Molecular Dynamics Study of Carbon Nanotubes/Polyamide Reverse Osmosis Membranes: Polymerization, Structure, and Hydration

Molecular simulations and an experimental study of the oligopeptide-mediated fouling mechanisms of polyamide reverse-osmosis membranes

Fouling is a major problem in reverse-osmosis plants. Fouling is believed to be caused by interactions between membranes and foulants. Experimental observation of fouling is very difficult, however, and in this study molecular dynamics (MD) simulations are used to analyze fouling on a molecular scale to elucidate the adsorption mechanisms of polyamide (PA) reverse-osmosis (RO) membranes affected by the fouling phenomenon. Because proteins are common foulants, a dimeric dipeptide of amino acids constituting a protein was used as a model. The dissociation of the membrane and that of the foulants that results from changes in pH were investigated on a molecular scale and by experimental water permeation testing. At pH 2.5, the foulants L-leucyl-L-aspartic acid (Lasp) and L-leucyl-L-arginine (Larg) showed a tendency toward constant adsorption to the membrane with no significant difference in interaction energy. At pH 7.0, on the other hand, the membrane surface charge turned negative and the total charge of the Lasp and Larg foulants became negative and positive, respectively. Lasp was not close to the membrane surface and demonstrated repulsive and weak adsorption tendencies. On the other hand, Larg penetrated deeply into the membrane surface and showed a strong adsorption tendency. The fouling mechanism in the adsorption simulation varied depending on the conditions, and simulations confirmed that the fouling was very strong when the charges of a foulant and the PA membrane were opposite. These observed trends are similar to those reported from experimental water permeation testing.

Molecular modeling of thin-film nanocomposite membranes for reverse osmosis water desalination

The scarcity of freshwater resources is a major global challenge causedby population and economic growth. Water desalination using a reverse osmosis (RO) membrane is a promising technology to supply potable water from seawater and brackish water. The advancement of RO desalination highly depends on new membrane materials. Currently, the RO technology mainly relies on polyamide thin-film composite (TFC) membranes, which suffer from several drawbacks (e.g., low water permeability, permeability-selectivity tradeoff, and low fouling resistance) that hamper their real-world applications. Nanoscale fillers with specific characteristics can be used to improve the properties of TFC membranes. Embedding nanofillers into TFC membranes using interfacial polymerization allows the creation of thin-film nanocomposite (TFNC) membranes, and has become an emerging strategy in the fabrication of high-performance membranes for advanced RO water desalination. To achieve optimal design, it is indispensable to search for reliable methods that can provide fast and accurate predictions of the structural and transport properties of the TFNC membranes. However, molecular understanding of permeability-selectivity characteristics of nanofillers remains limited, partially due to the challenges in experimentally exploring microscopic behaviors of water and salt ions in confinement. Molecular modeling and simulations can fill this gap by generating molecular-level insights into the effects of nanofillers' characteristics (e.g., shape, size, surface chemistry, and density) on water permeability and ion selectivity. In this review, we summarize molecular simulations of a diverse range of nanofillers including nanotubes (carbon nanotubes, boron nitride nanotubes, and aquaporin-mimicking nanochannels) and nanosheets (graphene, graphene oxide, boron nitride sheets, molybdenum disulfide, metal and covalent organic frameworks) for water desalination applications. These simulations reveal that water permeability and salt rejection, as the major factors determining the desalination performance of TFNC membranes, significantly depend on the size, topology, density, and chemical modifications of the nanofillers. Identifying their influences and the physicochemical processes behind, via molecular modeling, is expected to yield important insights for the fabrication and optimization of the next generation high-performance TFNC membranes for RO water desalination.

Differentiation of Hebbian and homeostatic plasticity mechanisms within layer 5 visual cortex neurons

Cortical layer 5 contains two major types of projection neuron known as IB (intrinsic bursting) cells that project sub-cortically and RS (regular spiking) cells that project between cortical areas. This study describes the plasticity properties of RS and IB cells in the mouse visual cortex during the critical period for ocular dominance plasticity. We find that RS neurons exhibit synaptic depression in response to both dark exposure (DE) and monocular deprivation (MD), and their homeostatic recovery from depression is dependent on TNF-α. In contrast, IB cells demonstrate opposite responses to DE and MD, potentiating to DE and depressing to MD. IB cells' potentiation depends on CaMKII-autophosphorylation and not TNF-α. IB cells show mature synaptic properties at the start of the critical period while RS cells mature during the critical period. Together with observations in somatosensory cortex, these results suggest that differences in RS and IB plasticity mechanisms are a general cortical property.

Molecular Methods for Assessing the Morphology, Topology, and Performance of Polyamide Membranes

The molecular-scale morphology and topology of polyamide composite membranes determine the performance characteristics of these materials. However, molecular-scale simulations are computationally expensive and morphological and topological characterization of molecular structures are not well developed. Molecular dynamics simulation and analysis methods for the polymerization, hydration, and quantification of polyamide membrane structures were developed and compared to elucidate efficient approaches for producing and analyzing the polyamide structure. Polymerization simulations that omitted the reaction-phase solvent did not change the observed hydration, pore-size distribution, or water permeability, while improving the simulation efficiency. Pre-insertion of water into the aggregate pores (radius ≈ 4 Å) of dry domains enabled shorter hydration simulations and improved simulation scaling, without altering pore structure, properties, or performance. Medial axis and Minkowski functional methods were implemented to identify permeation pathways and quantify the polyamide morphology and topology, respectively. Better agreement between simulations and experimentally observed systems was accomplished by increasing the domain size rather than increasing the number of ensemble realizations of smaller systems. The largest domain hydrated was an order of magnitude larger by volume than the largest domain previously reported. This work identifies methods that can enable more efficient and meaningful fundamental modeling of membrane materials.

Nanocomposite desalination membranes made of aromatic polyamide with cellulose nanofibers: synthesis, performance, and water diffusion study

Reverse osmosis membranes of aromatic polyamide (PA) reinforced with a crystalline cellulose nanofiber (CNF) were synthesized and their desalination performance was studied. Comparison with plain PA membranes shows that the addition of CNF reduced the matrix mobility resulting in a molecularly stiffer membrane because of the attractive forces between the surface of the CNFs and the PA matrix. Fourier transform-infrared spectroscopy and X-ray photoelectron spectroscopy results showed complex formation between the carboxy groups of the CNF surface and the m- phenylenediamine monomer in the CNF-PA composite. Molecular dynamics simulations showed that the CNF-PA had higher hydrophilicity which was key for the higher water permeability of the synthesized nanocomposite membrane. The CNF-PA reverse osmosis nanocomposite membranes also showed enhanced antifouling performance and improved chlorine resistance. Therefore, CNF shows great potential as a nanoreinforcing material towards the preparation of nanocomposite aromatic PA membranes with longer operation lifetime due to its antifouling and chlorine resistance properties.

Desalination behavior analysis of interior-modified carbon nanotubes doped membrane by dielectric spectrum and molecular simulation

Carbon nanotube (CNT)-doped polyamide (PA) membranes have attracted much attention in reverse osmosis (RO) membranes due to their significant advantages of water flux and desalination. In this study, we synthesized multi-walled carbon nanotube (MWNT)/PA RO membrane by 12-oxodidodecanoic acid methyl ester group interior-modified MWNTs (MWNT-C₁₄H₂₅O₄). Then, their mechanism of desalination behavior was successfully analyzed by combining dielectric relaxation spectrum (DRS) and molecular dynamics (MD) simulation. DRS analysis mainly focuses on two aspects: (1) the water volume fraction, average pore size and dielectric parameters of MWNT-C₁₄H₂₅O₄/PA and PA membranes were obtained by model analysis of DRS data. These data of MWNT-C₁₄H₂₅O₄/PA membrane are higher than PA membrane, which indicates that the water flux of the MWNT-C₁₄H₂₅O₄/PA membrane was higher than that of the PA membrane. (2) Further analysis shows that the MWNT-C₁₄H₂₅O₄/PA membranes have high average charge density, ion solvation barrier and reflection coefficient, which indicates that the added interior-modified MWNT can improve the salt rejection of PA membranes. In the microscopic aspect, the desalination behavior of the MWNT-C₁₄H₂₅O₄/PA and PA membrane was analyzed from the aspects of free volume distribution, the dynamic diffusion process of water and ions. The results show that the microscopic data of dynamic simulation well support the conclusion of the DRS method. This study provides a convenient methodology to characterize the properties of the membrane from the aspect of membrane structure.

Recent Advances in Applications of Carbon Nanotubes for Desalination: A Review

As a sustainable, cost-effective and energy-efficient method, membranes are becoming a progressively vital technique to solve the problem of the scarcity of freshwater resources. With these critical advantages, carbon nanotubes (CNTs) have great potential for membrane desalination given their high aspect ratio, large surface area, high mechanical strength and chemical robustness. In recent years, the CNT membrane field has progressed enormously with applications in water desalination. The latest theoretical and experimental developments on the desalination of CNT membranes, including vertically aligned CNT (VACNT) membranes, composited CNT membranes, and their applications are timely and comprehensively reviewed in this manuscript. The mechanisms and effects of CNT membranes used in water desalination where they offer the advantages are also examined. Finally, a summary and outlook are further put forward on the scientific opportunities and major technological challenges in this field.

In silico study of structure and water dynamics in CNT/polyamide nanocomposite reverse osmosis membranes

A comprehensive understanding of the role of CNTs in the reverse osmosis process is disclosed through in silico study.

The role of interaction between low molecular weight neutral organic compounds and a polyamide RO membrane in the rejection mechanism

Reverse osmosis (RO) is a membrane technology that separates dissolved species from water.

New Insights in the Natural Organic Matter Fouling Mechanism of Polyamide and Nanocomposite Multiwalled Carbon Nanotubes-Polyamide Membranes

Polyamide (PA) membranes comprise most of the reverse osmosis membranes currently used for desalination and water purification. However, their fouling mechanisms with natural organic matter (NOM) is still not completely understood. In this work, we studied three different types of PA membranes: a laboratory made PA, a commercial PA, and a multiwalled carbon nanotube (CNT-PA nanocomposite membrane during cross-flow measurements by NaCl solutions including NOM, humic acid (HA), or alginate, respectively). Molecular dynamic simulations were also used to understand the fouling process of NOM down to its molecular scale. Low molecular weight humic acid binds to the surface cavities on the PA structures that leads to irreversible adsorption induced by the high surface roughness. In addition, the larger alginate molecules show a different mechanism, due to their larger size and their ability to change shape from the globule type to the uncoiled state. Specifically, alginate molecules either bind through Ca²⁺ bridges or they uncoil and spread on the surface. This work shows that carbon nanotubes can help to decrease roughness and polymer mobility on the surfaces of the membranes at the molecular scale, which represents a novel method to design antifouling membranes.

Molecular Insights into the Composition-Structure-Property Relationships of Polyamide Thin Films for Reverse Osmosis Desalination

A molecular-level understanding of the structure-property relationship of polyamide (PA) active layers in thin-film-composite membranes remains unclear. We developed an approach to build and hydrate the PA layer in molecular dynamics simulations and reproduced realistic membrane properties, which enabled us to examine the composition-structure-permeability relationships at the molecular level. We discovered the variation of pore size distributions in the dry PA structures at different monomer compositions, leading to different water cluster distributions and wetting properties of hydrated PA films. Membrane swelling was linearly dependent on the degree of cross-linking (DC), and higher water flux was obtained in the more swelling-prone PA films because of the transition in water transport mechanisms. Continuum-like and jumping transport both occurred in PA films with smaller DC, where visible and more persistent channels existed. In the denser films, water molecules relied only on the on-and-off channels to jump from one cavity to another; however, jumping transport was more pronounced even in the less dense PA films, and all the PA structures exhibited oscillations, which provided evidence for the solution-diffusion model rather than the pore-flow model. The results not only contribute to fundamental understanding but also provide insights into the molecule-level design for next-generation membranes.

Data for molecular recognition between polyamide thin film composite on the polymeric subtract by molecular dynamic

This paper focus to examine the best molecular interaction between Polyamide Thin Film Composite (PA TFC) layers with different properties of the support membrane. The support membrane of Nylon 66 (N66) and Polyvinylidene fluoride (PVDF) was chosen to represent the hydrophilic and hydrophobic model respectively in the Molecular Dynamic (MD) simulation. The Condensed-Phase Optimized Molecular Potential for Atomistic Simulation Studies (COMPASS) force field was used with the total simulation runs were set 1000 picoseconds run production ensembles. The temperature and pressure set for both ensembles were 298 K and 1 atm respectively. The validity of our model densities data was check and calculated where the deviation must be less than 6%. The comparison between hydrophobic and hydrophilic of the support membrane data was examined by the distance and magnitude of intensity of the Radial Distribution Function (RDF's) trends.

Nanoscopic insights of saline water in carbon nanotube appended filters using molecular dynamics simulations

Nanotube appended membranes are shown to be very promising due to their ultrafast water transport and very high salt rejection ability. Using classical molecular dynamics, the present study reports the nanoscopic assessment of various molecular events for nanotube-based desalination, which might be useful for nanoscale devices during process operation at the macroscopic scale. The characteristics of water and ion flow are discussed with varied strength of pressure gradient and salt concentration for different scales of confinement. The results revealed that the membranes comprising nanotubes of 1.0-1.1 nm diameter can be optimized for efficient water desalination with more than >95% salt rejection. Furthermore, the anomalies in water flux through nanotubes are linked with the hydration characteristics of ions inside CNTs. The results show the maximum hydration of confined ions inside the nanotubes, which indicated the minimum permeability of water due to freezing effects. Furthermore, the MD results revealed that akin to bulk phases, the mass transport through nanotubes can be linked with the component diffusivity in the medium. It has been demonstrated that not only the diffusivities of water and ions, but even the gradient of water to ion diffusivity might be utilized to predict and explore the experimental observations, which might be helpful in optimizing the operational regime in nanotube-based filtrations. Moreover, the thermodynamic characteristics of the flow are discussed in terms of the entropy of water and ions using the robust two-phase thermodynamic (2PT) method. The results reflect that the entropy of water is linked to the distortion of the hydrogen bond network inside the nanotube confinement, at the nanotube-water interface and at the bulk solution, whereas the entropy of ions seems to be majorly dominated by their oscillation. Also, the interconnection of hydration structure, mass flux and the diffusivity of water and ions along with their thermodynamic origin are discussed.

Transport mechanism of water molecules passing through polyamide/COF mixed matrix membranes

Mixed matrix membranes (MMMs) have gained significant attention due to their high water permeability without the cost of salt rejection. The mechanism of permeability promotion for PA/COFs MMMs is investigated in this work from molecular insights.

Molecular Dynamics Study on the Reverse Osmosis Using Multilayer Porous Graphene Membranes

In this study, the reverse osmosis (RO) of a salt solution was investigated using a molecular dynamics method to explore the performance of a multilayer porous graphene membrane. The effects of the salt solution concentration, pressure, layer separation and pore offset on the RO performance of the membrane were investigated and the influences of the number of layers and the gradient structure were determined. The results show that as the salt solution concentration increases, the energy barrier of the water molecules passing through the bilayer porous graphene membranes changes slightly, indicating that the effect of the water flux on the membrane can be ignored. The salt rejection performance of the membrane improves with an increase in the concentration of the salt solution. When the pressure is increased, the energy barrier decreases, the water flux increases and the salt rejection decreases. When the layer separation of the bilayer porous graphene membrane is the same as the equilibrium spacing of the graphene membrane, the energy barrier is the lowest and the membrane water flux is the largest. The energy barrier of the bilayer porous graphene membrane increases with increasing layer separation, resulting in a decrease in the water flux of the membrane. The salt rejection increases with increasing layer separation. The water flux of the membrane decreases as the energy barrier increases with increasing pore offset and the salt rejection increases. The energy barrier effect is more pronounced for a larger number of graphene layers and the water flux of the membrane decreases because it is more difficult for the water molecules to pass through the porous graphene membrane. However, the salt rejection performance improves with the increase in the number of layers. The gradient pore structure enhances the energy barrier effect of the water molecules permeating through the membrane and the water flux of the membrane decreases. The salt rejection performance is improved by the gradient pore structure. The research results provide theoretical guidance for research on the RO performance of porous graphene membranes and the design of porous graphene membranes.

Effective Antiscaling Performance of Reverse-Osmosis Membranes Made of Carbon Nanotubes and Polyamide Nanocomposites

The antiscaling properties of multiwalled carbon nanotube (MWCNT)-polyamide (PA) nanocomposite reverse-osmosis (RO) desalination membranes (MWCNT-PA membranes) were studied. An aqueous solution of calcium chloride (CaCl₂) and sodium bicarbonate (NaHCO₃) was used to precipitate in situ calcium carbonate (CaCO₃) to emulate scaling. The MWCNT contents of the studied nanocomposite membranes prepared by interfacial polymerization ranged from 0 wt % (plain PA) to 25 wt %. The inorganic antiscaling performances were compared for the MWCNT-PA membranes to laboratory-made plain and commercial PA-based RO membranes. The scaling process on the membrane surface was monitored by fluorescence microscopy after labeling the scale with a fluorescent dye. The deposited scale on the MWCNT-PA membrane was less abundant and more easily detached by the shear stress under cross-flow compared to other membranes. Molecular dynamics simulations revealed that the attraction of Ca²⁺ ions was hindered by the interfacial water layer formed on the surface of the MWCNT-PA membrane. Together, our findings revealed that the observed outstanding antiscaling performance of MWCNT-PA membranes results from (i) a smooth surface morphology, (ii) a low surface charge, and (iii) the formation of an interfacial water layer. The MWCNT-PA membranes described herein are advantageous for water treatment.

Enhanced Chemical Separation by Freestanding CNT-Polyamide/Imide Nanofilm Synthesized at the Vapor-Liquid Interface

In chemical separation, thin membranes exhibit high selectivity, but often require a support at the expense of permeance. Here, we report a pinhole-free polymeric layer synthesized within freestanding carbon nanotube buckypaper through vapor-liquid interfacial polymerization (VLIP). The VLIP process results in thin, smooth and uniform polyamide and imide films. The scaffold reinforces the nanofilm, defines the membrane thickness, and introduces an additional transport mechanism. Our membranes exhibit superior gas selectivity and osmotic semipermeability. Plasticization resistance and high permeance in hydrocarbon separation together with a considerable improvement in water-salt permselectivity highlight their potential as new membrane architecture for chemical separation.

Robust water desalination membranes against degradation using high loads of carbon nanotubes

Chlorine resistant reverse osmosis (RO) membranes were fabricated using a multi-walled carbon nanotube-polyamide (MWCNT-PA) nanocomposite. The separation performance of these membranes after chlorine exposure (4800 ppm·h) remained unchanged (99.9%) but was drastically reduced to 82% in the absence of MWCNT. It was observed that the surface roughness of the membranes changed significantly by adding MWCNT. Moreover, membranes containing MWCNT fractions above 12.5 wt.% clearly improved degradation resistance against chlorine exposure, with an increase in water flux while maintaining salt rejection performance. Molecular dynamics and quantum chemical calculations were performed in order to understand the high chemical stability of the MWCNT-PA nanocomposite membranes, and revealed that high activation energies are required for the chlorination of PA. The results presented here confirm the unique potential of carbon nanomaterials embedded in polymeric composite membranes for efficient RO water desalination technologies.

Computer simulation of water desalination through boron nitride nanotubes

Development of high-efficiency and low-cost seawater desalination technologies is critical to solving the global water crisis. Herein we report a fast water filtering method with high salt rejection by boron nitride nanotubes (BNNTs). The effect of the radius of BNNTs on water filtering and salt rejection was investigated by molecular dynamics (MD) simulation. Our simulation results demonstrate that fast water permeation and high salt rejection could be achieved by BNNT(7,7) under both high pressure and low pressure. The potential of mean force (PMF) of Na⁺ ion and water molecule through BNNT(7,7) further revealed the mechanism of seawater desalination by BNNT(7,7). Using BNNT(7,7) array, a 10 cm² nanotube membrane with 1.5 × 10¹³ pores per cm² will produce freshwater with a flow rate of 98 L per day per MPa under 100 MPa. Our study shows the potential application of BNNTs membrane for fast and efficient desalination.

Antiorganic Fouling and Low-Protein Adhesion on Reverse-Osmosis Membranes Made of Carbon Nanotubes and Polyamide Nanocomposite

We demonstrate efficient antifouling and low protein adhesion of multiwalled carbon nanotubes-polyamide nanocomposite (MWCNT-PA) reverse-osmosis (RO) membranes by combining experimental and theoretical studies using molecular dynamics (MD) simulations. Fluorescein isothiocyanate (FITC)-labeled bovine serum albumin (FITC-BSA) was used for the fouling studies. The fouling was observed in real time by using a crossflow system coupled to a fluorescence microscope. Notably, it was observed that BSA anchoring on the smooth MWCNT-PA membrane was considerably weaker than that of other commercial/laboratory-made plain PA membranes. The permeate flux reduction of the MWCNT-PA nanocomposite membranes by the addition of FITC-BSA was 15% of its original value, whereas those of laboratory-made plain PA and commercial membranes were much larger at 34%-50%. Computational MD simulations indicated that the presence of MWCNT in PA results in weaker interactions between the membrane surface and BSA molecule due to the formation of (i) a stiffer PA structure resulting in lower conformity of the molecular structure against BSA, (ii) a smoother surface morphology, and (iii) an increased hydrophilicity involving the formation of an interfacial water layer. These results are important for the design and development of promising antiorganic fouling RO membranes for water treatment.

Nanofiltration Membrane with a Mussel-Inspired Interlayer for Improved Permeation Performance

A mussel-inspired interlayer of polydopamine (PDA)/polyethylenimine (PEI) is codeposited on the ultrafiltration substrate to tune the interfacial polymerization of piperazine and trimesoyl chloride for the preparation of thin-film composite (TFC) nanofiltration membranes (NFMs). This hydrophilic interlayer results in an efficient adsorption of piperazine solution in the substrate pores. The solution height increases with the PDA/PEI codeposition time from 45 to 135 min due to the capillary effect of the substrate pores. The prepared TFC NFMs are characterized with thin and smooth polyamide selective layers by ATR/IR, XPS, FESEM, AFM, zeta potential, and water contact angle measurements. Their water permeation flux measured in a cross-flow process increases to two times as compared with those TFC NFMs without the mussel-inspired interlayer. These TFC NFMs also show a high rejection of 97% to Na₂SO₄ and an salt rejection order of Na₂SO₄ ≈ MgSO₄ > MgCl₂ > NaCl.