Ph.D Thesis Colloquium : Dhondi Pradeep

Functionalized Nanoporous 2D Membranes for Water Desalination: Ab Initio Force-Field Development and Insights from Molecular Dynamics Simulations

Ultrathin nanoporous membranes composed of two-dimensional (2D) materials exhibit exceptional properties, including high specific surface areas and superior separation efficiency, making them promising candidates for seawater desalination and osmotic power harvesting applications. Among these materials, molybdenum disulfide (MoS₂), hexagonal boron nitride (hBN), and graphene oxide (GO) stand out due to chemical tunability and heteropolar nature. While recent studies highlight the potential of nanoporous 2D materials for desalination, molecular dynamics (MD) simulations, particularly for MoS₂ and hBN, have primarily focused on unfunctionalized 2D nanopores. This major limitation arises from the lack of classical force fields that can accurately describe functionalized nanopores. Indeed, interactions between water molecules and edge atoms (e.g., B and N in hBN and Mo and S in MoS₂) can promote water dissociation, thus enabling functionalization with hydrogen (H), oxo (O), and hydroxyl (OH) groups under experimental conditions. In contrast, GO-based membranes are extensively studied for water purification but face challenges such as nanochannel swelling due to water intercalation and membrane disintegration under high pressure, limiting their scalability. To address these several research gaps, in this thesis, we not only carried out force-field development for modeling functionalized 2D nanopores, but also conducted extensive MD simulations to understand water and ion transport through nanoporous and nanocomposite membranes.

First, we employed density functional theory (DFT)-based ab initio molecular dynamics (AIMD) simulations to investigate the functionalization tendencies of hBN nanopores in aqueous environments. We found that when boron edges in hBN are exposed to water, they favor hydrogen and hydroxyl functionalization, but when nitrogen edges are exposed they exhibit hydrogenation and occasional oxygen functionalization. This study establishes functionalization as a key tuning parameter for post-graphene 2D membranes. Additionally, we also demonstrate the role of the Grotthuss mechanism in the functionalization process of hBN edges in water. After obtaining insights from AIMD simulations, to enable realistic MD simulations of functionalized 2D membranes, we developed high-fidelity force fields (FF) for H-, O-, and OH-functionalized hBN using potential energy surfaces (PES) derived from DFT calculations. Additionally, nonbonded parameters for H-functionalized hBN were obtained by training a force field for borazine (B₃N₃H₆).

Subsequently, using the developed FF for functionalized hBN systems, we performed MD simulations for the separation of salt from NaCl solutions through functionalized (B-OH, B-H, N-H, and N-O) nanopores in hBN. Our findings indicate that previous studies, which considered only bare nanopores, likely overestimated water flux and underestimated ion rejection in hBN membranes. Furthermore, we demonstrate that functional groups modulate charge distributions at boron- and nitrogen-terminated triangular nanopores in hBN, allowing for highly selective membranes for salt and boron rejection. Using quantum-mechanically computed charge distributions in MD simulations, we showed that hydrogen-functionalized boron-terminated pores and oxygen-functionalized nitrogen-terminated pores exhibit near-perfect salt rejection due to their neutral edge charges. Additionally, through hydrogen bond lifetime analysis and potential of mean force calculations, we rationalized how water permeance varies with different functional groups at the nanopore edges. Meanwhile, hydrogen-functionalized nitrogen-terminated pores enable 100% rejection of boron, a challenging-to-remove neutral contaminant in seawater, by repelling boric acid molecules with positively charged termini.

Next, we employed DFT-based AIMD simulations to investigate the functionalization tendencies of MoS₂ nanopores in aqueous environments and found that water exposure at edges induces spontaneous, site-selective, and nanopore shape-dependent functionalization. These include Mo-O and S-H edges in hexagonal pores, and Mo-OH and bare S edges in triangular pores. Based on these AIMD results, we identified possible functionalization types at different edges and developed a high-quality force field for H-, O-, and OH-functionalized MoS₂. The force field was fitted to the PES obtained from DFT calculations, similar to what we did earlier for the hBN system. Using this developed FF, we then carried out MD simulations for NaCl solution separation through functionalized hexagonal and triangular nanopores. The results show that unfunctionalized S-terminated triangular pores and functionalized hexagonal pores in MoS₂ allow high water permeance while still blocking salt effectively. In contrast, triangular pores functionalized with Mo-OH exhibit reduced transport due to the high charge magnitude of the OH groups at the pore edges and the associated reduction in the effective pore diameter. This functionalization alters both the accessible pore area and edge electrostatics, resulting in suppressed water permeance and reduced salt rejection compared to other pore configurations.

Finally, we simulated nanocomposite GO membranes to understand their desalination performance. MD simulations were carried out for two types of functionalized GO in a polymer matrix consisting of polydopamine and polyvinylidene fluoride. The functional group combinations studied include carboxylic acid, hydroxyl, and amide in one case (with the resulting membrane called rGO-I), and (hydroxyethyl)methacrylate (HEMA) and hydroxyl in the second case (with the resulting membrane named GO-HEMA). This work was done in collaboration with Prof. Suryasarathi Bose from the Department of Materials Engineering, IISc, whose group has developed GO nanocomposite membranes based on a sequential interpenetrating polymeric network (IPN). Our MD simulations provided insights into the ionic sieving mechanism and the water flux trends of the GO-based IPN membranes at different concentrations. MD simulations supported the water flux reduction seen upon arresting the rGO-I sheets within IPN, which scales with the concentration of rGO-I. In addition, simulations show that this confinement at molecular length scales leads to a reduction in the number of ions residing within the membrane region, favouring ion retention within the feed region. For GO-HEMA-based IPN membranes, simulations indicate more effective ion exclusion and sustained water transport under confinement, consistent with their enhanced desalination performance. Among the studied systems, MD simulations indicate that GO-HEMA membranes achieve enhanced desalination performance compared to rGO-I-based membranes. These results corroborate the observed experimental evidence.

Overall, this thesis establishes a framework for accurate modeling of edge-functionalized 2D membranes and provides detailed atomic-scale mechanisms underlying water, ion, and boron transport through functionalized nanporous hBN and MoS₂ membranes, as well as water and ion transport through nanocomposite GO membranes.  The molecular-level insights presented in this work will pave the way for potential deployment of promising 2D membrane materials, including hBN, MoS₂, and GO, in sustainable water treatment applications.