Molecular insights into the structure-property relationships of 3D printed polyamide reverse-osmosis membrane for desalination

Toward Molecular Simulation Guided Design of Next-Generation Membranes: Challenges and Opportunities

Membranes provide energy-efficient solutions for separating ions from water, ion-ion separation, neutral or charged molecules, and mixed gases. Understanding the fundamental mechanisms and design principles for these separation challenges has significant applications in the food and agriculture, energy, pharmaceutical, and electronics industries and environmental remediation. In situ experimental probes to explore Angstrom-nanometer length-scale and pico-nanosecond time-scale phenomena remain limited. Currently, molecular simulations such as density functional theory, ab initio molecular dynamics (MD), all-atom MD, and coarse-grained MD provide physics-based predictive models to study these phenomena. The status of molecular simulations to study transport mechanisms and state-of-the-art membrane separation is discussed. Furthermore, limitations and open challenges in molecular simulations are discussed. Finally, the importance of molecular simulations in generating data sets for machine learning and exploration of membrane design space is addressed.

Role of Transmembrane Pressure and Water Flux in Reverse Osmosis Composite Membrane Compaction and Performance

This study explores the compaction behavior of thin-film composite reverse osmosis (TFC RO) membranes for different combinations of transmembrane pressure (TMP) and transmembrane water flux. Operating a crossflow system at constant feed pressure (60 bar) but different feed solution osmotic pressures enabled adjusting the TMP─the difference between hydraulic and osmotic pressure─and water flux. The extent of membrane compaction increases as TMP (and flux) increases. Both commercial and hand-cast TFC RO membranes showed substantial compaction at high TMP (up to 30% compaction at 50 bar TMP) compared to less than 10% at 10 bar TMP. Scanning electron microscope (SEM) images reveal a direct relationship between TMP and polysulfone (PSU) support layer compaction, while molecular dynamics (MD) simulations confirmed decreased porosity and reduced thickness in the polyamide (PA) active layer as TMP increases. Combined findings from wet-testing and MD simulations confirm a hydraulic pressure drop occurs across both the PA active layer and the meso-to-macro-porous support layer; higher TMP exacerbates compaction in both layers resulting in lower water permeability but higher water flux, observed salt rejection, and salt permeability. Transitioning from high TMP to low TMP or vice versa did not notably alter the extent of membrane compaction. This observation is attributed to the highly cross-linked PA active layer's ability to recover after pressure is released, whereas the compaction in the PSU support layer is largely irreversible. While TMP dictates the overall pressure gradient, our findings suggest that flux-induced frictional forces play a crucial role in compaction dynamics. Specifically, higher flux generates additional drag forces on the polymer matrix of both the PSU support layer and the PA selective layer, intensifying structural deformation. Overall, our findings offer critical insights into the mechanisms of membrane compaction, providing a foundation for optimizing RO membrane performance and advancing next-generation membrane technologies.

Membrane for Pressure-Driven Separation Prepared with a Method of 3D Printing: Performance in Concentrating Orange Peel Extract

3D-printing enables the fabrication of membranes with desired shapes and geometrical parameters. In this study, a membrane for pressure-driven processes was manufactured in a single step using the fused deposition modeling (FDM) technique. The membrane was produced from a mixture of polylactic acid (PLA) with sucrose as a pore-forming agent. Sucrose was removed from the final membrane by washing it with water. The membrane consists of three layers, and this sandwich-like structure ensures its mechanical stability. The material obtained was characterized using SEM and AFM imaging, as well as nitrogen adsorption-desorption and contact angle measurements. The porosity of each layer of the membrane is due to a loose region, which is coated on both sides with a dense film formed during printing. The pores responsible for rejection capability can be found in grooves between the polymer stripes in the dense layer. The membrane exhibits a water permeability of 64 L m-2h-1bar-1, with a molecular weight cut-off of 69 kDa. The PLA membrane can be used for polyphenol concentration, demonstrating a permeability of 2-3.4 L m-2h-1bar-1 and a selectivity towards these compounds of 78-98% at 0.5 bar, with a flux decline ratio of up to 50%.

PX-MDsim: a rapid and efficient platform for large-scale construction of polyamide membranes via automated molecular dynamics simulations

Polyamide (PA) membranes have attracted extensive attention due to their excellent separation performance in water treatment through reverse osmosis and nanofiltration processes. Although numerous molecular simulation studies attempt to explore their advantages from the microstructure, large-scale construction and simulation of PA membranes remain challenging, mainly due to the complexity and computational intensity of cross-linking reactions of polymers in molecular dynamics simulations. This paper introduces an automated platform called PX-MDsim for modeling and simulation of PA membranes. PX-MDsim is based on the PXLink framework and extends its applicability to any monomer with amino (-NH2) and carboxyl (-COOH) groups. The platform, combined with the PXLink program, realizes the full-process automated cross-linking simulation from input preparation, initial system construction, force field generation, functional group identification, and charge distribution update. Moreover, the software was used to cross-link m-phenylenediamine and 1,4-bis(3-aminopropyl)piperazine with trimesic acid, respectively, and multiple membrane structures with different cross-linking degrees were obtained. Furthermore, the generated membrane microstructure was analyzed using methods such as pore size distribution and order parameter, and the obtained results verified the applicability and accuracy of PX-MDsim in constructing PA membrane structures. The platform is user-friendly and accessible to researchers without prior expertise in molecular dynamics simulation, and it offers new possibilities for polymer research and applications.

Elucidating governing factors of PFAS removal by polyamide membranes using machine learning and molecular simulations

Per- and polyfluoroalkyl substances (PFASs) have recently garnered considerable concerns regarding their impacts on human and ecological health. Despite the important roles of polyamide membranes in remediating PFASs-contaminated water, the governing factors influencing PFAS transport across these membranes remain elusive. In this study, we investigate PFAS rejection by polyamide membranes using two machine learning (ML) models, namely XGBoost and multimodal transformer models. Utilizing the Shapley additive explanation method for XGBoost model interpretation unveils the impacts of both PFAS characteristics and membrane properties on model predictions. The examination of the impacts of chemical structure involves interpreting the multimodal transformer model incorporated with simplified molecular input line entry system strings through heat maps, providing a visual representation of the attention score assigned to each atom of PFAS molecules. Both ML interpretation methods highlight the dominance of electrostatic interaction in governing PFAS transport across polyamide membranes. The roles of functional groups in altering PFAS transport across membranes are further revealed by molecular simulations. The combination of ML with computer simulations not only advances our knowledge of PFAS removal by polyamide membranes, but also provides an innovative approach to facilitate data-driven feature selection for the development of high-performance membranes with improved PFAS removal efficiency.

Novel Treatment of 3D-Printed Short-Carbon-Fiber-Reinforced Polyamide (3D-SCFRPA66) Using Homogeneous Low-Potential Electron Beam Irradiation (HLEBI) and Ductility Enhancement

In short-carbon-fiber-reinforced polyamide 66 articles shaped by 3D printing (3D-SCFRPA66), the interfaces between printed layers are often susceptible to damage, and the composite is excessively brittle. Therefore, a novel treatment for 3D-printed short-carbon-fiber-reinforced polyamide (3D-SCFRPA66) using homogeneous low-potential electron beam irradiation (HLEBI) to enhance tensile properties was investigated. In 3D-SCFRPA66 samples, ductility was measured based on the following parameters: strain at tensile strength (corresponding to homogeneous deformation) (εts) and resistance energy to homogeneous deformation, a measure of toughness (Ehd), which were both substantially increased. An HLEBI dose of 43.2 kGy at an acceleration potential of 210 kV for the finished 3D-SCFRPA66 samples increased the εts and Ehd values from 0.031 and 1.20 MPa·m for the untreated samples to 0.270 and 6.05 MPa·m for the treated samples, increases of 771% and 504%, respectively. Higher HLEBI doses of 86, 129, or 215 kGy also increased the εts and Ehd values to lesser degrees. Electron spin resonance (ESR) data in the literature show that HLEBI creates dangling bonds in Nylon 6. Since PA66 and Nylon 6 are constructed of C, N, and O and have similar molecular structures, HLEBI apparently severs the (-C-N-) bonds in the backbone of PA66, which have the lowest bond-dissociation energy (BDE) of ~326 to 335 kJ mol-1. This shortens the PA66 chains for higher ductility. In addition, for Nylon 6, X-ray photoelectron spectroscopy (XPS) data in the literature show that HLEBI reduces the N peak while increasing the C peak, indicating the occurrence of shortening chains via dangling bond formation accompanied by increases in crosslinking with carbon bonds. However, caution is advised, since HLEBI was found to decrease the tensile strength (σts) and initial elasticity ([dσ/dε]i) of 3D-SCFRPA66. This tradeoff can possibly allow the HLEBI dose to be adjusted for the desired ductility and strength while minimizing energy consumption.

Nonpreferential Solvent Transport through an Intrinsic Cyclodextrin Pore in a Polyester Film

We performed equilibrium molecular dynamics simulations to study the transport of water and hexane solvents through cyclodextrin(CD)-based membranes (α-/β-/γ-CD/TMC). Although it is known that water and hexane can permeate through the macrocyclic cavity, surprisingly, when it is present in the CD-based membrane (α-/β-/γ-CD/TMC), these solvents are not permeating through the CD cavity. Interactions between membrane functional group atoms with the water and hexane suggest that these solvents primarily permeate through the polar aggregate pores formed via ester-linkage rather than the CD cavity. Our observation reveals that both solvents can permeate through the membrane; however, the hexane flux was one order of magnitude lower than water flux. Our study suggests that further work is needed to confirm the functional significance of the macrocyclic cavity in solvent permeation and the existence of Janus pathways.

Water transport in reverse osmosis membranes is governed by pore flow, not a solution-diffusion mechanism

We performed nonequilibrium molecular dynamics (NEMD) simulations and solvent permeation experiments to unravel the mechanism of water transport in reverse osmosis (RO) membranes. The NEMD simulations reveal that water transport is driven by a pressure gradient within the membranes, not by a water concentration gradient, in marked contrast to the classic solution-diffusion model. We further show that water molecules travel as clusters through a network of pores that are transiently connected. Permeation experiments with water and organic solvents using polyamide and cellulose triacetate RO membranes showed that solvent permeance depends on the membrane pore size, kinetic diameter of solvent molecules, and solvent viscosity. This observation is not consistent with the solution-diffusion model, where permeance depends on the solvent solubility. Motivated by these observations, we demonstrate that the solution-friction model, in which transport is driven by a pressure gradient, can describe water and solvent transport in RO membranes.

Correlating the Role of Nanofillers with Active Layer Properties and Performance of Thin-Film Nanocomposite Membranes

Thin-film nanocomposite (TFN) membranes are emerging water-purification membranes that could provide enhanced water permeance with similar solute removal over traditional thin-film composite (TFC) membranes. However, the effects of nanofiller incorporation on active layer physico-chemical properties have not been comprehensively studied. Accordingly, we aimed to understand the correlation between nanofillers, active layer physico-chemical properties, and membrane performance by investigating whether observed performance differences between TFN and control TFC membranes correlated with observed differences in physico-chemical properties. The effects of nanofiller loading, surface area, and size on membrane performance, along with active layer physico-chemical properties, were characterized in TFN membranes incorporated with Linde Type A (LTA) zeolite and zeolitic imidazole framework-8 (ZIF-8). Results show that nanofiller incorporation up to ~0.15 wt% resulted in higher water permeance and unchanged salt rejection, above which salt rejection decreased 0.9-25.6% and 26.1-48.3% for LTA-TFN and ZIF-8-TFN membranes, respectively. Observed changes in active layer physico-chemical properties were generally unsubstantial and did not explain observed changes in TFN membrane performance. Therefore, increased water permeance in TFN membranes could be due to preferential water transport through porous structures of nanofillers or along polymer-nanofiller interfaces. These findings offer new insights into the development of high-performance TFN membranes for water/ion separations.