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.

Exploring fast water permeation through aquaporin-mimicking membranes.

Using molecular dynamics simulations, herein, we illustrate that a bending structure shows different behaviors for fast water transport through aquaporin-mimicking membranes in multilayer graphene and tubular structures. This suggests that the bending structure enhances water transport through multilayer membranes, indicating the optimum state at θ = 45°. Disruption of the single-file water arrangement inside the membrane can contribute to promoting water transport in this system. However, a bending structure reduces the rate of water transport in tubular systems. Our results exhibit that a straight tubular membrane transfers water molecules faster than its non-straight counterpart. A stabilized form of the single-file water structure was observed in the membrane. Interestingly, we found that the tubular hourglass-shaped membranes possessed a lower free energy than the multilayer membranes with an hourglass shape. This can be attributed to the accommodation of the single-file water configuration in a confined space with hydrophobic characteristics. Accordingly, integrating an hourglass shape pore in a tubular structure in an impermeable membrane provides high water permeability compared with its multilayer counterpart. We also found that the wide variation in the dipole orientation of water molecules and the energy barrier have dominant effects in determining fast water transport through multilayer and tubular membranes, respectively. The contribution of interlayer spacing on fast water transport through multilayer membranes was also studied.

Transport of water molecules through noncylindrical pores in multilayer nanoporous graphene.

In this study, molecular dynamics (MD) simulations are used to examine the water transport properties through asymmetric hourglass-shaped pores in multilayer nanoporous graphene with a constant interlayer separation of 6 Å. The properties of the tested asymmetric hourglass-shaped pores [with the models having long cone (l1, -P) and short cone (l2, +P) entrances] are compared to a symmetric pore model. The study findings indicate that the water occupancy increases across the asymmetric pore (l1, -P) compared to (l2, +P), because of the length effect. The asymmetric pore, (l1, -P), yields higher flux compared to (l2, +P) and even the symmetric model, which can be attributed to the increase in the hydrogen bonds. In addition, the single-file water molecules across the narrowest pore diameter inside the (l2, +P) pore exhibit higher viscosity compared to those in the (l1, -P) pore because of the increase in the water layering effect. Moreover, it is found that the permeability inside the multilayer hourglass-shaped pore depends on the length of the flow path of the water molecules before approaching the layer with the smallest pore diameter. The probability of dipole orientation exhibits wider distribution inside the (l1, -P) system compared to (l2, +P), implying an enhanced formation of hydrogen bonding of water molecules. This results in the fast flow of water molecules. The MD trajectory shows that the dipole orientation across the single-layer graphene has frequently flipped compared to the dipole orientation across the pores in multilayer graphene, which is maintained during the whole simulation time (although the dipole orientation has flipped for a few picoseconds at the beginning of the simulation). This can be attributed to the energy barrier induced by the individual layer. The diffusion coefficient of water molecules inside the (l2, +P) system increases with pressure difference, however, it decreases inside the (l1, -P) system because of the increase in the number of collisions. It was found that the velocity in the axial direction (z-direction) has a significant impact on the permeation ability of water molecules across the asymmetric nanopores examined in this study. Finally, the study results suggest that the appropriate design of an asymmetric hourglass-shaped nanopore in multilayer graphene can significantly improve the water permeation rate even compared to a symmetric structure.

Molecular Dynamics Simulation of Water Transport Mechanisms through Nanoporous Boron Nitride and Graphene Multilayers.

In this study, molecular dynamics simulations are used to investigate water transport mechanisms through hourglass-shaped pore structure in nanoporous boron nitride (BN) and graphene multilayers. An increase in water flux is evidenced as the gap between the layers increases, reaching a maximum of 41 and 43 ns-1 at d = 6 Å in BN and graphene multilayers, respectively. Moreover, the BN multilayer exhibits less flux compared to graphene due to large friction force and energy barrier. In BN, the friction force dramatically increases when the layers are strongly stacked (d = 3.5 Å), whereas it would be independent of the layer separation when the layers are sufficiently spaced (d ≥ 5 Å). In contrast, it was shown that the friction force is independent of the layer spacing in graphene. On the other hand, water molecules across the BN exhibits larger energy barriers compared to graphene when the layers are highly spaced at d = 8 Å. Consistent with the result reported for the flux, the axial diffusion coefficient of water molecules in graphene increases with layer spacing, reaching a maximum of 6.8 × 10-5 cm2/s when the layers are spaced at d = 6 Å.

Molecular Dynamics Simulation of the Effect of Angle Variation on Water Permeability through Hourglass-Shaped Nanopores.

Water transport through aquaporin water channels occurs extensively in cell membranes. Hourglass-shaped (biconical) pores resemble the geometry of these aquaporin channels and therefore attract much research attention. We assumed that hourglass-shaped nanopores are capable of high water permeation like biological aquaporins. In order to prove the assumption, we investigated nanoscale water transport through a model hourglass-shaped pore using molecular dynamics simulations while varying the angle of the conical entrance and the total nanopore length. The results show that a minimal departure from optimized cone angle (e.g., 9° for 30 Å case) significantly increases the osmotic permeability and that there is a non-linear relationship between permeability and the cone angle. The analysis of hydrodynamic resistance proves that the conical entrance helps to reduce the hydrodynamic entrance hindrance. Our numerical and analytical results thus confirm our initial assumption and suggest that fast water transport can be achieved by adjusting the cone angle and length of an hourglass-shaped nanopore.