Computer simulations of water flux and salt permeability of the reverse osmosis FT-30 aromatic polyamide membrane

Experimental and Theoretical Insights into Interfacial Properties of 2D Materials for Selective Water Transport Membranes: A Critical Review

Interfacial properties, such as wettability and friction, play critical roles in nanofluidics and desalination. Understanding the interfacial properties of two-dimensional (2D) materials is crucial in these applications due to the close interaction between liquids and the solid surface. The most important interfacial properties of a solid surface include the water contact angle, which quantifies the extent of interactions between the surface and water, and the water slip length, which determines how much faster water can flow on the surface beyond the predictions of continuum fluid mechanics. This Review seeks to elucidate the mechanism that governs the interfacial properties of diverse 2D materials, including transition metal dichalcogenides (e.g., MoS2), graphene, and hexagonal boron nitride (hBN). Our work consolidates existing experimental and computational insights into 2D material synthesis and modeling and explores their interfacial properties for desalination. We investigated the capabilities of density functional theory and molecular dynamics simulations in analyzing the interfacial properties of 2D materials. Specifically, we highlight how MD simulations have revolutionized our understanding of these properties, paving the way for their effective application in desalination. This Review of the synthesis and interfacial properties of 2D materials unlocks opportunities for further advancement and optimization in desalination.

Quantitatively Unveiling the Structure-Activity Relationship of Polyamide Nanofiltration Membranes with Complex Structures

Nanofiltration polyamide (NF PA) membranes are widely used in seawater desalination and wastewater treatment due to their excellent permeability. The structure-activity relationship of PA membranes has attracted extensive attention in decades. In this work, NF PA membranes with planar structure, nodular structure, and peak-valley structure were constructed, and the pure water permeance was calculated by nonequilibrium molecular dynamics simulation to quantitatively investigate the structure-activity relationship between the microstructure and water permeance. Results showed that the peak-valley structure had the highest effective utilization rate of the membrane surface, which had the highest number of water molecules that passed through membranes per unit cross-sectional area (7.09). Furthermore, with the increase of the specific surface area ratio, the water permeance of the NF PA with peak-valley increased at a rate about 2.5 times than that of the planar NF PA. Therefore, some molecular scale insights were supplied about the structure-activity relationship of NF PA membranes, which is helpful for the fabrication of high-performance NF PA membranes.

When self-assembly meets interfacial polymerization

Interfacial polymerization (IP) and self-assembly are two thermodynamically different processes involving an interface in their systems. When the two systems are incorporated, the interface will exhibit extraordinary characteristics and generate structural and morphological transformation. In this work, an ultrapermeable polyamide (PA) reverse osmosis (RO) membrane with crumpled surface morphology and enlarged free volume was fabricated via IP reaction with the introduction of self-assembled surfactant micellar system. The mechanisms of the formation of crumpled nanostructures were elucidated via multiscale simulations. The electrostatic interactions among m-phenylenediamine (MPD) molecules, surfactant monolayer and micelles, lead to disruption of the monolayer at the interface, which in turn shapes the initial pattern formation of the PA layer. The interfacial instability brought about by these molecular interactions promotes the formation of crumpled PA layer with larger effective surface area, facilitating the enhanced water transport. This work provides valuable insights into the mechanisms of the IP process and is fundamental for exploring high-performance desalination membranes.

Molecular Simulations to Elucidate Transport Phenomena in Polymeric Membranes

Despite decades of dominance in separation technology, progress in the design and development of high-performance polymer-based membranes has been incremental. Recent advances in materials science and chemical synthesis provide opportunities for molecular-level design of next-generation membrane materials. Such designs necessitate a fundamental understanding of transport and separation mechanisms at the molecular scale. Molecular simulations are important tools that could lead to the development of fundamental structure-property-performance relationships for advancing membrane design. In this Perspective, we assess the application and capability of molecular simulations to understand the mechanisms of ion and water transport across polymeric membranes. Additionally, we discuss the reliability of molecular models in mimicking the structure and chemistry of nanochannels and transport pathways in polymeric membranes. We conclude by providing research directions for resolving key knowledge gaps related to transport phenomena in polymeric membranes and for the construction of structure-property-performance relationships for the design of next-generation membranes.

Effect of an ionic environment on membrane fouling: a molecular dynamics study

The effect of the ionic environment on membrane fouling was investigated for polyamide (PA) and graphene oxide (GO) membranes using equilibrium molecular dynamics (MD) simulations. For each of these membranes, bovine serum albumin (BSA) was considered as the model foulant. The effect of the foulant on the membranes is investigated at seawater concentration and also in a normal aqueous environment. We investigated the translational and rotational motion of the protein relative to the membrane, interaction energy between the protein and the membrane surface, structural changes in the protein, and ion distribution around the protein and the membrane surface for all the systems. We found that the effects of ions were very different on both the membranes. Specifically, with an increase in ionic strength, the repulsion between the protein and membrane was observed in the case of GO, while for PA, no significant changes were observed for the same. Also, the ion distribution around the protein and the membrane surface were found to be different. In particular, for GO, there were more number of chloride ions around the protein and the membrane than that of sodium ions, which was probably the reason for the repulsion in the case of GO. However, in the case of PA, the membrane surface did not exhibit any affinity towards a specific ion, and the protein in the case of PA was surrounded by more number of sodium ions than chloride ions.

Unveiling the Molecular Mechanisms of Thickness-Dependent Water Dynamics in an Ultrathin Free-Standing Polyamide Membrane

Aromatic polyamide (PA) membranes fabricated from interfacial polymerization are widely used for desalination and water treatment. The fabrication of the high-flux PA membrane requires a fundamental understanding of the molecular mechanisms of water dynamics in the PA, which is still obscure due to the limited experimental methods. Herein, molecular dynamics (MD) simulations were employed to establish an atomic model of ultrathin free-standing PA membranes with various thickness and to explore the thickness-dependent dynamics of water molecules in the PA membrane. Simulation results illustrate that the simulated PA membrane has an average pore radius of 3 Å similar to the free volume size of the experimental PA membrane measured by PALS. The PA could be identified as the swelling layer (SL) and the confined layer (CL) based on their water diffusion rates. The diffusivity of water in the confined layer of PA membrane was much lower than that in the swelling layer and thus determined the water flux of the PA membrane. The water diffusivity in the sub-8 nm PA membrane is greatly enhanced due to a very thin confined layer thickness, illustrating the mechanism of the experimentally fabricated sub-8 nm PA membrane having the dramatically enhanced water permeability. Furthermore, results show that water molecules tend to transport rapidly in the free space inside the PA membrane. Our results provide some insights into the thickness-dependent water dynamics in the PA on a molecular level and may help to design the next generation of high-flux PA membranes.

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.

Group Contribution Method to Predict the Mass Transfer Coefficients of Organics through Various RO Membranes

Reverse osmosis (RO) is a membrane technology that separates dissolved species from water. RO has been applied for the removal of chemical contaminants from water for potable reuse applications. The presence of a wide variety of influent chemical contaminants and the insufficient rejection of low-molecular-weight neutral organics by RO calls for the need to develop a model that predicts the rejection of various organics. In this study, we develop a group contribution method (GCM) to predict the mass transfer coefficients by fragmenting the structure of low-molecular-weight neutral organics into small parts that interact with the RO membrane. Overall, 54 organics including 26 halogenated and oxygenated alkanes, 8 alkenes, and 20 alkyl and halobenzenes were used to determine 39 parameters to calibrate for 6 different RO membranes, including 4 brackish water and 2 seawater membranes. Through six membranes, approximately 80% of calculated rejection was within an error goal (i.e., ±5%) from the experimental observation. To extend the GCM for a reference RO membrane, ESPA2-LD, 14 additional organics were included from the literature to calibrate nitrogen-containing functional groups of nitrosamine, nitriles, and amide compounds. Overall, 49 organics (72% of 68 compounds) from calibration and 7 compounds (87.5% of 8 compounds) from prediction were within the error goal.

Influence of the presence of cations on the water and salt dynamics inside layered graphene oxide (GO) membranes

Although over the past few years, graphene oxide (GO) has emerged as a promising membrane material, the applicability of layered GO membranes in water purification/seawater desalination is still a challenging issue because of the undesirable swelling of GO laminates in the aqueous environment. One of the ways to tune the interlayer spacing and to arrest the undesirable swelling of layered GO membranes in the aqueous environment is to intercalate the interlayer spacing of the GO laminates with cations. Although the cation intercalation imparts stabilization to GO laminates in the aqueous environment, their effect on the performance of the membrane is yet to be addressed in detail. In the present study we have investigated the effect of cation intercalation on the performance of layered GO membranes using molecular dynamics simulation. For the same interlayer spacing, the cation intercalated layered GO membranes have a higher water flux as compared to the corresponding pristine layered GO membranes. In the presence of the cations, the water molecules inside the interlayer gallery get more compactly packed. The presence of the cations also increases the stability of the hydrogen bond network among the water molecules inside the membrane. This can be attributed to slow water reorientation dynamics inside the interlayer gallery in the presence of the cations. The synergistic effect of all these changes is that the water permeability through the cation intercalated layered GO membranes is higher as compared to that through the corresponding pristine layered GO membranes. On the other hand, the intercalation of the cations (K+, Mg2+) leads to higher rejection of Na+ ions whereas the rejection of Cl- ions slightly decreases.

Ion partitioning and permeation in charged low-T* membranes

Understanding ion transport in membrane materials is key to engineering and development of desalination and water purification technologies as well as electro-membrane applications. To date, modeling of ion transport has mainly relied on mean-field approaches, originally intended for weak inter-ionic interactions, i.e., high reduced temperature T*. This condition is violated in many membranes, which could explain disagreement between predicted trends and experiments. The paper highlights observed discrepancies and develops a new approach based on the concept of ion association, more adequate in the low-T^⁎ limit. The new model addresses ion binding and mobility consistently within the same physical picture, applied to different types of single and mixed salts. The resulting relations show a significantly weaker connection between ion partitioning and permeability than the standard ones. Estimates using primitive model (PM) of ions in a homogeneous dielectric suggest that non-PM mechanisms, originating from the molecular structure of the ion-solvating environment, might enhance ion association in membranes. PM analysis also predicts that ion solvation and association must be rigidly related, yet non-PM effects may decouple these phenomena and allow a crossover to non-trivial regimes consistent with experiments and simulations. Despite the crude nature of the presented approach and some questions remaining open, it appears to explain most available experimental data and presents a step towards predictive modeling of ion-selective membrane separations in water-, environment- and energy-related applications.

Enantioseparation processes and mechanisms in functionalized graphene membranes: Facilitated or retarded transport?

Up to date, functionalized graphene-based membranes have exhibited a promising potential in the enantioseparation. However, since precisely controlling the interlayer distance of two-dimensional materials is a great challenge in practical experiments, the transport mechanism of chiral guests in such membranes, together with various critical parameters that play a controlling role in the transport behaviors of the preferentially binding enantiomer in narrow channels, remains to be explored. The molecular dynamics (MD) simulation, especially using the steered MD (SMD) method, might be an alternative way to investigate the enantioseparation processes and mechanisms of layered membranes with different interlayer distances. In this work, D-alanine modified graphene sheets with different interlayer distances were built as membrane models, whereas D- and L-phenylalanine were selected as chiral probes. The effect of the interlayer distance and the applied external force on the enantioseparation performance was examined. Results show that such two parameters exert a significant influence on the enantioseparation performance: (a) Increasing the interlayer distance would result in a conversion from the retarded to the facilitated mechanism at a proper external force (medium); (b) both the large and small driving forces would only lead to the appearance of the retarded transport for the preferential enantiomer, unlike the moderate force; (c) the interaction energy of L-phenylalanine with D-isomer selector decreases with the rising interlayer distances studied in this work, regardless of what the external force is. Our findings can provide guidance on the practical applications in the membrane-based chiral separation.

Anomalous Dynamics of Water in Polyamide Matrix

Water in polymer matrixes is likely to show anomalous dynamics, a problem that has not been well understood yet. Here, we performed atomistic molecular dynamics simulations to study the water dynamics in a polyamide (PA) matrix, the bulk phase of well-known reverse osmosis membranes. For time-dependent ensemble average, water molecules experienced ballistic diffusion at a shorter time scale, followed by a crossover from subdiffusion to Brownian diffusion at a time scale ∼10 ns, and non-Gaussian diffusion, an indication of anomalous dynamics, sticks on even in the Brownian diffusion region. The anomalous dynamics mainly originates from two distinct motions including small-step continuous diffusion and jumping diffusion. The jumping motion has a mean length of 3.08 ± 0.31 Å and characteristic relaxation time of 0.218 ± 0.040 ns, which dominates the water diffusion in a fully hydrated PA matrix. It comprised low- and high-frequency jumps; the former is almost unchanged, and the latter remarkably increases with the increase of the hydration level. Surrounding neighbors of water strongly affect the jumping frequency, which exponentially or linearly decays with the increase in the number of atoms from the PA matrix. Although the PA matrix is flexible, associated with the water dynamics, the translocation of water is mainly through either tracing the position of neighboring water or jumping into the adjacent accommodation space.

Molecular dynamics simulation studies of the structure and antifouling performance of a gradient polyamide membrane

The structure and the antifouling performance of the first gradient polyamide layer model are systematically disclosed using molecular dynamics simulations.

Molecular Dynamics Simulation Study of Polyamide Membrane Structures and RO/FO Water Permeation Properties

Polyamide (PA) membranes possess properties that allow for selective water permeation and salt rejection, and these are widely used for reverse osmotic (RO) desalination of sea water to produce drinking water. In order to design high-performance RO membranes with high levels of water permeability and salt rejection, an understanding of microscopic PA membrane structures is indispensable, and this includes water transport and ion rejection mechanisms on a molecular scale. In this study, two types of virtual PA membranes with different structures and densities were constructed on a computer, and water molecular transport properties through PA membranes were examined on a molecular level via direct reverse/forward osmosis (RO/FO) filtration molecular dynamics (MD) simulations. A quasi-non-equilibrium MD simulation technique that uses applied (RO mode) or osmotic (FO mode) pressure differences of several MPa was conducted to estimate water permeability through PA membranes. A simple NVT (Number, Volume, and Temperature constant ensemble)-RO MD simulation method was presented and verified. The simulations of RO and FO water permeability for a dense PA membrane model without a support layer agreed with the experimental value in the RO mode. This PA membrane completely rejected Na⁺ and Cl⁻ ions during a simulation time of several nano-seconds. The naturally dense PA structure showed excellent ion rejection. The effect that the void size of PA structure exerted on water permeability was also examined.

Selectivity and polarization in water channel membranes: lessons learned from polymeric membranes and CNTs

The aspects of ion exclusion and concentration polarization are highlighted as critical for achieving high selectivity in an artificial water channel.

Understanding the temperature effect on transport dynamics and structures in polyamide reverse osmosis system via molecular dynamics simulations

Molecular simulations could disclose the transport dynamics, membrane structures and temperature effect on reverse osmosis process.

Development of Nanostructured Water Treatment Membranes Based on Thermotropic Liquid Crystals: Molecular Design of Sub-Nanoporous Materials

Supply of safe fresh water is currently one of the most important global issues. Membranes technologies are essential to treat water efficiently with low costs and energy consumption. Here, the development of self-organized nanostructured water treatment membranes based on ionic liquid crystals composed of ammonium, imidazolium, and pyridinium moieties is reported. Membranes with preserved 1D or 3D self-organized sub-nanopores are obtained by photopolymerization of ionic columnar or bicontinuous cubic liquid crystals. These membranes show salt rejection ability, ion selectivity, and excellent water permeability. The relationships between the structures and the transport properties of water molecules and ionic solutes in the sub-nanopores in the membranes are examined by molecular dynamics simulations. The results suggest that the volume of vacant space in the nanochannel greatly affects the water and ion permeability.

Model-based performance and energy analyses of reverse osmosis to reuse wastewater in a PVC production site

A pilot-scale reverse osmosis (RO) followed behind a membrane bioreactor (MBR) was developed for the desalination to reuse wastewater in a PVC production site. The solution-diffusion-film model (SDFM) based on the solution-diffusion model (SDM) and the film theory was proposed to describe rejections of electrolyte mixtures in the MBR effluent which consists of dominant ions (Na⁺ and Cl^-) and several trace ions (Ca²⁺, Mg²⁺, K⁺ and SO₄^2-). The universal global optimisation method was used to estimate the ion permeability coefficients (B) and mass transfer coefficients (K) in SDFM. Then, the membrane performance was evaluated based on the estimated parameters which demonstrated that the theoretical simulations were in line with the experimental results for the dominant ions. Moreover, an energy analysis model with the consideration of limitation imposed by the thermodynamic restriction was proposed to analyse the specific energy consumption of the pilot-scale RO system in various scenarios.

Dynamics of Water Monolayers Confined by Chemically Heterogeneous Surfaces: Observation of Surface-Induced Anisotropic Diffusion

Water present in confining geometries plays key roles in many systems of scientific and technological relevance. Prominent examples are living cells and nanofluidic devices. Despite its importance, a complete understanding of the dynamics of water in nanoscale confinement remains elusive. In this work, we use molecular dynamics (MD) simulation to investigate the diffusive dynamics of water monolayers confined in chemically heterogeneous silica slit pores. The effect of chemical heterogeneity is systematically investigated through the fraction f_SiOH of randomly distributed surface sites that possess hydroxyl functional groups. Partial hydroxylation results in heterogeneous surfaces comprising nanoscale hydrophobic and hydrophilic regions. We find that the in-plane diffusivity of water increases monotonically with f_SiOH; at low surface hydroxylation (f_SiOH ≤ 50%), slow water dynamics arise due to the formation of icelike structures in the hydrophobic regions, while at f_SiOH ≥ 75%, surface-water H-bonds in the hydrophilic regions result in faster dynamics. We show that surface patterning with ordered hydrophobic and hydrophilic "stripes" can be used to induce one-dimensional diffusion, with water diffusing through the slit pore preferentially along the direction of the hydrophilic surface patterns.

Fundamental transport mechanisms, fabrication and potential applications of nanoporous atomically thin membranes

Graphene and other two-dimensional materials offer a new approach to controlling mass transport at the nanoscale. These materials can sustain nanoscale pores in their rigid lattices and due to their minimum possible material thickness, high mechanical strength and chemical robustness, they could be used to address persistent challenges in membrane separations. Here we discuss theoretical and experimental developments in the emerging field of nanoporous atomically thin membranes, focusing on the fundamental mechanisms of gas- and liquid-phase transport, membrane fabrication techniques and advances towards practical application. We highlight potential functional characteristics of the membranes and discuss applications where they are expected to offer advantages. Finally, we outline the major scientific questions and technological challenges that need to be addressed to bridge the gap from theoretical simulations and proof-of-concept experiments to real-world applications.

Aromatic Polyamide Reverse-Osmosis Membrane: An Atomistic Molecular Dynamics Simulation

Polyamide (PA) membrane-based reverse-osmosis (RO) serves as one of the most important techniques for water desalination and purification. Fundamental understanding of PA RO membranes at the atomistic level is critical to enhance their separation capabilities, leading to significant societal and commercial benefits. In this paper, a fully atomistic molecular dynamics simulation was performed to investigate PA membrane. Our simulated cross-linked membrane exhibits structural properties similar to those reported in experiments. Our results also reveal the presence of small local two-layer slip structures in PA membrane with 70% cross-linking, primarily due to short-range anisotropic interactions among aromatic benzene rings. Inside the inhomogeneous polymeric structure of the membrane, water molecules show heterogeneous diffusivities and converge adjacent to polar groups. Increased diffusion of water molecules is observed through the less cross-linked pathways. The existence of the fast pathways for water permeation has no effect on membrane's salt rejections.

Ultrathin Molecular-Layer-by-Layer Polyamide Membranes: Insights from Atomistic Molecular Simulations

In this study, we present an atomistic simulation study of several physicochemical properties of polyamide (PA) membranes formed from interfacial polymerization or from a molecular-layer-by-layer (mLbL) on a silicon wafer. These membranes are composed of meta-phenylenediamine (MPD) and benzene-1,3,5-tricarboxylic acid chloride (TMC) for potential reverse osmosis (RO) applications. The mLbL membrane generation procedure and the force field models were validated, by comparison with available experimental data, for hydrated density, membrane swelling, and pore size distributions of PA membranes formed by interfacial polymerization. Physicochemical properties such as density, free volume, thickness, the degree of cross-linking, atomic compositions, and average molecular orientation (which is relevant for the mLbL membranes) are compared for these different processes. The mLbL membranes are investigated systematically with respect to TMC monomer growth rate per substrate surface area, MPD/TMC ratio, and the number of mLbL deposition cycles. Atomistic simulations show that the mLbL deposition generates membranes with a constant film growth if both the TMC monomer growth rate and MPD/TMC monomer ratio are kept constant. The film growth rate increases with TMC monomer growth rate or MPD/TMC ratio. Furthermore, it was found on one hand that the mLbL membrane density and free volume varies significantly with respect to the TMC monomer growth rate, while on the other hand the degree of cross-linking and the atomic composition varies considerably with the MPD/TMC ratio. Additionally, it was found that both TMC and MPD orient at a tilted angle with respect to the substrate surface, where their angular distribution and average angle orientation depend on both the TMC growth rate and the number of deposition cycles. This study illustrates that molecular simulations can play a crucial role in the understanding of structural properties that can empower the design of the next generation RO membranes created from molecular-layer-by-layer (mLbL) on a silicon wafer.

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

Carbon nanotubes/polyamide (PA) nanocomposite thin films have become very attractive as reverse osmosis (RO) membranes. In this work, we used molecular dynamics to simulate the influence of single walled carbon nanotubes (SWCNTs) in the polyamide molecular structure as a model case of a carbon nanotubes/polyamide nanocomposite RO membrane. It was found that the addition of SWCNTs decreases the pore size of the composite membrane and increases the Na and Cl ion rejection. Analysis of the radial distribution function of water confined in the pores of the membranes shows that SWCNT+PA nanocomposite membranes also exhibit smaller clusters of water molecules within the membrane, thus suggesting a dense membrane structure (SWCNT+PA composite membranes were 3.9% denser than bare PA). The results provide new insights into the fabrication of novel membranes reinforced with tubular structures for enhanced desalination performance.