Magnesium Ion Gated Ion Rejection through Carboxylated Graphene Oxide Nanopore: A Theoretical Study

While nanoporous graphene oxide (GO) is recognized as one of the most promising reverse osmosis desalination membranes, limited attention has been paid to controlling desalination performance through the large GO pores, primarily due to significant ion leakage resulting in the suboptimal performance of these pores. In this study, we employed a molecular dynamics simulation approach to demonstrate that Mg2+ ions, adhered to carboxylated GO nanopores, can function as gates, regulating the transport of ions (Na+ and Cl-) through the porous GO membrane. Specifically, the presence of divalent cations near a nanopore reduces the concentration of salt ions in the vicinity of the pore and prolongs their permeation time across the pore. This subsequently leads to a notable enhancement in salt rejection rates. Additionally, the ion rejection rate increases with more adsorbed Mg2+ ions. However, the presence of the adsorbed Mg2+ ions compromises water transport. Here, we also elucidate the impact of graphene oxidation degree on desalination. Furthermore, we design an optimal combination of adsorbed Mg2+ ion quantity and oxidation degree to achieve high water flux and salt rejection rates. This work provides valuable insights for developing new nanoporous graphene oxide membranes for controlled water desalination.

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.

Mechanistic studies on the anomalous transport behaviors of water molecules in nanochannels of multilayer graphynes

An in-depth understanding of directed transport behaviors of water molecules through nanoporous materials is essential for the design and development of next-generation filtration devices. In this work, we perform molecular dynamics (MD) simulations to explore transport properties of water molecules through nanochannels of multilayer graphyne with different pore sizes. Our simulation results reveal that the orientations of confined water molecules would periodically reverse between two opposite directions as they diffuse along the nanochannels, and such a transport mechanism shows similarities with water transport in aquaporin channels. Further, we observe that, for each orientation reversal, there is an obvious difference in the HB breaking frequency among the three graphyne systems, with an order of graphyne-4 > graphyne-5 > graphyne-3. Besides, the average HB number is found to display a periodic fluctuation with a pulse-like pattern along the diffusion direction, wherein the graphyne-4 system has the maximum fluctuation, while the graphyne-3 system has the minimum one. Such anomalous HB breaking frequency and average HB number fluctuation results finally lead to a nonmonotonic relationship between water diffusion rate and graphyne pore size, and the diffusion order follows graphyne-4 > graphyne-5 > graphyne-3. Herein, we provide a new insight into the transport mechanisms of water molecules through nanoporous materials and our findings open up opportunities for the design and development of high-performance graphyne-based membranes used for water purification and desalination.

Controlling ion transport in a C2N-based nanochannel with tunable interlayer spacing

Selective ion transport through a nanochannel formed by stacked two-dimensional materials plays a key role in water desalination, nanofiltration, and ion separation. Although there have been many functional nanomaterials used in these applications, how to well control ion transport in a laminar structure so as to obtain the desired selectivity still remains a challenging problem. In the present work, the transport of ions through a C₂N-based nanochannel is investigated by using all-atom molecular dynamics simulation. It is found that C₂N-based nanochannels with different interlayer spacing posses diverse ion selectivity, which is mainly attributed to the distinct loading capability among ions and the different velocity of ions inside the nanochannel. Moreover, we also find that the ion selectivity is dependent on the electric field, but nearly independent of the salt concentration. The present study may provide some physical insights into the experimental design of C₂N-based nanodevices in nanofiltration.

Theory and simulation developments of confined mass transport through graphene-based separation membranes

The perspectives of graphene-based membranes based on confined mass transport from simulations and experiments for 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.