property, membranes having sub-5 nm CNTs as conductive pores feature outstanding breathability while maintaining a high degree of protection from biothreats by size exclusion.

From dimers to collective dipoles: Structure and dynamics of methanol/ethanol partition by narrow carbon nanotubes

Alcohol partitioning by narrow single-walled carbon nanotubes (SWCNTs) holds the promise for the development of novel nanodevices for diverse applications. Consequently, in this work, the partition of small alcohols by narrow tubes was kinetically and structurally quantified via molecular dynamics simulations. Alcohol partitioning is a fast process in the order of 10 ns for diluted solutions but the axial-diffusivity within SWCNT is greatly diminished being two to three orders of magnitude lower with respect to bulk conditions. Structurally, alcohols form a single-file conformation under confinement and more interestingly, they exhibit a pore-width dependent transition from dipole dimers to a single collective dipole, for both methanol and ethanol. Energetic analyses demonstrate that this transition is the result of a detailed balance between dispersion and electrostatics interactions, with the latter being more pronounced for collective dipoles. This transition fully modifies the reorientational dynamics of the loaded particles, generating stable collective dipoles that could find usage in signal-amplification devices. Overall, the results herein have shown distinct physico-chemical features of confined alcohols and are a further step towards the understanding and development of novel nanofluidics within SWCNTs.

Computational study of pressure-driven methane transport in hierarchical nanostructured porous carbons

Using the reflecting particle method together with a perturbation-relaxation loop developed in our previous work, we studied pressure-driven methane transport in hierarchical nanostructured porous carbons (HNPCs) containing both mesopores and micropores in non-equilibrium molecular dynamics simulations. The surface morphology of the mesopore wall was systematically varied by tuning interaction strength between carbon atoms and the template in a mimetic nanocasting process. Effects of temperature and mesopore size on methane transport in HNPCs were also studied. Our study shows that increased mesopore wall surface roughness changes the character of the gas-wall interaction from specular to diffuse, while the gas-gas interaction is diminished due to the decrease of adsorption density. Effects of the mesopore wall surface morphology are the most significant at low temperatures and in small channels. Our systematic study provides a better understanding of the transport mechanisms of light gases through carbon nanotube composite membranes in experiments.

Wetting dynamics of a water nanodrop on graphene

Spreading of water nanodrop on supported and unsupported graphene reveals inertia-dominated behavior.

Fabrication of reduced graphene oxide membranes for highly efficient water desalination

The resultant PDA–RGO membranes allow faster permeation of water compared with GO membranes, but a higher retention rate of solutes.

Confinement effects on liquid-flow characteristics in carbon nanotubes

Liquid flow dynamics through the armchair (6,6)-(160,160) carbon nanotubes (CNTs) is elucidated by molecular dynamics simulations. The liquid is modeled by nonpolar argon atoms to understand the fundamental flow physics. The velocity profiles and slip lengths are discussed considering the radial distributions of the fluid density by the presently proposed finite difference-based velocity fitting method. It is found that as the CNT diameter D increases, the slip length and the flow rate enhancement show three-step transitional profiles in the region of D≤2.3 nm. The slip length and the flow rate stepwise increase at the first transition while they drop at the second and third transitions. The first transition corresponds to the structural change from the single-file chain to single-ring structures of the molecule cluster. The second and third transitions take place when the ring structure starts to develop another inner layer.

Preparation and Transport Performances of High-Density, Aligned Carbon Nanotube Membranes

We report a simple and effective method for the preparation of high-density and aligned carbon nanotube (CNT) membranes. The CNT arrays were prepared by water-assisted chemical vapor deposition (CVD) and were subsequently pushed over and stacked into dense membranes by mechanical rolling. It was demonstrated that various gases and liquids, including H2, He, N2, O2, Ar, water, ethanol, hexane, and kerosene, could effectively pass through the aligned carbon nanotube membranes. The membranes exhibited different selections on different gases, indicating that there was a separation potential for the gas mixtures. The selectivities (H2 relative to other gases) of H2/He, H2/N2, H2/O2, and H2/Ar were found to be lower than that of the ideal Knudsen model. For pure water, the permeability was measured to be 3.23 ± 0.05 ml·min(-1)·cm(-2) at 1 atm, indicating that the CNT membranes were promising for applications in liquid filtration and separation.

Computational Study of Pressure-Driven Gas Transport in Nanostructured Carbons: An Alternative Approach

We demonstrated a computationally efficient method in nonequilibrium molecular dynamics (NEMD) simulations to study pressure-driven gas transport in porous media. The reflecting particle method (RPM)14 was used to establish a steady-state gas flow along the transport channel, and the gas density in the feed chamber was properly adjusted to allow a constant pressure drop under various conditions by using a perturbation-relaxation loop developed here. This method was validated for methane flow through carbon nanotubes over a wide range of temperatures, giving results comparable to those of the commonly used dual control volume grand canonical molecular dynamics (DCV-GCMD) method but at least 20 times more efficient, even though the transport condition tested is favorable for the latter. This made it possible to perform systematic studies on the effects of temperature, pressure, and channel size on the transport behaviors. Our study shows that adsorption density varies significantly with temperature, which dramatically influences the transport mechanisms, especially in small channels at low temperatures and under high pressures. This newly developed NEMD method can be readily extended to study gas transport through channels with more complex surface morphology.

Selective gas diffusion in graphene oxides membranes: a molecular dynamics simulations study

Designing membrane materials from one-atom-thick structures is highly promising in separation and filtration applications for the reason that they offer the ultimate precision in modifying the atomic structures and chemistry for optimizing performance, and thus resolving the permeation-selectivity trade-off. In this work, we explore the molecular dynamics of gas diffusion in the gallery space between functionalized graphene layers as well as within nanopores across the multilayers. We have identified highly selective gas permeation that agrees with recent experimental measurements and is promising for advancing gas separation technologies such as hydrogen separation, helium/nitrogen generation, and CO2 sequestration. The roles of structural and chemical factors are discussed by considering different types of gases including H2, He, CH4, N2, O2, CO, CO2, and H2O. The overall performance of graphene oxide membranes is also discussed with respect to their microstructures, and compared with recent experimental measurements. These understandings could advise high-performance gas-separation membrane development by engineering assemblies of two-dimensional layered structures.

Tailoring the water structure and transport in nanotubes with tunable interiors

Self-assembly of cyclic peptide nanotubes (CPNs) in polymer thin films has opened up the possibility of creating separation membranes with tunable nanopores that can differentiate molecules at the sub-nanometer level. While it has been demonstrated that the interior chemistry of the CPNs can be tailored by inserting functional groups in the nanopore lumen (mCPNs), a design strategy for picking the chemical modifications that lead to particular transport properties has not been established. Drawing from the knowledgebase of functional groups in natural amino acids, here we use molecular dynamics simulations to elucidate how bioinspired mutations influence the transport of water through mCPNs. We show that, at the nanoscale, factors besides the pore size, such as electrostatic interactions and steric effects, can dramatically change the transport properties. We recognize a novel asymmetric structure of water under nanoconfinement inside the chemically functionalized nanotubes and identify that the small non-polar glycine-mimic groups that minimize the steric constraints and confer a hydrophobic character to the nanotube interior are the fastest transporters of water. Our computationally developed experiments on a realistic material system circumvent synthetic challenges, and lay the foundation for bioinspired principles to tailor artificial nanochannels for separation applications such as desalination, ion-exchange and carbon capture.

From hydrophobic to hydrophilic polyvinylidenefluoride (PVDF) membranes by gaining new insight into material's properties

From hydrophobic to hydrophilic PVDF membranes by a combination of functionalization by blending chemical additives and selection of manufacturing procedure.

Temperature-dependent gas transport performance of vertically aligned carbon nanotube/parylene composite membranes

A novel composite membrane consisting of vertically aligned carbon nanotubes (CNTs) and parylene was successfully fabricated. Seamless filling of the spaces in CNT forests with parylene was achieved by a low-pressure chemical vapor deposition (CVD) technique and followed with the Ar/O2 plasma etching to expose CNT tips. Transport properties of various gases through the CNT/parylene membranes were explored. And gas permeances were independent on feed pressure in accordance with the Knudsen model, but the permeance values were over 60 times higher than that predicted by the Knudsen diffusion kinetics, which was attributed to specular momentum reflection inside smooth CNT pores. Gas permeances and enhancement factors over the Knudsen model firstly increased and then decreased with rising temperature, which confirmed the existence of non-Knudsen transport. And surface adsorption diffusion could affect the gas permeance at relatively low temperature. The gas permeance of the CNT/parylene composite membrane could be improved by optimizing operating temperature.

The mechanism of selective molecular capture in carbon nanotube networks

Recently, air pollution issues have drawn significant attention to the development of efficient air filters, and one of the most promising materials for this purpose is nanofibers. We explore here the mechanism of selective molecular capture of volatile organic compounds in carbon nanotube networks by performing atomistic simulations. The results are discussed with respect to the two key parameters that define the performance of nanofiltration, i.e. the capture efficiency and flow resistance, which demonstrate the advantages of carbon nanotube networks with high surface-to-volume ratio and atomistically smooth surfaces. We also reveal the important roles of interfacial adhesion and diffusion that govern selective gas transport through the network.

Experimental evidence of rapid water transport through carbon nanotubes embedded in polymeric desalination membranes

As water molecules permeate ultrafast through carbon nanotubes (CNTs), many studies have prepared CNTs-based membranes for water purification as well as desalination, particularly focusing on high flux membranes. Among them, vertically aligned CNTs membranes with ultrahigh water flux have been successfully demonstrated for fundamental studies, but they lack scalability for bulk production and sufficiently high salt rejection. CNTs embedded in polymeric desalination membranes, i.e., polyamide thin-film composite (TFC) membranes, can improve water flux without any loss of salt rejection. This improved flux is achieved by enhancing the dispersion properties of CNTs in diamine aqueous solution and also by using cap-opened CNTs. Hydrophilic CNTs were prepared by wrapping CNT walls via bio-inspired surface modification using dopamine solution. Cap-opening of pristine CNTs is performed by using a thermo-oxidative process. As a result, hydrophilic, cap-opened CNTs-embedded polyamide TFC membranes are successfully prepared, which show much higher water flux than pristine polyamide TFC membrane. On the other hand, less-disperse, less cap-opened CNTs-embedded TFC membranes do not show any flux improvement and rather lead to lower salt rejection properties.

Efficacy of CNT-bound polyelectrolyte membrane by spray-assisted layer-by-layer (LbL) technique on water purification

This study demonstrates anti-fouling properties of surface-modified polyethersulfone composite ultrafiltration membranes prepared by a spray-assisted layer-by-layer technique.

Development of stretchable membrane based nanofilters using patterned arrays of vertically grown carbon nanotubes

A unique process which utilizes membrane based vertically grown carbon nanotubes (CNTs) as nanofilters for a mass transport study is presented here. By using ions, ss-DNA and haemagglutinin as testing molecules of different dimensions, the mass transport function of the CNT membrane is investigated under pressure difference and/or electric field.

How fast does water flow in carbon nanotubes?

The purpose of this paper is threefold. First, we review the existing literature on flow rates of water in carbon nanotubes. Data for the slip length which characterizes the flow rate are scattered over 5 orders of magnitude for nanotubes of diameter 0.81-10 nm. Second, we precisely compute the slip length using equilibrium molecular dynamics (EMD) simulations, from which the interfacial friction between water and carbon nanotubes can be found, and also via external field driven non-equilibrium molecular dynamics simulations (NEMD). We discuss some of the issues in simulation studies which may be reasons for the large disagreements reported. By using the EMD method friction coefficient to determine the slip length, we overcome the limitations of NEMD simulations. In NEMD simulations, for each tube we apply a range of external fields to check the linear response of the fluid to the field and reliably extrapolate the results for the slip length to values of the field corresponding to experimentally accessible pressure gradients. Finally, we comment on several issues concerning water flow rates in carbon nanotubes which may lead to some future research directions in this area.