Effect of nanochannel dimension on the transport of water molecules

Ferroelectricity of ice nanotube forests grown in three-dimensional graphene: the electric field effect

Generating diverse ferroelectric ice nanotubes (NTs) efficiently has always been challenging, but matters in nanomaterial synthesis and processing technology. In the present work, we propose a method of growing ice NT forests in a single cooling process. A three-dimensional (3D) graphene structure was selected to behave as a representative container in which a batch of (5, 0) ice NTs was formed simultaneously under the cooling process from molecular dynamics simulation. Other similar 3D graphene structures but with different hole configurations, like uniform triangle or both triangle and pentagon, were also tested, revealing that ice NTs with different tube indices, i.e. both (3, 0) and (5, 0), could also be formed at the same time. Intriguingly, the orientations of the dipole moments of the water molecules of an ice NT formed were independent of each other, making the net ferroelectricity of the whole system weakened or even cancelled. An electric field could help change the orientation of the water molecules of the already obtained ice NTs and even twist the tube to be a spiral (5, 1) one if it was applied during the cooling process, such that the net ferroelectricity was greatly improved. The underlying physical mechanism of all phase transition phenomena, including the improvement of the ferroelectricity under an electric field, were explored in depth from the phase transition curves and structural point of view. The obtained results are of significant application value for improving the preparation efficiency of nano-ferroelectric materials, which are prosperous in nano-devices.

Modeling the influence of the external electric fields on water viscosity inside carbon nanotubes

Equilibrium molecular dynamics simulations were performed to explore the effects of external electric fields and confinement on water properties inside various carbon nanotubes (CNTs). Using different GHz electric field frequencies as well as various constant electric field strengths, the radial distribution function and density profile were investigated, by which the impact of the electric fields and confinement on the water structure are revealed. The results indicated water molecules inside the CNT form layered structures due to topological confinement applying external electric fields can disturb this ordered water molecules structure and increase the viscosity of confined water, particularly in the case of CNTs with a radius less than 13.5 Å. Conversely, for CNTs with a radius greater than13.5 Å, the viscosity decreases under the influence of external oscillating or constant electric fields. How dose the synergism of confinement and external electric fields affect the water properties inside the CNTs?

Modeling Ultrafast Transport of Water Clusters in Carbon Nanotubes

Carbon nanotubes can be used as ultrafast liquid transporters for water purification and drug delivery applications. In this study, we mathematically model the interaction between water clusters and carbon nanotubes using a continuum approach with the Lennard-Jones potential. Since the structure of water clusters depends on the confining material, this paper models the cluster as a cylindrical column of water molecules located inside a carbon nanotube. By assuming the system of two concentric cylinders, we derive analytical expressions for the interaction energy and force, which are used to determine the mechanics and physical parameters that optimize water transport in the nanotubes. Additionally, we adopt Verlet algorithm to investigate the ultrahigh-speed dynamics of water clusters inside carbon nanotubes. For a given carbon nanotube, we find that the cluster's length and the surface's wettability are important factors in controlling the dynamics of water transport. Our findings here demonstrate the possibility of using carbon nanotubes as effective nanopumps in water purification and nanomedical devices.

Structure-Thermodynamic Relationship of a Polysaccharide Gel (Alginate) as a Function of Water Content and Counterion Type (Na vs Ca)

Biofilms are the predominant mode of microbial life on Earth, and so a deep understanding of microbial communities─and their impacts on environmental processes─requires a firm understanding of biofilm properties. Because of the importance of biofilms to their microbial inhabitants, microbes have evolved different ways of engineering and reconfiguring the matrix of extracellular polymeric substances (EPS) that constitute the main non-living component of biofilms. This ability makes it difficult to distinguish between the biotic and abiotic origins of biofilm properties. An important route toward establishing this distinction has been the study of simplified models of the EPS matrix. This study builds on such efforts by using atomistic simulations to predict the nanoscale (≤10 nm scale) structure of a model EPS matrix and the sensitivity of this structure to interpolymer interactions and water content. To accomplish this, we use replica exchange molecular dynamics (REMD) simulations to generate all-atom configurations of ten 3.4 kDa alginate polymers at a range of water contents and Ca-Na ratios. Simulated systems are solvated with explicitly modeled water molecules, which allows us to capture the discrete structure of the hydrating water and to examine the thermodynamic stability of water in the gels as they are progressively dehydrated. Our primary findings are that (i) the structure of the hydrogels is highly sensitive to the identity of the charge-compensating cations, (ii) the thermodynamics of water within the gels (specific enthalpy and free energy) are, surprisingly, only weakly sensitive to cation identity, and (iii) predictions of the differential enthalpy and free energy of hydration include a short-ranged enthalpic term that promotes hydration and a longer-ranged (presumably entropic) term that promotes dehydration, where short and long ranges refer to distances shorter or longer than ∼0.6 nm between alginate strands.

Desalination Potential of Aquaporin-Inspired Functionalization of Carbon Nanotubes: Bridging Between Simulation and Experiment

Outstanding water/ion selectivity of aquaporins paves the way for bioinspired desalination membranes. Since the amino acid asparagine (Asn) plays a critical role in the fast water conduction of aquaporins through hydrogen bonding interactions, we adapted this feature by functionalizing carbon nanotubes (CNTs) with Asn. We also studied a nonpolar amino acid and carboxylate functional groups for comparison. Computation of the ideal performance of individual CNTs at atomistic scale is a powerful tool for probing the effect of tip-functionalized CNTs on water and ion transport mechanism. Molecular simulation study suggests that steric effects required for ion rejection compromise fast water conductivity; however, an Asn functional group having polarity and hydrogen bonding capability can be used to balance this trade-off to some extent. To test our hypothesis, we incorporated functionalized CNTs (f-CNTs) into the in situ polymerized selective polyamide (PA) layer of thin film nanocomposite membranes and compared their experimental RO desalination performance. The f-CNTs were found to change the separation environment through modification of cross-linking density, thickness, and hydrophilicity of the PA layer. Asn functionalization led to more cross-linked and thinner PA layer while hydrophilicity is improved compared to other functional groups. Accordingly, water permeance is increased by 25% relative to neat PA with a salt rejection above 98%. Starting from the nanomaterial itself and benefiting from molecular simulation, it is possible to design superior membranes suited for practical applications.

Flow-Induced Autonomic Ordering of Hydrogen Molecules under a Non-Equilibrium Flow

A non-equilibrium molecular flow through a carbon nanotube (CNT) serves as a key system for revealing molecular transport and establishing nanofluidics. It has been challenging to simulate a non-equilibrium flow of hydrogen molecules exhibiting strong nuclear quantumness. Taking advantage of the quantum molecular dynamics method that can calculate real-time trajectories of hydrogen molecules even under a non-equilibrium flow, we found that the non-equilibrium flow makes hydrogen molecules more condensed and accelerates their adsorption near a CNT surface, letting the molecules flow more smoothly by propagating velocity momenta more efficiently along the CNT axis and by suppressing transverse molecular dynamics on the CNT cross section. Such flow-induced autonomic ordering indicates the importance of monitoring and investigating dynamics and adsorption of hydrogen molecules under a non-equilibrium circumstance as well as in a quiet equilibrium state, opening a new strategy for efficient hydrogen liquefaction and storage.

A gel-like condensation of Cidec generates lipid-permeable plates for lipid droplet fusion

Membrane contact between intracellular organelles is important in mediating organelle communication. However, the assembly of molecular machinery at membrane contact site and its internal organization correlating with its functional activity remain unclear. Here, we demonstrate that a gel-like condensation of Cidec, a crucial protein for obesity development by facilitating lipid droplet (LD) fusion, occurs at the LD-LD contact site (LDCS) through phase separation. The homomeric interaction between the multivalent N terminus of Cidec is sufficient to promote its phase separation both in vivo and in vitro. Interestingly, Cidec condensation at LDCSs generates highly plastic and lipid-permeable fusion plates that are geometrically constrained by donor LDs. In addition, Cidec condensates are distributed unevenly in the fusion plate generating stochastic sub-compartments that may represent unique lipid passageways during LD fusion. We have thus uncovered the organization and functional significance of geometry-constrained Cidec phase separation in mediating LD fusion and lipid homeostasis.

Water molecules in CNT-Si3N4 membrane: Properties and the separation effect for water-alcohol solution

Water confined in carbon nanotubes (CNTs) has been intensively studied because of its unique properties and potential for various applications and is often embedded in silicon nitride (Si₃N₄) membranes. However, the understanding of the influence of Si₃N₄ on the properties of water in CNTs lacks clarity. In this study, we performed molecular dynamics simulations to investigate the effect of the Si₃N₄ membrane on water molecules inside CNTs. The internal electric field generated in the CNTs by the point charges of the Si₃N₄ membrane changes the structure and dynamical properties of water in the nanotubes, causing it to attain a disordered structure. The Si₃N₄ membrane decreases the diffusivity of water in the CNTs; this is because the Coulomb potential energy (i.e., electrostatic interaction) of water decreases owing to the presence of Si₃N₄, whereas the Lennard-Jones potential energy (i.e., van der Waals interaction) does not change significantly. Furthermore, electrostatic interactions make the water structure more stable in the CNTs. As a result, the Si₃N₄ membrane enhances the separation effect of the water-methanol mixture with CNTs in the presence of an external electric field. Furthermore, the threshold of the external electric field strength to induce water-methanol separation with CNTs is reduced owing to the presence of a silicon nitride membrane.

Highly efficient desalination performance of carbon honeycomb based reverse osmosis membranes unveiled by molecular dynamics simulations

Seawater desalination is vital to our modern civilization. Here, we report that the carbon honeycomb (CHC) has an outstanding water permeability and salt rejection in the seawater desalination, as revealed by molecular dynamics simulations. More than 92% of ions are rejected by CHC at applied pressures ranging from 50 to 250 MPa. CHC has a perfect salt rejection at pressures below 150 Mpa. On increasing the applied pressure up to 150 MPa, the salt rejection reduces only to 92%. Pressure, temperature and temperature gradient are noted to play a significant role in modulating the water flux. The water flux increases with pressure and temperature. With the introduction of a temperature gradient of 3.5 K nm^-1, the seawater permeability increases by 33% as compared to room temperature. The water permeability of the CHC is greater than other carbon materials and osmosis membranes including graphene (8.7 times) and graphyne (2.1 times). It indicates the significant potential of the CHC for commercial application in water purification.

Chemically Converted Graphene Nanosheets for the Construction of Ion-Exclusion Nanochannel Membranes

Water and ion transport in nanochannels is an intriguing topic that has been extensively investigated in several energy- and environment-related research fields. Recently developed two-dimensional (2D) materials are ideal building blocks for constructing confined nanochannels by self-stacking. Among these, graphene oxide (GO) is the most frequently employed as the starting material because of its excellent solution processability. Since solvation of the GO nanostructure usually impairs the function of nanochannels, in this study, chemically converted graphene was prepared using a one-step method, to simultaneously acquire the desired stability and functionality of the nanochannels. The confined channels with high charge densities are capable of excluding ∼90% NaCl solutes from water in a pressure-driven filtration process. This surpasses the performance of most GO desalination membranes reported in the literature. Thus, this study provides useful information for the feasible development of ion-exclusion nanochannel membranes based on the proposed nanochannel-confined charge repulsion mechanism.

Elucidation of high permeability water among VACNFs using molecular dynamics

The cause of the high permeability in the flow of water in CNT (carbon nanotube)-based nanoscale materials remains to be elucidated. In this study, water impregnation simulations outside the VACNFs were performed using the molecular dynamics method to investigate the factors that cause high permeability by virtually changing the force field parameters. As a result, the permeability coefficient increased with increasing CNT content (VC) in the slip flow region. For the constant VC, the smaller the intermolecular force between water and CNTs, the higher the permeability coefficient. Because the intermolecular forces between water and CNTs are smaller than those between water and water, it may have an effect on the high permeability phenomenon. Furthermore, in the present VC change, the arrangement structure of the water molecules changed from a disordered structure, such as bulk flow, to a chain structure in the impregnation direction, which is also considered a factor for the increase in the permeability. Therefore, both the intermolecular forces between water and CNTs and structural change in the arrangement of water molecules were factors in the high permeability phenomenon.

Metallicity-Dependent Ultrafast Water Transport in Carbon Nanotubes

Carbon nanotubes (CNTs) with hydrophobic and atomically smooth inner channels are promising for building ultrahigh-flux nanofluidic platforms for energy harvesting, health monitoring, and water purification. Conventional wisdom is that nanoconfinement effects determine water transport in CNTs. Here, using full-atomistic molecular dynamics simulations, it is shown that water transport behavior in CNTs strongly correlates with the electronic properties of single-walled CNTs (metallic (met) vs semiconducting (s/c)), which is as dominant as the effect of nanoconfinement. Three pairs of CNTs (i.e., (8,8)_met , 10.85 Å vs (9,7)_s/c , 10.88 Å; (9,8)_s/c , 11.53 Å vs (10,7)_met , 11.59 Å; and (9,9)_met , 12.20 Å vs (10,8)_s/c , 12.23 Å) are used to investigate the roles of diameter and metallicity. Specifically, the (9,8)_s/c can restrict the hydrogen-bonding-mediated structuring of water and give the highest reduction in carbon-water interaction energy, providing an extraordinarily high water flux, around 250 times that of the commercial reverse osmosis membranes and approximately fourfold higher than the flux of the state-of-the-art boron nitrate nanotubes. Further, the high performance of (9,8)_s/c is also reproducible when embedded in lipid bilayers as synthetic high-water flux porins. Given the increasing availability of high-purity CNTs, these findings provide valuable guides for realizing novel CNT-enhanced nanofluidic systems.

The Role of Interface Ions in the Control of Water Transport through a Carbon Nanotube

Controlling the water transport toward a given direction is still challenging, particularly due to thermal fluctuations of water motion at the nanoscale. While most of the previous works focus on the symmetric hydrophobic membrane systems, the role of the membrane in affecting the water transport remains largely unexplored. In this work, by using extensive molecular dynamics simulations, we find an interesting electropumping phenomenon, that is, the flowing counterions on an asymmetric hydrophobic-hydrophilic membrane can significantly drive the single-file water transport through a carbon nanotube, suggesting a nanometer water pump in a highly controllable fashion. The ion-water coupling motion in electric fields on the charged surface provides an indirect driving force for this pumping phenomenon. The water dynamics and thermal dynamics demonstrate a unique behavior with the change in electric fields, surface charge density, and even charge species. Particularly, due to the ion flux bifurcation for the positive and negative surfaces, the water dynamics such as the water flow, flux, and translocation time also exhibit similar asymmetry. Surprisingly, the positive surface charge induces an abnormal three-peak dipole distribution for the confined water and subsequent high flipping frequency. This can be attributed to the competition between the surface charge and interface water orientation on it. Our results indicate a new strategy to pump water through a nanochannel, making use of the counterion flowing on an asymmetric charged membrane, which are promising for future studies.

Nanoscopic insights of saline water in carbon nanotube appended filters using molecular dynamics simulations

Nanotube appended membranes are shown to be very promising due to their ultrafast water transport and very high salt rejection ability. Using classical molecular dynamics, the present study reports the nanoscopic assessment of various molecular events for nanotube-based desalination, which might be useful for nanoscale devices during process operation at the macroscopic scale. The characteristics of water and ion flow are discussed with varied strength of pressure gradient and salt concentration for different scales of confinement. The results revealed that the membranes comprising nanotubes of 1.0-1.1 nm diameter can be optimized for efficient water desalination with more than >95% salt rejection. Furthermore, the anomalies in water flux through nanotubes are linked with the hydration characteristics of ions inside CNTs. The results show the maximum hydration of confined ions inside the nanotubes, which indicated the minimum permeability of water due to freezing effects. Furthermore, the MD results revealed that akin to bulk phases, the mass transport through nanotubes can be linked with the component diffusivity in the medium. It has been demonstrated that not only the diffusivities of water and ions, but even the gradient of water to ion diffusivity might be utilized to predict and explore the experimental observations, which might be helpful in optimizing the operational regime in nanotube-based filtrations. Moreover, the thermodynamic characteristics of the flow are discussed in terms of the entropy of water and ions using the robust two-phase thermodynamic (2PT) method. The results reflect that the entropy of water is linked to the distortion of the hydrogen bond network inside the nanotube confinement, at the nanotube-water interface and at the bulk solution, whereas the entropy of ions seems to be majorly dominated by their oscillation. Also, the interconnection of hydration structure, mass flux and the diffusivity of water and ions along with their thermodynamic origin are discussed.

Cosolvent Impurities in SWCNT Nanochannel Confinement: Length Dependence of Water Dynamics Investigated with Atomistic Simulations

The advent of nanotechnology has seen a growing interest in the nature of fluid flow and transport under nanoconfinement. The present study leverages fully atomistic molecular dynamics (MD) simulations to study the effect of nanochannel length and intrusion of molecules of the organic solvent, hexafluoro-2-propanol (HFIP), on the dynamical characteristics of water within it. Favorable interactions of HFIP with the nanochannels comprised of single-walled carbon nanotubes traps them over time scales greater than 100 ns, and confinement confers small but distinguishable spatial redistribution between neighboring HFIP pairs. Water molecules within the nanochannels show clear signatures of dynamical slowdown relative to bulk water even for pure systems. The presence of HFIP causes further rotational and translational slowdown in waters when the nanochannel dimension falls below a critical length of 30 Å. The enhanced slowdown in the presence of HFIP is quantified from characteristic relaxation parameters and diffusion coefficients in the absence and presence of HFIP. It is finally seen that the net flow of water between the ends of the nanochannel shows a decreasing dependence with nanochannel length only when the number of HFIP molecules is small. These results lend insights into devising ways of modulating solvent properties within nanochannels with cosolvent impurities.

Prediction of fluid slip in cylindrical nanopores using equilibrium molecular simulations

We introduce an analytical method to predict the slip length (L _s) in cylindrical nanopores using equilibrium molecular dynamics (EMD) simulations, following the approach proposed by Sokhan and Quirke for planar channels [39]. Using this approach, we determined the slip length of water in carbon nanotubes (CNTs) of various diameters. The slip length predicted from our method shows excellent agreement with the results obtained from nonequilibrium molecular dynamics (NEMD) simulations. The data show a monotonically decreasing slip length with an increasing nanotube diameter. The proposed EMD method can be used to precisely estimate slip length in high slip cylindrical systems, whereas, L _s calculated from NEMD is highly sensitive to the velocity profile and may cause large statistical errors due to large velocity slip at the channel surface. We also demonstrated the validity of the EMD method in a BNNT-water system, where the slip length is very small compared to that in a CNT pore of similar diameter. The developed method enables us to calculate the interfacial friction coefficient directly from EMD simulations, while friction can be estimated using NEMD by performing simulations at various external driving forces, thereby increasing the overall computational time. The EMD analysis revealed a curvature dependence in the friction coefficient, which induces the slip length dependency on the tube diameter. Conversely, in flat graphene nanopores, both L _s and friction coefficient show no strong dependency on the channel width.

Effects of gas reservoir configuration and pore radius on shale gas nanoflow: A molecular dynamics study

We calculated methane transport through cylindrical graphite nanopores in cyclical steady-state flows using non-equilibrium molecular dynamics simulations. First, two typical gas reservoir configurations were evaluated: open (OS) and closed (CS) systems in which pores connect to the gas reservoir without/with a graphite wall parallel to the gas flow. We found that the OS configuration, which is commonly used to study nanoflows, exhibited obvious size effects. Smaller gas reservoir cross-sectional areas were associated with faster gas flows. Because Knudsen diffusion and slip flow in pores are interrupted in a gas reservoir that does not have walls as constraints, OSs cannot be relied upon in cyclical nanoflow simulations. Although CSs eliminated size effects, they introduced surface roughness effects that stem from the junction surface between the gas reservoir and the pore. To obtain a convergent nanoflow, the length of a side of the gas reservoir cross-section should be at least 2 nm larger than the pore diameter. Second, we obtained methane flux data for various pore radii (0.5-2.5 nm) in CSs and found that they could be described accurately using the Javadpour formula. This is the first direct molecular simulation evidence to validate this formula. Finally, the radial density and flow-velocity distributions of methane in CS pores were analyzed in detail. We tested pores with a radius between 0.5 nm and 2.5 nm and determined that the maximum ratio (∼34%) of slip flow to overall flow occurred in the pore with a radius of 1.25 nm. This study will aid in the design of gas reservoir configurations for nanoflow simulations and is helpful in understanding shale gas nanoflows.

Water flow in carbon nanotubes: The effect of tube flexibility and thermostat

Although the importance of temperature control in nonequilibrium molecular dynamics simulations is widely accepted, the consequences of the thermostatting approach in the case of strongly confined fluids are underappreciated. We show the strong influence of the thermostatting method on the water transport in carbon nanotubes (CNTs) by considering simulations in which the system temperature is controlled via the walls or via the fluid. Streaming velocities and mass flow rates are found to depend on the tube flexibility and on the thermostatting algorithm, with flow rates up to 20% larger when the walls are flexible. The larger flow rates in flexible CNTs are explained by a lower friction coefficient between water and the wall. Despite the lower friction, a larger solid-fluid interaction energy is found for flexible CNTs than for rigid ones. Furthermore, a comparison of thermostat schemes has shown that the Berendsen and Nosé-Hoover thermostats result in very similar transport rates, while lower flow rates are found under the influence of the Langevin thermostat. These findings illustrate the significant influence of the thermostatting methods on the simulated confined fluid transport.

Understanding the role of pore size homogeneity in the water transport through graphene layers

Graphene is a versatile 2D material and attracts an increasing amount of attention from a broad scientific community, including novel nanofluidic devices. In this work, we use molecular dynamics simulations to study the pressure driven water transport through graphene layers, focusing on the pore size homogeneity, realized by the arrangement of two pore sizes. For a given layer number, we find that water flux exhibits an excellent linear behavior with pressure, in agreement with the prediction of the Hagen-Poiseuille equation. Interestingly, the flux for concentrated pore size distribution is around two times larger than that of a uniform distribution. More surprisingly, under a given pressure, the water flux changes in an opposite way for these two distributions, where the flux ratio almost increases linearly with the layer number. For the largest layer number, more distributions suggest the same conclusion that higher water flux can be attained for more concentrated pore size distributions. Similar differences for the water translocation time and occupancy are also identified. The major reason for these results should clearly be due to the hydrogen bond and density profile distributions. Our results are helpful to delineate the exquisite role of pore size homogeneity, and should have great implications for the design of high flux nanofluidic devices and inversely the detection of pore structures.

A Visualization Technique of a Unique pH Distribution around an Ion Depletion Zone in a Microchannel by Using a Dual-Excitation Ratiometric Method

The ion depletion zone of ion concentration polarization has a strong potential to act as an immaterial barrier, separating delicate submicron substances, including biomolecules, without causing physical damage. However, the detailed mechanisms of the barrier effect remain incompletely understood because it is difficult to visualize the linked behavior of protons, cations, anions, and charged molecules in the thin ion depletion zone. In this study, pH distribution in an ion depletion zone was measured to estimate the role of proton behavior. This was done in order to use it as a tool with good controllability for biomolecule handling in the future. As a result, a unique pH peak was observed at several micrometers distance from the microchannel wall. The position of the peak appeared to be in agreement with the boundary of the ion depletion zone. From this agreement, it is expected that the pH peak has a causal connection to the barrier effect of the ion depletion zone.

Engineered Transport in Microporous Materials and Membranes for Clean Energy Technologies

Many forward-looking clean-energy technologies hinge on the development of scalable and efficient membrane-based separations. Ongoing investment in the basic research of microporous materials is beginning to pay dividends in membrane technology maturation. Specifically, improvements in membrane selectivity, permeability, and durability are being leveraged for more efficient carbon capture, desalination, and energy storage, and the market adoption of membranes in those areas appears to be on the horizon. Herein, an overview of the microporous materials chemistry driving advanced membrane development, the clean-energy separations employing them, and the theoretical underpinnings tying membrane performance to membrane structure across multiple length scales is provided. The interplay of pore architecture and chemistry for a given set of analytes emerges as a critical design consideration dictating mass transport outcomes. Opportunities and outstanding challenges in the field are also discussed, including high-flux 2D molecular-sieving membranes, phase-change adsorbents as performance-enhancing components in composite membranes, and the need for quantitative metrologies for understanding mass transport in heterophasic materials and in micropores with unusual chemical interactions with analytes of interest.

Investigation of Transport Properties of Water-Methanol Solution through a CNT with Oscillating Electric Field

Molecular dynamics simulations were used to investigate the transport properties of water-methanol solution getting through a carbon nanotube (CNT) with an oscillating electric field. Eight alternating electric fields with different oscillation periods were used in this work. Under the oscillating electric field, water molecules have the advantage of occupying a CNT over methanol molecules. Meanwhile, the space occupancy of water-methanol solution in the CNT increases as the oscillating period increases. More importantly, we found that the oscillating period of electric field affects the van der Waals interaction of the solution inside the CNT and the shell of the CNT, which results in the change in the number of hydrogen bonds in the water-methanol solution confined in the CNT. And the change in the hydrogen-bond network leads to the change in transport properties of water-methanol solution.

Thermal conductivity of liquid argon in nanochannels from molecular dynamics simulations

The thermal conductivity of liquid argon in nanochannels has been calculated over a wide range of densities using two independent methods-the Green-Kubo approach in equilibrium molecular dynamics simulations and the Müller-Plathe method in non-equilibrium molecular dynamics simulations. The Lennard-Jones potential was used to model interatomic interactions. The influence of transversal size and shape of a nanochannel on the thermal conductivity of liquid argon along the length of the channel has been investigated. The transversal size of nanochannel varied from 2.25 nm to 15 nm. The simulations revealed that the thermal conductivity weakly depends on the shape (square vs circular) of channel and scales with a cross-sectional area of nanochannel. It has been observed that thermal conductivity increases with an increase of the transversal size of the channel. Also, it reaches bulk values for some characteristic size of channel that depends strongly on density. Good agreement of the computed thermal conductivities of liquid argon over a wide density range with the experimental data allowed the value of the characteristic size of channel as a function of density to be estimated. This value depends on density and varies from 5 nm to 11 nm.

Structure and dynamics of water confined in a graphene nanochannel under gigapascal high pressure: dependence of friction on pressure and confinement

The friction coefficient at the water/graphene interface is dependent on the lateral pressure and nanochannel height under gigapascal high-pressure.

Interface nanoparticle control of a nanometer water pump

A nanoparticle is forced to move on a membrane surface, inducing considerable water flux through a carbon nanotube, suggesting a controllable nanometer water pump.

Simulation of water impregnation through vertically aligned CNT forests using a molecular dynamics method

The flow rate of water through carbon nanotube (CNT) membranes is considerably large. Hence, CNT membranes can be used in nanofluidic applications. In this work, we performed a molecular dynamics (MD) simulation of the introduction of water into CNTs in the CNT membranes, especially in vertically aligned CNT forests. The results showed that the Knudsen number (Kn) increased with an increasing volume fraction of CNT (V_C) and was greater than 10⁻³ for each V_C. Beyond this value, the flow became a slip flow. Further, the permeability increased as V_C increased in the actual state calculated by the MD simulation, whereas the permeability in the no-slip state predicted by the Hagen–Poiseuille relationship decreased. Thus, a clear divergence in the permeability trend existed between the states. Finally, the flow enhancement ranged from 0.1 to 23,800, and the results show that water easily permeates as V_C increases.

A current-driven nanometer water pump

The design of a water pump, which has huge potential for applications in nanotechnology and daily life, is the dream of many scientists. In this paper, we successfully design a nanometer water pump by using molecular dynamics simulations. Ions of either sodium or chlorine in a narrow channel will generate electric current under electric fields, which then drives the water through a wider channel, similar to recent experimental setups. Considerable water flux is achieved within small field strengths that are accessible by experimentation. Of particular interest, is that for sodium the water flux increases almost linearly with field strengths; while for chlorine there exists a critical field strength, the water flux exhibits a plateau before the critical value and increases linearly after it. This result follows the behavior of ion velocity, which is related to friction behavior. We also estimate the power and energy consumption for such a pump, and compare it to the macroscopic mechanical pumps. A further comparison suggests that different ions will have different pumping abilities. This study not only provides new, significant results with possible connection to existing research, but has tremendous potential application in the design of nanofluidic devices.

A computational assessment of the permeability and salt rejection of carbon nanotube membranes and their application to water desalination

Membranes made from nanomaterials such as nanotubes and graphene have been suggested to have a range of applications in water filtration and desalination, but determining their suitability for these purposes requires an accurate assessment of the properties of these novel materials. In this study, we use molecular dynamics simulations to determine the permeability and salt rejection capabilities for membranes incorporating carbon nanotubes (CNTs) at a range of pore sizes, pressures and concentrations. We include the influence of osmotic gradients and concentration build up and simulate at realistic pressures to improve the reliability of estimated membrane transport properties. We find that salt rejection is highly dependent on the applied hydrostatic pressure, meaning high rejection can be achieved with wider tubes than previously thought; while membrane permeability depends on salt concentration. The ideal size of the CNTs for desalination applications yielding high permeability and high salt rejection is found to be around 1.1 nm diameter. While there are limited energy gains to be achieved in using ultra-permeable CNT membranes in desalination by reverse osmosis, such membranes may allow for smaller plants to be built as is required when size or weight must be minimized. There are diminishing returns in further increasing membrane permeability, so efforts should focus on the fabrication of membranes containing narrow or functionalized CNTs that yield the desired rejection or selection properties rather than trying to optimize pore densities.

The good, the bad and the user in soft matter simulations

Molecular dynamics (MD) simulations have become popular in materials science, biochemistry, biophysics and several other fields. Improvements in computational resources, in quality of force field parameters and algorithms have yielded significant improvements in performance and reliability. On the other hand, no method of research is error free. In this review, we discuss a few examples of errors and artifacts due to various sources and discuss how to avoid them. Besides bringing attention to artifacts and proper practices in simulations, we also aim to provide the reader with a starting point to explore these issues further. In particular, we hope that the discussion encourages researchers to check software, parameters, protocols and, most importantly, their own practices in order to minimize the possibility of errors. The focus here is on practical issues. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Molecular dynamics study of pressure-driven water transport through graphene bilayers

Water molecules form layered structures inside graphene bilayers and ultra-high pressure-driven flow rates can be observed.

Tailoring the water structure and transport in nanotubes with tunable interiors

Self-assembly of cyclic peptide nanotubes (CPNs) in polymer thin films has opened up the possibility of creating separation membranes with tunable nanopores that can differentiate molecules at the sub-nanometer level. While it has been demonstrated that the interior chemistry of the CPNs can be tailored by inserting functional groups in the nanopore lumen (mCPNs), a design strategy for picking the chemical modifications that lead to particular transport properties has not been established. Drawing from the knowledgebase of functional groups in natural amino acids, here we use molecular dynamics simulations to elucidate how bioinspired mutations influence the transport of water through mCPNs. We show that, at the nanoscale, factors besides the pore size, such as electrostatic interactions and steric effects, can dramatically change the transport properties. We recognize a novel asymmetric structure of water under nanoconfinement inside the chemically functionalized nanotubes and identify that the small non-polar glycine-mimic groups that minimize the steric constraints and confer a hydrophobic character to the nanotube interior are the fastest transporters of water. Our computationally developed experiments on a realistic material system circumvent synthetic challenges, and lay the foundation for bioinspired principles to tailor artificial nanochannels for separation applications such as desalination, ion-exchange and carbon capture.

Removal of heavy metals from water through armchair carbon and boron nitride nanotubes: a computer simulation study

Number of heavy metals permeation from the (7,7) CNT and the (7,7) BNNT in the applied voltages.

Electricity resonance-induced fast transport of water through nanochannels

We performed molecular dynamics simulations to study water permeation through a single-walled carbon nanotube with electrical interference. It was found that the water net flux across the nanochannel is greatly affected by the external electrical interference, with the maximal net flux occurred at an electrical interference frequency of 16670 GHz being about nine times as high as the net flux at the low or high frequency range of (<1000 GHz or >80,000 GHz). The above phenomena can be attributed to the breakage of hydrogen bonds as the electrical interference frequency approaches to the inherent resonant frequency of hydrogen bonds. The new mechanism of regulating water flux across nanochannels revealed in this study provides an insight into the water transportation through biological water channels and has tremendous potential in the design of high-flux nanofluidic systems.

What have we learnt about the mechanisms of rapid water transport, ion rejection and selectivity in nanopores from molecular simulation?

Nanopores have demonstrated an extraordinary ability to allow water molecules to pass through their interiors at rates far exceeding expectations based on continuum theory. Moreover, simulation studies suggest that particular nanoscale pores have the potential to discriminate between water and salts as well as to distinguish between a range of different ion types. Some of the unusual features of transport in these nanopores have been elucidated with molecular dynamics simulation, specifically the spontaneous filling and rapid transport of water, the rejection of ions and the selection between ions. The main focus of this review, however, is the physical mechanisms which act to produce such remarkable behaviour at this scale, drawing on the many studies that have been conducted in the last decade. Since molecular dynamics simulations allow the motion of individual atoms to be followed over time, they have the potential to provide fundamental insight into the reasons why transport in nanoscale pores differs from expectations based on macroscopic theory. Gaining an understanding of the mechanisms of transport in these tiny pores should guide future experiments in this area aimed at developing novel technologies and improving existing membrane separation techniques.

Water desalination through armchair carbon nanotubes: a molecular dynamics study

Separation of ions from water using armchair carbon nanotubes.

Long-term live cell microscopy studies of lipid droplet fusion dynamics in adipocytes

During the adipogenic differentiation process of mesenchymal stem cells, lipid droplets (LDs) grow slowly by transferring lipids between each other. Recent findings hint at the possibility that a fusion pore is involved. In this study, we analyze lipid transfer data obtained in long-term label-free microscopy studies in the framework of a Hagen-Poiseuille model. The data obtained show a LD fusion process in which the lipid transfer directionality depends on the size difference between LDs, whereas the respective rates depend on the size difference and additionally on the diameter of the smaller LDs. For the data analysis, the viscosity of the transferred material has to be known. We demonstrate that a viscosity-dependent molecular rotor dye can be used to measure LD viscosities in live cells. On this basis, we calculate the diameter of a putative lipid transfer channel which appears to have a direct dependence on the diameter of the smaller of the two participating LDs.