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.

Surface modification of polyamide reverse osmosis membranes with small-molecule zwitterions for enhanced fouling resistance: a molecular simulation study

Surface modification with small-molecule zwitterions is experimentally proved to be an effective solution to improve the antifouling performance of polyamide membranes. However, there is no comprehensive understanding of their microscopic mechanism. In order to address this issue, in this work we constructed two atomistic models, PA (a pure polyamide membrane) and QDAP-PA (a polyamide membrane surface-modified with QDAP), where QDAP was a zwitterion that was prepared by 2,6-daaminopyridine quaternized with 3-bromopropionic acid experimentally. Density functional theory was adopted to elucidate the variations in the electrostatic potential before and after modification. Then, equilibrium molecular dynamics (EMD) simulations were conducted to investigate the structure and hydrophobic/hydrophilic nature of the membrane surface in the two models. Finally, we introduced two typical organic foulants, sodium dodecyl sulfonate (SDS) and dodecyl trimethyl ammonium chloride (DTAC), to evaluate the antifouling performance of the membranes with the umbrella sampling method. The analyses of the membrane structure and properties show that surface modification with small-molecule zwitterions can densify the membrane surface as well as enlarge the distribution of electrostatic potential on the membrane surface. Water molecules tend to have more interactions with the membrane and more hydrogen bonds near the membrane surface are observed in QDAP-PA. The antifouling test supports that QDAP-PA shows a better antifouling performance, as the surface-modified membrane exhibits a stronger resistance to SDS and DTAC. Even if the foulant is adsorbed to the membrane surface, the denser interface region can prevent a further pollution of the foulant. Also, the free energy needed during the process for QDAP-PA to desorb a foulant is relatively small, indicating that this kind of membrane is easy to clean. The current work might provide a comprehensive understanding of the enhanced fouling resistance of polyamide membranes after surface modification with small-molecule zwitterions.