Enhancing the Dynamics of Water Confined between Graphene Oxide Surfaces with Janus Interfaces: A Molecular Dynamics Study

Influence of CTAB Reverse Micellar Confinement on the Tetrahedral Structure of Liquid Water

The effect of confinement on the tetrahedral ordering of liquid water plays a vital role in controlling their microscopic structure and dynamics as well as their spectroscopic properties. In this article, we have performed the classical molecular dynamics simulations of four different CTAB/water/chloroform reverse micelles with varied water content to study how the tetrahedral ordering of nanoscale water inside reverse micellar confinement influences the microscopic dynamics and the structural relaxation of water···water hydrogen bonds and its impact on the low-frequency intermolecular vibrational bands. We have noticed from the results obtained from simulated trajectories the lowering trends of tetrahedral ordering of water pools in reverse micellar confinements as we move from bulk to confined and strictly confined environments. We have observed that the order of confinements significantly altered the relaxation pattern of water···water hydrogen bonds present in the nanoscale water pool of reverse micelles. The recrossing related to hydrogen bond dynamics can effectively explain the relaxation pattern of C HB WW ( t ) under confinement. The Br-1···water hydrogen bond depicts a much slower relaxation compared to the water···water hydrogen bonds inside reverse micelles. We have also explored the correlation between the tetrahedral ordering of nanoscale water pools and the relaxation of water···water hydrogen bonds with the 50 cm-1 band for water inside reverse micelles. The computations reported that compared to bulk water, the band appearing at 50 cm-1 for O···O···O triplet bending is nonuniformly blue-shifted by 18-45 cm-1 for the nanoscale water pool inside reverse micelles, and the intensity of the band drops from bulk to confined and strictly confined environments, which indicates the reduced tendency of such triplet formation. It is observed that a significant intensity variation at the 200 cm-1 band correlates with the effect of confinement on the tetrahedral ordering of the water pool inside reverse micelles. So, our observations support the influence of strictly confined environments on the tetrahedral water structure to adopt the quasi-two-dimensional water network and experience restricted longitudinal translations. It is further noticed that the 500 cm-1 librational band is also found to be blue-shifted by 71-112 cm-1 for the water pool in reverse micelles, and the extent of the shift being more noticeable for strictly confined environments correlates excellently with the sluggish relaxation of water···water hydrogen bonds in such environments.

Similar structure but different thermodynamic, dielectric, and frictional properties of confined water in twisted 2D materials: MoS2vs. graphene

Water-based nanofluidic devices, where water is confined in Angstrom scale nanochannels, are widely encountered in nanotechnology. Although it is known that the material of confinement has a significant influence on the properties of confined water, much less is known of the relationship between the structure of nanoconfined water and its properties, impacting the design of nanofluidic devices. We explore the behavior of a confined water monolayer within a bilayer molybdenum disulfide (MoS2) structure, comparing its behavior with that within bilayer graphene. We find that only ∼2% of the entire structure has nearly perfect square order and the rest is filled with rhombus ordering. Surprisingly, we find that although the structure of monolayer confined water remains the same in both the 2D materials, thermodynamic analysis shows that confined water has a more favorable potential environment in MoS2 than graphene for all twists explored here. However, with increasing twist angle, the encapsulating effect of water diminishes slightly in the case of graphene than MoS2. Interestingly, the dielectric constant is anomalously lower in MoS2 by ∼22% compared to the confined water dielectric constant in a graphene nanochannel. Finally, we show that the static friction coefficient of confined water in bilayer MoS2 does not change with twist. However, unlike graphene, it does not show an order of magnitude reduction due to this extreme confinement. Overall, we show, counter-intuitively, that although confined water structures are similar in different 2D materials considered here, there exist differences in other properties of this structured water.

Desalination Performance in Janus Graphene Oxide Channels: Geometric Asymmetry vs Charge Polarity

Improving the desalination performance of membranes is always in the spotlight of scientific research; however, Janus channels with polarized surface charge as nanofiltration membranes are still unexplored. In this work, using molecular dynamics simulations, we demonstrate that Janus graphene oxide (GO) channels with appropriate geometry and surface charge can serve as highly efficient nanofiltration membranes. We observe that the water permeability of symmetric Janus GO channels is significantly superior to that of asymmetric channels without sacrificing much ion rejection, owing to weakened ion blockage and electrostatic effects. Furthermore, in symmetric Janus GO channels, the transport of water and ions is sensitive to the charge polarity of the channel inner surface, which is realized by tuning the ratio of cationic and anionic functionalization. Specifically, with the increase in cationic functionalization, the water flux decreases monotonously, while ion rejection displays an interesting maximum behavior that indicates desalination optimization. Moreover, the trade-off between water permeability and ion rejection suggests that the Janus GO channels have an excellent desalination potential and are highly tunable according to the specific water treatment requirements. Our work sheds light on the key role of channel geometry and charge polarity in the desalination performance of Janus GO channels, which paves the way for the design of novel desalination devices.

Does twist angle affect the properties of water confined inside twisted bilayer graphene?

Graphene nanoslit pores are used for nanofluidic devices, such as, in water desalination, ion-selective channels, ionic transistors, sensing, molecular sieving, blue energy harvesting, and protein sequencing. It is a strenuous task to prepare nanofluidic devices, because a small misalignment leads to a significant alteration in various properties of the devices. Here, we focus on the rotational misalignment between two parallel graphene sheets. Using molecular dynamics simulation, we probe the structure and dynamics of monolayer water confined inside graphene nanochannels for a range of commensurate twist angles. With SPC/E and TIP4P/2005 water models, our simulations reveal the independence of the equilibrium number density- n ∼ 13 nm^-2 for SPC/E and n ∼ 11.5 nm^-2 for TIP4P/2005- across twists. Based on the respective densities of the water models, the structure and dielectric constant are invariant of twist angles. The confined water structure at this density shows square ice ordering for SPC/E water only. TIP4P/2005 shows ordering at the vicinity of a critical density (n ∼ 12.5 nm^-2). The average perpendicular dielectric constant of the confined water remains anomalously low (∼2 for SPC/E and ∼6 for TIP4P/2005) for the studied twist angles. We find that the friction coefficient of confined water molecules varies for small twist angles, while becoming independent for twists greater than 5.1°. Our results indicate that a small, angular misalignment will not impair the dielectric properties of monolayer water within a graphene slit-pore, but can significantly influence its dynamics.

Water confinement in small polycyclic aromatic hydrocarbons

The confinement of water molecules is vital in fields from biology to nanotechnology. The conditions allowing confinement in small finite polycyclic aromatic hydrocarbons (PAHs) are unclear, yet are crucial for understanding confinement in larger systems. Here, we report a computational study of water cluster confinement within PAHs dimers. Our results serve as a model for larger carbon allotropes and for understanding molecular interactions in confined systems. We identified size and structural motifs allowing confinement and demonstrated the motifs in various PAHs systems. We show that optimal OH⋯π interactions between water clusters and the PAH dimer permit optimal confinement to occur. However, the lack of such interactions leads to the formation of CH⋯O interactions, resulting in less ideal confinement. Confinement of layered clusters is also possible, provided that the optimal OH⋯π interactions are conserved.

Influence of the extent of hydrophobicity on water organization and dynamics on 2D graphene oxide surfaces

Graphene oxide (GO) nanomaterials are being extensively explored for a wide spectrum of applications, ranging from water desalination to fuel cell applications due to their tunable mechanical, thermal, and electrical...

Amphiphilicity of Intricate Layered Graphene/g-C3N4 Nanosheets

The hybrid heterostructure of the tri-s-triazine form of graphitic carbon nitride (g-C₃N₄), a stable two-dimensional material, results from intricate layer formation with graphene. In this material, g-C₃N₄, an amphiphilic material, stabilizes Pickering emulsions as an emulsifier and can effectively disperse graphene. Due to the various technological applications of the hybrid nanosheets in an aqueous environment, it is essential to study the interaction of water molecules with graphene and g-C₃N₄ (Gr/g-C₃N₄)-combined heterostructure. Although few studies have been performed signifying the water orientation in the interfacial layer, we find that there is a lack of detailed studies using various dynamical and structural properties of the interfacial water molecules. The interface of the Gr/g-C₃N₄ hybrid structure, one of the rarely found amphiphilic interfaces (on the g-C₃N₃ side), is appropriate for exploring the water affinity due to the availability of heterogeneous interfacial aqueous interactions. We adopted classical molecular dynamics simulations using two models for water molecules to study the structure and dynamics of an aqueous interface. We have correlated the structural properties to dynamics and spectral properties to understand the overall behavior of the amphiphilic interface. Our results branch into two significant hydrogen bond (HB) properties in HB count and HB strength among the water molecules in the different layers. The HB counts in the different layers of water are correlated using the average distance distribution (P_rO4), tetrahedral order parameters, HB donor/acceptor count, and total HBs per water molecule. A conspicuous difference is found in the HB count and related dynamics of the system. The HB lifetime and diffusion coefficient hint at the equivalent strength of HBs in the different layers. All the findings conclude that the amphiphilicity of the Gr/g-C₃N₄ interface can help in understanding various interfacial physical and chemical processes.

Aqueous Affinity and Interfacial Dynamics of Anisotropic Buckled Black Phosphorous

The structure of black phosphorous (BP) is similar to the honeycomb arrangement of graphene, but the layered BP is found to be buckled and highly anisotropic. The buckled surface structure affects interfacial molecule mobility and plays a vital role in various nanomaterial applications. The BP is also known for wettability, droplet formation, stability, and hydrophobicity in the aqueous environment. However, there is a gap concerning the structural and dynamical behavior of water molecules, which is available in abundance for other monoatomic and polyatomic two-dimensional (2D) materials. Motivated by the technological importance, we try to bridge the gap by explaining the surface anisotropy-facilitated behavior of water molecules on bilayer BP using classical and first principles molecular dynamics (MD) simulations. From our classical MD study, we find three distinct layers of water molecules. The water layer closest to the interface is L1, followed by L2 and L3/bulk perpendicular to the BP surface. Water molecules in the L1 layer experience some structural disintegration in hydrogen bond (HB) phenomena compared to the bulk. There is a loss of HB donor-acceptor count per water molecule. The average HB count decreases because of an elevated rate of HB formation and deformation; this would affect the dynamic properties in terms of HB lifetime. Therefore, we observe the reduced lifetime of HB in the layer in close contact with BP, which again complements our finding on the diffusion coefficient of water molecules in distinct layers. Water diffuses relatively faster with diffusion coefficient 3.25 × 10^-9 m² s^-1 in L1, followed by L2 and L3. The BP layer shows moderate hydrophobic nature. Our results also indicate the anisotropic behavior as the diffusion along the x-direction is faster than that along the y-direction. The gap in the slope of the x and y components of mean-squared displacement (MSD) complements the pinning effect in an aqueous environment. We observe blue-shifted and red-shifted libration and O-H stretching modes from the calculated power spectra for the L1 water molecules compared to the L2 and L3 molecules from first principles MD simulations. Our analysis may help understand the physical phenomena that occur during the surface wetting of the predroplet formation process observed experimentally.

Dielectric Profile and Electromelting of a Monolayer of Water Confined in Graphene Slit Pore

A monolayer of water confined between two parallel graphene sheets exists in many different phases and exhibits fascinating dielectric properties that have been studied in experiments. In this work, we use molecular dynamics simulations to study how the dielectric properties of a confined monolayer of water is affected by its structure. We consider six of the popular nonpolarizable water models-SPC/E, SPC/Fw, TIP3P, TIP3P_M (modified), TIP4P-2005, and TIP4P-2005f-and find that the in-plane structure of the water molecules at ambient temperature and pressure is strongly dependent on the water model: all the 3-point water models considered here show square ice formation, whereas no such structural ordering is observed for the 4-point water models. This allows us to investigate the role of the in-plane structure of the water monolayer on its dielectric profile. Our simulations show an anomalous perpendicular dielectric constant compared to the bulk, and the models that do not exhibit ice formation show very different dielectric response along the channel width compared to models that exhibit square ice formation. We also demonstrate the occurrence of electromelting of the in-plane ordered water under the application of a perpendicular electric field and find that the critical field for electromelting strongly depends on the water model. Together, we have shown the dependence of confined water properties on the different water structures that it may take when sandwiched between bilayer graphene. These remarkable properties of confined water can be exploited in various nanofluidic devices, artificial ion channels, and molecular sieving.

Comparative first principles-based molecular dynamics study of catalytic mechanism and reaction energetics of water oxidation reaction on 2D-surface

The study of the water-splitting process, which can proceed in 2e^- as well as 4e^- pathway, reveals that the process is entirely an uphill process, and the third step, that is, the oxooxo bond formation is the rate-determining step. The kinetic barrier of the oxygen evolution reaction (OER) on the 2D material catalysts in the presence of explicit solvents is scarcely studied. Here, we investigate the dynamics of the OER on the undoped graphene and the activation energy barrier of each step using first principles molecular dynamics simulations. Here we provide a detailed analysis of the kinetics of all the 4e^- transfer steps of OER on the graphene surface. We also compare the accuracy of one of the density functional theory (DFT) functionals and density functional based tight binding (DFTB) method in explaining the OER steps. The comparative study reveals that DFTB can be used for performing metadynamics simulations quipped with much less computational cost than DFT functionals. By both Perdew-Burke-Ernzerhof and DFTB methods, the third step is revealed to be the rate-determining step with an energy barrier of 21.19 ± 0.51 and 20.23 ± 0.20 kcal mol^-1 , respectively. DFTB gives an impression of being successful in predicting the energy barriers of OER in 4e- transfer pathway and comparable to the DFT method, and we would like to extend the use of DFTB for further studies with a sizable and complex system.

Self-assembly of graphene oxide sheets: the key step toward highly efficient desalination

Lamellar graphene oxide (GO) membranes are new membrane materials for seawater desalination due to their selective sub-nanometer interlayer two-dimensional channels. In general, the reliable and precise desalination of GO membranes is still heavily dependent on thick membranes that usually have a low water flux. The trade-off between the water flux and ion rejection is a long-lasting problem that restricts the development of highly efficient desalination membranes. In this work, we theoretically predicted that this trade-off can be broken by the self-assembly of GO sheets during the membrane preparation. Our molecular dynamics (MD) simulations indicate that the high-water permeability of the GO membrane is due to the frictionless flow of water in the 2D nanochannels enclosed by the non-oxidized regions of neighboring GO sheets, while the oxidized regions are responsible for the high ion rejection rate. Meanwhile, the MD simulations of the self-assembly processes of GO sheets in aqueous solutions just demonstrate that the oxidized regions of neighboring GO sheets are prone to stacking with each other, while the non-oxidized regions of neighboring GO sheets are inclined to matching with each other. Therefore, more interlayer nanochannels for fast water flow and ion rejection will be formed, respectively, after the full assembly of GO sheets during membrane preparation. Finally, based on our results, a new but simple method has been proposed to prepare GO membranes with superior desalination performance via deposition rate control.

Relationship between Desalination Performance of Graphene Oxide Membranes and Edge Functional Groups

High desalination efficiency in principle could be achieved by layer-by-layer graphene oxide (GO) membranes, which benefits from their entrance-functionalized channels assembled by edge-functionalized GO nanosheets. The effects of these edge functional groups on desalination, however, are not fully understood yet. To study the isolated influence of three typical edge functional groups, namely, carboxyl (-COOH), hydroxyl (-OH), and hydrogen (-H), molecular dynamics simulation was used in this work. The results revealed that the edge volumetric blockage effect, resulting in ion permeability at G-H > G-OH > G-COOH membranes, was the dominant mechanistic effect inside the GO membranes with 7 Å interlayer channels. The OH edge has the same effect as the H edge in NaCl/water selectivity because of a unique "ion pulling" effect. Moreover, the OH and H edge-functionalized membranes with 7 Å interlayer channels showed preferential Na⁺ and Cl^- rejections, respectively. This kind of preference leads to a cycle of charging and neutralization in the penetrant reservoir throughout the filtration process. The results from this work suggested that it would be strategic to keep the COOH and H edge functional groups, to maintain the size of interlayer channels in order to stimulate the effects of edge functional groups, and to increase the membrane porosity for designing higher desalination efficiency GO membranes.

Influence of surface hydrophilicity and hydration on the rotational relaxation of supercooled water on graphene oxide surfaces

The presence of a bulk water film influences the dynamical transitions of supercooled water on graphene oxide surfaces.