Comparative Retention Analysis of Intercalated Cations Inside the Interlayer Gallery of Lamellar and Nonlamellar Graphene Oxide Membranes in Reverse Osmosis Process: A Molecular Dynamics Study.

Over the past decade, multilayered graphene oxide (GO) membranes have emerged as promising candidates for desalination applications. Despite their potential, a comprehensive understanding of separation mechanisms remains elusive due to the intricate morphology and structural arrangement of interlayer galleries. Moreover, a critical concern of multilayered GO membranes is their susceptibility to swelling within aqueous environments, which hinders their practical implementation. Therefore, this study introduces cation intercalation within GO laminates to elucidate the underlying factors governing swelling behavior and subsequently mitigate it. Moreover, this study performed nonequilibrium molecular dynamics simulations on the cation (Mg2+ or K+)-intercalated lamellar and nonlamellar GO membranes to understand the effect of the arrangement of GO sheets on the retention time of intercalated cations within GO layers, water permeance, and salt rejection mechanism in the reverse osmosis process using cation-intercalated GO membranes. Our results highlight that lamellar GO membranes exhibit higher water permeance, attributed to their well-defined interlayer gallery structure. On the other hand, nonlamellar GO membranes display superior salt rejection due to their complex interlayer gallery structure that impedes salt permeation. Moreover, the structural complexity of nonlamellar GO membranes contributes to greater stability by retention of the more intercalated cations for a longer time within the layers. Furthermore, it is observed that a higher percentage of Mg2+ cations remained inside the GO laminates as compared to K+ cations, hence resulting in the greater stability of the Mg2+-intercalated GO membrane in the aqueous environment.

Semiaromatic Polyamide-Based Membrane in Forward Osmosis: Molecular Insights.

Despite the increased interest in forward osmosis (FO) in recent years, the technology's advancement in commercial and industrial applications has been hampered by the absence of suitable FO membranes and ideal draw solutes, which demands the exploration of new membranes and novel draw solutes targeted for some specific applications. In this context, we considered a semiaromatic polyamide (SAPA) for an application where monovalent salt can be permeated but has high selectivity toward divalent salt and excellent water permeability. In this regard, we constructed an atomistic model for the membrane via a heuristic approach using an equilibrated mixture of hydrolyzed trimesoyl chloride and piperazine monomers and performed nonequilibrium molecular dynamics simulations on the SAPA membrane in the FO process to understand the structural properties and performance of the membrane at the atomistic level. We used pure water as the feed and Na2SO4 as the draw solution. It is observed that the SAPA membrane shows excellent water permeability and no reverse draw solute flux. To further test the dynamics of salt ions inside the membranes, we performed two distinct equilibrium simulations on systems consisting of either monovalent salt, such as NaCl, or divalent salt, such as Na2SO4. The atomistic details of the interactions between the functional groups of the membrane and salt ions provided in this work can inspire further experiments on SAPA membranes in the context of separation of monovalent and divalent salts, which have applications in the treatment of textile industry wastewater.

Fundamental Understanding of Ultrathin, Highly Stable Self-Assembled Liquid Crystalline Graphene Oxide Membranes Leading to Precise Molecular Sieving through Non-equilibrium Molecular Dynamics.

Self-assembled graphene oxide lyotropic liquid crystal (GO LLC) structures are mostly formed in aqueous medium; however, most GO derivatives are water insoluble, so processing GO LLCs in water poses a practical limitation. The use of polar aprotic solvent (like dimethyl sulfoxide) for the formation of GO LLC structures would be interesting, because it would allow incorporating additives, like photoinitiators or cross-linkers, or blending with polymers that are insoluble in water, which hence would expand its scope. The well-balanced electrostatic interaction between DMSO and GO can promote and stabilize the GO nanosheets' alignment even at lower concentrations. With this in mind, herein we report mechanically robust, chlorine-tolerant, self-assembled nanostructured GO membranes for precise molecular sieving. Small-angle X-ray scattering and polarized optical microscopy confirmed the alignment of the modified GO nanosheets in polar aprotic solvent, and the LLC structure was effectively preserved even after cross-linking under UV light. We found that the modified GO membranes exhibited considerably improved salt rejection for monovalent ions (99%) and water flux (120 LMH) as compared to the shear-aligned GO membrane, which is well supported by forward osmosis simulation studies. Additionally, our simulation studies indicated that water molecules traveled a longer path while permeating through the GO membrane compared to the GO LLC membrane. Consequently, salt ions permeate slowly across the GO LLC membrane, yielding higher salt rejection than the GO membrane. This begins to suggest strong electrostatic repulsion with the salt ions, causing higher salt rejection in the GO LLC membrane. We foresee that the ordered cross-linked GO sheets contributed to excellent mechanical stability under a high-pressure, cross-flow, chlorine environment. Overall, these membranes are easily scalable, exhibit good mechanical stability, and represent a breakthrough for the potential use of polymerized GO LLC membranes in practical water remediation applications.

Structural and Dynamic Insights into SARS-CoV-2 Spike-Protein-Montmorillonite Interactions.

The spike (S) protein of SARS-CoV-2 has been found to play a decisive role in the cell entry mechanism of the virus and has been the prime target of most vaccine development efforts. Although numerous vaccines are already in use and more than half of the world population has been fully vaccinated, the emergence of new variants of the virus poses a challenge to the existing vaccines. Hence, developing an effective drug therapy is a crucial step in ending the pandemic. Nanoparticles can play a crucial role as a drug or a drug carrier and help tackle the pandemic effectively. Here, we performed explicit all-atom molecular dynamics simulations to probe interactions between S protein and Montmorillonite (MMT) nano clay surface. We built two systems with different counterions (Na+ and Ca2+), namely Na-MMT and Ca-MMT, to investigate the effect of different ions on S protein-MMT interaction. Structural modification of S protein was observed in the presence of MMT surface, particularly the loss of helical content of S protein. We revealed that electrostatic and hydrophobic interactions synergistically govern the S protein-MMT interactions. However, hydrophobic interactions were more pronounced in the Na-MMT system than in Ca-MMT. We also revealed residues and glycans of S protein closely interacting with the MMT surface. Interestingly, N165 and N343, which we found to be closely interacting with MMT in our simulations, also have a critical role in cell entry and in thwarting the cell's immune response in recent studies. Overall, our work provides atomistic insights into S protein-MMT interaction and enriches our understanding of the nanoparticle-S protein interaction mechanism, which will help develop advanced therapeutic techniques in the future.

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.

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.

The effect of the stoichiometric ratio of zinc towards the fibrillation of Bovine Serum Albumin (BSA): A mechanistic insight.

Fibrillation of proteins is a major cause of various neurodegenerative diseases and its exact mechanism of formation is yet unclear instead of extensive research. However, the role of metal ions influencing fibrillation of proteins is gaining more attention recently. Herein, we have investigated the role of various concentrations of the transition metal, Zn(II), on the fibrillation of Bovine Serum Albumin (BSA) at the physiological pH 7.4. Several biophysical and simulation techniques were employed in order to analyze the same. Thioflavin T intensity and residual protein investigations revealed that fibrillation of BSA was significantly decelerated and accelerated at 1:3 and 1:4 ratios of BSA-Zn(II), respectively; while it was found to be independent at other ratios (1:1 and 1:2). Fourier transform infrared spectroscopy analysis revealed that the transition of BSA from α-helical conformation to the ß-sheet rich structure is greatly resisted at 1:3 ratio, however, the same is promoted at 1:4 ratio. Similarly, dynamic light scattering and field emission transmission electron microscopy analyses further confirmed the above observations. Furthermore, Isothermal Titration Calorimetry revealed the interaction of Zn(II) towards four binding sites of BSA with preferential affinities. Molecular dynamics studies predicted that at 1:3 ratio, the C- and N-terminal zones of BSA were least flexible owing to more stable conformation. Moreover, the solvent accessible surface area and structural analyses showed increase in hydrophilicity and more conserved secondary structure, respectively at 1:3 ratio. We propose that BSA fibrillation is indeed dependent on particular Zn(II) concentration, the temperature of the microenvironment of BSA, the number of binding sites exposed due to unfolding and the conformation after metal binding.

Effect of graphene oxide (GO) nanosheet sizes, pinhole defects and non-ideal lamellar stacking on the performance of layered GO membranes: an atomistic investigation.

The effect of non-idealities, namely pinhole defects and non-ideal lamellar stacking of nanosheets, on the performance of size-differentiated graphene oxide (GO) laminates is investigated using equilibrium molecular dynamics (MD) simulations. With the increase in sizes of the constituent GO nanosheets the water permeability of the layered GO membranes decreases and salt rejection increases. But with the inclusion of non-idealities the difference in water permeability between these membranes substantially reduced. The pinholes on the GO nanosheets provide shorter routes for trans-sheet flow, thereby increasing the water permeability of the membranes. The non-ideal stacking of the nanosheets without pinhole defects results in slight reduction in water permeability because of blockage of permeation pathways inside the membranes. However, with pinhole defects non-ideal stacking becomes favorable for water permeation through the layered GO membranes; as this time the non-ideal stacking leads to formation of voids inside the membranes, which act as routes for shorter permeation pathways. The effect of these non-idealities is more significant for layered GO membranes composed of large GO nanosheets. Although the water permeability through the layered GO membrane is greatly enhanced because of these non-idealities (about 10 times), the corresponding variation in the salt rejection is very low (<2%).

Granular silo flow of inelastic dumbbells: Clogging and its reduction.

We study the discharge of inelastic, two-dimensional dumbbells through an orifice in the bottom wall of a silo using discrete element method (DEM) simulations. As with spherical particles, clogging may occur due to the formation of arches of particles around the orifice. The clogging probability decreases with increasing orifice width in both cases. For a given width, however, the clogging probability is much higher for the nonspherical particles due to their arbitrary orientations and the possibility of geometrical interlocking. We also examine the effect of placing a fixed, circular obstacle above the orifice. The clogging probability depends strongly on the vertical and lateral position of the obstacle, as well as its size. By suitably placing the obstacle the clogging probability can be significantly reduced compared to a system with no obstacle. We attempt to elucidate the clogging reduction mechanism by examining the packing fraction, granular temperature, and velocity distributions of the particles in the vicinity of the orifice.