Carbon Nanotubes in Water: Structural Characteristics and Energetics

Molecular Simulation Study on the Wettability of a Surface Texturized with Hierarchical Pillars

By using molecular dynamics simulation, we investigate the wettability of a surface texturized with a periodic array of hierarchical pillars. By varying the height and spacing of the minor pillars on top of major pillars, we investigate the wetting transition from the Cassie-Baxter (CB) to Wenzel (WZ) states. We uncover the molecular structures and free energies of the transition and meta-stable states existing between the CB and WZ states. The relatively tall and dense minor pillars greatly enhance the hydrophobicity of a pillared surface, in that, the CB-to-WZ transition requires an increased activation energy and the contact angle of a water droplet on such a surface is significantly larger.

Water flow in graphene nanochannels driven by imposed thermal gradients: the role of flexural phonons

Accurate control of fluid transport in nanoscale structures is key to enable the design of foreseeable nanofluidic devices with applications in many fields such as chip cooling, energy conversion, drug delivery and medical diagnosis. Here, inspired by the experimental observation of intrinsic thermal ripples in graphene and by recent advances in the manipulation of 2D nanomaterials, we introduce a graphene-based thermal nanopump which produces controlled and continuous liquid flow in nanoslit channels. We investigate the performance of this thermal nanopump employing large scale molecular dynamics simulations. Upon systematically imposing thermal gradients, a net water flow towards the low-temperature zone is observed, achieving flow velocities up to 4 m s^-1. We observe that water flow rates increase monotonically due to larger ripple fluctuations on the graphene layers as higher thermal gradients are applied. Moreover, we find that the out-of-plane flexural phonons in graphene are responsible for flow generation wherein lower frequency phonon branches are activated with higher imposed thermal gradients. Furthermore, by modifying the wettability of the channel walls, an increase of 50% in the water flow rates is observed, showing that the efficiency of the proposed thermal pump can be enhanced by tuning the channel wall hydrophobicity. Our results indicate that thermal gradients can be employed to drive continuous water flow in graphene nanoslit channels with potential applications in nanofluidic devices.

Adjustable high-speed and directional diffusion of water nanodroplets confined by graphene sheets

Diffusion of confined water is important in nanofluidic and other water transport systems. In this study, the diffusion of water nanodroplets confined by graphene sheets is investigated based on molecular dynamics simulations. We find that the confined water nanodroplets can achieve a high-speed and directional motion. The impact of the size of water nanodroplets and distance of graphene sheets on diffusion is studied. The results show that the diffusion of confined water nanodroplets is adjustable and the speed is about 3 orders of magnitude faster than that of the self-diffusing water molecules in liquid water. Subsequently, the most suitable morphology of confined nanodroplets for rapid movement is found. We also find that the direction of diffusion of confined water nanodroplets is affected by the thermal vibrations of carbon atoms. Finally, the interaction energy and friction coefficient between confined nanodroplets and graphene sheets are analyzed to give an insight into the fast and directional diffusion behaviors of water nanodroplets. Our results reveal that a variation in the structure of interfacial water molecules with the distance of graphene sheets is the key to the rapid movement of confined water nanodroplets. The phenomena reported here can enrich the knowledge of molecular mechanisms for nanoconfined water systems, and may stimulate more ideas for the rapid removal of confined water.

A data driven approach to model thermal boundary resistance from molecular dynamics simulations

A new method is proposed to model the thermal boundary resistance (TBR) at the nanoscale, solid-liquid interface from macroscopic observables that characterize a nanoscale interface. We correlated the TBR with thermodynamic state variables, material properties, and geometric parameters to derive a generalized relationship with the help of data-driven heuristic algorithms. The results show that TBR can be expressed in terms of physical observables of the systems and material-specific parameters. We investigated the mutual independence of descriptor variables and quantified the weightage for each observable parameter in the TBR models. The interfacial liquid layering has a robust correlation with TBR. However, for systems with phonon size effects and under extreme thermodynamic conditions, the work of adhesion and system geometry also affects the variation in TBR. The data-driven approach followed in this study helps us gain better insight into the mechanism of TBR at nanoscale solid-liquid interfaces and shows significant improvement in our knowledge about interfacial thermal transport.

Development of accurate potentials for the physisorption of water on graphene

From coupled-cluster singles and doubles model including connected triples corrections [CCSD(T)] calculations on the water dimer and B97D/CC on the water-circumcoronene complex at a large number of randomly generated conformations, interaction potentials for the physisorption of water on graphene are built, accomplishing almost sub-chemical accuracy. The force fields were constructed by decomposing the interaction into electrostatic and van der Waals contributions, the latter represented through improved Lennard-Jones potentials. Besides, a Chemistry at Harvard Macromolecular Mechanics (CHARMM)-like term was included in the water-water potential to improve the description of hydrogen bonds, and an induction term was added to model the polarization effects in the interaction between water and polyaromatic hydrocarbons (PAHs) or graphene. Two schemes with three and six point charges were considered for the interactions water-water and water-PAH, as Coulomb contributions are zero in the water-graphene system. The proposed fitted potentials reproduce the ab initio data used to build them in the whole range of distances and conformations and provide results for selected points very close to CCSD(T) benchmarks. When applied to the water-graphene system, the obtained results are in excellent agreement with p-CCSD(T), revised symmetry-adapted perturbation theory based on density functional theory monomer properties (DFT-SAPT), and diffusion Monte Carlo reference values. Furthermore, the stability of the various conformers water-PAH and water-graphene, as well as the different trends observed between these systems are rationalized in terms of the modifications of the electrostatic contribution.

Formation of Graphene Nanoscrolls and Their Electronic Structures Based on Ab Initio Calculations

Rolling up two-dimensional (2D) materials can form quasi-one-dimensional nanoscrolls, which are expected to have novel properties due to their larger space of structural parameters. In this Letter, the structural dependence of formation energy was investigated based on more than 90 different graphene nanoscrolls (GNSs) through ab initio calculations. A quantified relationship between formation energy and structural parameters is discovered, which could provide universal description of rolling up 2D materials beyond graphene. Further calculations on electronic structures show the opening of bandgap in GNSs with ultrahigh carrier mobilities up to 10⁷ cm² V^-1 s^-1. The structural stability under room temperature was also testified by using molecular dynamic simulations. This work provides general insights into the rolling-up strategy and demonstrates the tunable properties of GNSs, thus extending the scope of the research field for 2D materials.

Efficient water desalination through mono and bilayer carbon nitride nanosheet membranes: Insights from molecular dynamics simulation

Water desalination through membranes is an excellent way to access drinking water. Among two-dimensional nanosheet membranes for water desalination, carbon nitride (C2N) nanosheet has been considered as a promising membrane by researchers because of its inherent structure and mechanical strength. In this work, molecular simulations were used to study the efficiency of the pristine C2N nanosheet in the water desalination process using applied hydrostatic pressure to the system. In our simulation box, the C2N nanosheet was placed in the center of the simulation cell in an aqueous ionic solution. Due to the applied pressure to the system, water molecules overcame the forces that prevented them from passing through the C2N, and therefore, they passed through the C2N membrane. The water flux and water permeability of considered systems were obtained. Also, for more investigation, water density, radial distribution function of ions, the water density map, and hydrogen bonds of the system were conducted. The results demonstrated that the C2N membrane is an effective membrane for desalination even at low pressures with the acceptable water flux and salt rejection.

Efficient Removal of Heavy Metals from Aqueous Solutions through Functionalized γ-Graphyne-1 Membranes under External Uniform Electric Fields: Insights from Molecular Dynamics Simulations

Carbon-based nanosheet membranes with functionalized pores have great potential as water treatment membranes. In this study, the separation of Hg²⁺ and Cu²⁺ as heavy metal ions from aqueous solutions using a functionalized γ-graphyne-1 nanosheet membrane is investigated by molecular dynamics simulations. The simulation systems consist of a γ-graphyne-1 nanosheet with -COOH or -NH₂ functional groups on the edge of pores placed in an aqueous solution containing CuCl₂ and HgCl₂. An external electric field is applied as a driving force across the membrane for the separation of heavy metal ions using these functionalized pores. The ion-membrane and water molecule-membrane interaction energies, the radial distribution function of cations, the retention time and permeation of ions through the membrane, the density profile of water and ions, and the hydrogen bond in the system are investigated, and these results reveal that the performance of -NH₂-functionalized γ-graphyne-1 is better than that of -COOH-functionalized γ-graphyne-1 in the separation of Cu²⁺, while the Hg²⁺ cations encounter a high energy barrier as they pass through the membrane, especially in the -COOH-functionalized pore, due to their larger ionic radius and the smaller pore size of this membrane.

Release of an Encapsulated Peptide from Carbon Nanotubes Driven by Electric Fields: A Molecular Dynamics Study

Targeted drug delivery into cells has been of tremendous scientific interest, and carbon nanotubes (CNTs) can be deemed as a promising material for the loading and unloading of drugs. One of the major challenges is the release of drugs from CNTs, which have a great potential well to trap molecules. By performing molecular dynamics simulations, this work attempts to study the releasing process of encapsulated protein/peptide molecules from CNTs in the presence of uniform electric fields. Zadaxin serves as a model for protein/peptide drugs. External electric fields can assist the peptide in overcoming the potential well during its release. It is found that successful release of the peptide depends on the pore width, the pore length, and the net charges on the peptide. The peptide is less likely to be released either from CNTs with a smaller pore diameter due to a deeper potential well of the tubes or from CNTs with a longer pore length due to a broader and deeper potential well. Peptides with more net charges are ideal for the releasing process driven by electric fields. This work can provide insights into the design of an optimal tube size for effective release of a given protein/peptide.

Influence of stacking on the aqueous proton penetration behaviour across two-dimensional graphtetrayne

Two-dimensional (2D) graphtetrayne (G4) with intrinsic pattern triangular nanopores has been predicted to be an excellent candidate for next-generation proton exchange membranes due to its superior proton conductivity and selectivity. However, it is technically challenging to prepare a large area single-layer intact 2D material. A multi-layer stacked 2D material is a much more suitable choice, and the stacking can effectively shield the undesired defects and tears. In this work, we investigate the aqueous proton penetration behavior across multilayer-stacked two-dimensional G4 using extensive ReaxFF molecular dynamics simulations. We found that the G4 layers prefer a slightly misplaced stacking pattern which would cause only a slight reduction in the pore size. Detailed analyses indicate that the "water wires" across G4 remain continuous and can provide a low-barrier path for proton penetration until the number of stacking layers increases to three. In triple-layer G4, the "water wires" no longer exist and the aqueous phase will be separated by a wide vacuum area, thus significantly impeding the proton penetration behavior. Based on these results, we suggest that when serving as a proton exchange membrane, the number of stacking G4 layers should be fewer than three to achieve satisfactory conductivity. Our work provides guidance for the fabrication of next-generation proton exchange membranes based on nanoporous 2D materials.

Nanopore-Level Wood-Water Interactions—A Molecular Simulation Study

The nanoscale wood-water interaction strength, accessible sorption sites, and cell wall pore sizes are important factors that drive water sorption and the hysteresis phenomenon in wood. In this work, these factors were quantitatively studied using molecular simulations based on a cell wall pore model, previously developed by the authors. Specifically, the wall-water interaction strength, the sorption sites network including their number, interaction range, strength, and spatial distributions were set at a series of theoretical values as simulation input parameters. The results revealed that most of the investigated parameters significantly affected both sorption isotherms and hysteresis. Water monolayers and clusters were observed on the simulated pore surface when the wood-water interaction and sorption site strength were set at unrealistically high values. Furthermore, multiple linear regression models suggested that wood-water interaction and sorption site parameters were coupled in determining sorption isotherms, but not in determining hysteresis.

Gradient of Segmental Dynamics in Stereoregular Poly(Methyl Methacrylate) Melts Confined between Pristine or Oxidized Graphene Sheets

Segmental dynamics in unentangled isotactic, syndiotactic, and atactic poly(methyl methacrylate) (i-, a-, and s-PMMA) melts confined between pristine graphene, reduced graphene oxide, RGO, or graphene oxide, GO, sheets is studied at various temperatures, well above glass transition temperature, via atomistic molecular dynamics simulations. The model RGO and GO sheets have different degrees of oxidization. The segmental dynamics is studied through the analysis of backbone torsional motions. In the vicinity of the model nanosheets (distances less than ≈2 nm), the dynamics slows down; the effect becomes significantly stronger with increasing the concentration of the surface functional groups, and hence increasing polymer/surface specific interactions. Upon decreasing temperature, the ratios of the interfacial segmental relaxation times to the respective bulk relaxation times increase, revealing the stronger temperature dependence of the interfacial segmental dynamics relative to the bulk dynamics. This heterogeneity in temperature dependence leads to the shortcoming of the time-temperature superposition principle for describing the segmental dynamics of the model confined melts. The alteration of the segmental dynamics at different distances, d, from the surfaces is described by a temperature shift, ΔTseg(d) (roughly speaking, shift of a characteristic temperature). Next, to a given nanosheet, i-PMMA has a larger value of ΔTseg than a-PMMA and s-PMMA. This trend correlates with the better interfacial packing and longer trains of i-PMMA chains. The backbone torsional autocorrelation functions are shown in the frequency domain and are qualitatively compared to the experimental dielectric loss spectra for the segmental α-relaxation in polymer nanocomposites. The εT″(f) (analogous of dielectric loss, ε″(f), for torsional motion) curves of the model confined melts are broader (toward lower frequencies) and have lower amplitudes relative to the corresponding bulk curves; however, the peak frequencies of the εT″(f) curves are only slightly affected.

Adsorption-induced clustering of CO2 on graphene

Utilization of graphene-based materials for selective carbon dioxide capture has been demonstrated recently as a promising technological approach. In this study we report results from density functional theory calculations and molecular dynamics simulations on the adsorption of CO2, N2, and CH4 gases on a graphene sheet. We calculate adsorption isotherms of ternary and binary mixtures of these gases and reproduce the larger selectivity of CO2 to graphene relative to the other two gases. Furthermore it is shown that the confinement to two-dimensions, associated with adsorbing the CO2 gas molecules on the plane of graphene, increases their propensity to form clusters on the surface. Above a critical surface coverage (or partial pressure) of the gas, these CO2-CO2 interactions augment the effective adsorption energy to graphene, and, in part, contribute to the high selectivity of carbon dioxide with respect to nitrogen and methane. The origin of the attractive interaction between the CO2 molecules adsorbed on the surface is of electric quadrupole-quadrupole nature, in which the positively-charged carbon of one molecule interacts with the negatively-charged oxygen of another molecule. The energy of attraction of forming a CO2 dimer is predicted to be around 5-6 kJ mol-1, much higher than the corresponding values calculated for N2 and CH4. We also evaluated the adsorption energies of these gases to a graphene sheet and found that the attractions obtained using the classical force-fields might be over-exaggerated. Nevertheless, even when the magnitudes of these (classical force-field) graphene-gas interactions are scaled-down sufficiently, the tendency of CO2 molecules to cluster on the surface is still observed.

Mesoscopic method to study water flow in nanochannels with different wettability

Molecular dynamics (MD) simulations is currently the most popular and credible tool to model water flow in nanoscale where the conventional continuum equations break down due to the dominance of fluid-surface interactions. However, current MD simulations are computationally challenging for the water flow in complex tube geometries or a network of nanopores, e.g., membrane, shale matrix, and aquaporins. We present a novel mesoscopic lattice Boltzmann method (LBM) for capturing fluctuated density distribution and a nonparabolic velocity profile of water flow through nanochannels. We incorporated molecular interactions between water and the solid inner wall into LBM formulations. Details of the molecular interactions were translated into true and apparent slippage, which were both correlated to the surface wettability, e.g., contact angle. Our proposed LBM was tested against 47 published cases of water flow through infinite-length nanochannels made of different materials and dimensions-flow rates as high as seven orders of magnitude when compared with predictions of the classical no-slip Hagen-Poiseuille (HP) flow. Using the developed LBM model, we also studied water flow through finite-length nanochannels with tube entrance and exit effects. Results were found to be in good agreement with 44 published finite-length cases in the literature. The proposed LBM model is nearly as accurate as MD simulations for a nanochannel, while being computationally efficient enough to allow implications for much larger and more complex geometrical nanostructures.

Effects of Water and Different Solutes on Carbon-Nanotube Low-Voltage Field-Effect Transistors

Semiconducting single-walled carbon nanotubes (swCNTs) are a promising class of materials for emerging applications. In particular, they are demonstrated to possess excellent biosensing capabilities, and are poised to address existing challenges in sensor reliability, sensitivity, and selectivity. This work focuses on swCNT field-effect transistors (FETs) employing rubbery double-layer capacitive dielectric poly(vinylidene fluoride-co-hexafluoropropylene). These devices exhibit small device-to-device variation as well as high current output at low voltages (<0.5 V), making them compatible with most physiological liquids. Using this platform, the swCNT devices are directly exposed to aqueous solutions containing different solutes to characterize their effects on FET current-voltage (FET I-V) characteristics. Clear deviation from ideal characteristics is observed when swCNTs are directly contacted by water. Such changes are attributed to strong interactions between water molecules and sp² -hybridized carbon structures. Selective response to Hg²⁺ is discussed along with reversible pH effect using two distinct device geometries. Additionally, the influence of aqueous ammonium/ammonia in direct contact with the swCNTs is investigated. Understanding the FET I-V characteristics of low-voltage swCNT FETs may provide insights for future development of stable, reliable, and selective biosensor systems.

The production of graphene using impinging jet exfoliation in a binary system of CO2 and N-methyl pyrrolidone

High quality and high quantity few-layer graphene was successfully prepared using a new impinging jet method. Natural graphite flakes were first agitated in N-methyl pyrrolidone (NMP) with the assistance of supercritical CO₂, then the half-exfoliated graphite was further stripped using the shear stress derived from the impinging jets. After the energy conversion and stress analysis of the graphite particles during the whole exfoliation process, it was revealed that the size of the target mesh, the distance between the nozzle and the target, the decompression rate, and the size of the raw materials had a significant influence on the exfoliation process. Additionally, a microscopic view of the exfoliation and dispersion mechanism of graphene in the CO₂-NMP system was investigated using molecular dynamics simulation, and CO₂ was found to be beneficial for the penetration of NMP into the graphite sheets. Finally, the concentration and quality characteristics of the prepared graphene were characterized using ultraviolet-visible spectroscopy, transmission electron microscopy, Raman spectroscopy, and atomic force microscopy. The maximum concentration was as high as 0.689 mg ml^-1, the thickness of 68% of the product was less than 2.5 nm, and the lateral dimension was from 0.5 to 3.0 μm. These results indicate that this impinging jet method is promising for large-scale industrial production.

Exfoliation and stabilization mechanism of graphene in carbon dioxide expanded organic solvents: molecular dynamics simulations

CO₂ expanded organic solvents possess significant advantages in liquid-phase exfoliation to obtain monolayer/few-layer graphene from graphite. Further insights into the mechanism of graphene exfoliation in such solvents are essential to explore liquid-phase dispersion of graphene as a more potent alternative to chemical vapor deposition. In this study, dynamic processes of exfoliation and stabilization of graphene in CO₂-N,N-dimethylformamide (DMF), CO₂-N-methylpyrrolidone (NMP), CO₂-dimethyl sulfoxide (DMSO), and CO₂-ethanol (EtOH) were investigated using molecular dynamics simulations. The origin of the effect of each solvent on graphene exfoliation was analyzed quantitatively through potential mean force simulations. It has been found that the organic solvent in a CO₂ expanded solvent should be chosen with proper surface tension, and there exist two different graphene exfoliation processes in the effective solvents, which can be described as "burger dissociation" and "extrusion-taking away" processes, respectively. In the former process, a characteristic "super-burger-like" conformation with a semi-exfoliated structure was formed, which was the deciding factor to obtain high ratio of monolayer/few-layer graphene in dispersion product. A theoretical explanation has also been provided at the molecular level to the earlier experimental phenomena. A predicted simulation of the CO₂-3,3'-iminobis(N,N-dimethylpropylamine) (DMPA) system is also calculated. This investigation helps to avoid incompatible CO₂ expanded organic solvents employed in the experimental studies and provides theoretical clues to understand the mechanism of exfoliation and stabilization of graphene in such solvents.

Investigation of the properties of nanographene in polymer nanocomposites through molecular simulations: dynamics and anisotropic Brownian motion

The dynamical behavior of nanographene sheets dispersed in polymer matrices is investigated through united-atom molecular dynamics simulations. The Brownian motion of the sheet and the anisotropy in its translational and orientational diffusion are the topics of the current study. Different polymer matrices and pristine and functionalized graphene constitute various nanocomposite systems. Interactions between the nanographene flake and the matrix determine the dynamics of the systems. The dynamics is reduced in polyethylene oxide compared to polyethylene matrix, whereas carboxylated sheets move considerably slower than the pristine nanographene in any matrix. Diffusion is anisotropic for short times, while it becomes isotropic in the long time limit. The in-plane motion of the nanographene sheet is faster than the out-of-plane component, in agreement with the diffusion of perfectly oblate ellipsoids. In functionalized graphene, the anisotropy is suppressed. By exploring the temperature effect on both the nanographene sheet and polymer close to the surface, indications for coupling in the motion of the two components are revealed. The strong effect of edge functional groups on the dynamics can be used as a way to control the Brownian motion of nanographene sheets in polymer nanocomposites and consequently tailor the properties of the materials.

Tracing chirality, diameter dependence, and temperature-controlling of single-walled carbon nanotube non-covalent functionalization by biologically compatible peptide: insights from molecular dynamics simulations

Biological applications of single-walled carbon nanotubes (SWCNTs), including drug delivery, require their functionalization with various functional groups such as peptides. Recently, a biologically compatible peptide (named PW3 with the sequence of NH₂-Trp-Val-Trp-Val-Trp-Val-Lys-Lys-COOH) has been introduced as a good candidate for modification of carbon nanotubes due to its high affinity toward the exterior surface of these nano-carriers. In order to optimize the process of SWCNT peptide functionalization, the effects of chirality and diameter of SWCNTs as well as the temperature on PW3 adsorption were systematically investigated using molecular dynamics (MD) simulation. It was found that modification of chiral/zigzag SWCNT by PW3 peptide was more suitable compared with the armchair system due to the strong peptide-nanotube interactions and more water solubility at 310 K which can be well explained by microscopic structural investigations. Regarding the enhanced peptide-chiral nanotube interactions at the low temperature of 277 K, chiral nanotubes can be effective structures for SWCNT functionalization process at reduced temperatures. Our analysis indicated that disrupted PW3 and SWCNT hydration patterns and fewer internal interactions within the peptide could be responsible for the stronger peptide modification of SWCNT at higher temperatures. Additionally, "PW3/SWCNT" systems containing larger tube diameters formed more stable complexes owing to their effective surface area increment.

Molecular Dynamics of CH4/N2 Mixtures on a Flexible Graphene Layer: Adsorption and Selectivity Case Study

We theoretically investigate graphene layers, proposing them as membranes of subnanometer size suitable for CH₄/N₂ separation and gas uptake. The observed potential energy surfaces, representing the intermolecular interactions within the CH₄/N₂ gaseous mixtures and between these and the graphene layers, have been formulated by adopting the so-called Improved Lennard-Jones (ILJ) potential, which is far more accurate than the traditional Lennard-Jones potential. Previously derived ILJ force fields are used to perform extensive molecular dynamics simulations on graphene's ability to separate and adsorb the CH₄/N₂ mixture. Furthermore, the intramolecular interactions within graphene were explicitly considered since they are responsible for its flexibility and the consequent out-of-plane movements of the constituting carbon atoms. The effects on the adsorption capacity of graphene caused by introducing its flexibility in the simulations are assessed via comparison of different intramolecular force fields giving account of flexibility against a simplified less realistic model that considers graphene to be rigid. The accuracy of the potentials guarantees a quantitative description of the interactions and trustable results for the dynamics, as long as the appropriate set of intramolecular and intermolecular force fields is chosen. In particular it is shown that only if the flexibility of graphene is explicitly taken into account, a simple united-atom interaction potential can provide correct predictions. Conversely, when using an atomistic model, neglecting in the simulations the intrinsic flexibility of the graphene sheet has a minor effect. From a practical point of view, the global analysis of the whole set of results proves that these nanostructures are versatile materials competitive with other carbon-based adsorbing membranes suitable to cope with CH₄ and N₂ adsorption.

Optimization of Carbon Nanotube Dispersions in Water Using Response Surface Methodology

The aim of this work was to demonstrate an optimization methodology to reliably obtain stable macrodispersions (i.e., for ≥24 h) of carbon nanotubes in water using sonication. Response surface methodology (RSM) was utilized to assess and optimize the sonication parameters for the process. The studied input parameters were (i) sonication time (duration), (ii) amplitude (of vibration), and (iii) pulse-on/off (duration) of the sonicator. The analyzed responses were mean diameter and size distribution of multiwalled carbon nanotube (MWNT) aggregates in water, which were measured by the dynamic light scattering technique. A semiempirical model was developed and statistically tested to estimate the magnitude of sonicator parameters required to obtain specified MWNT macrodispersions (i.e., aggregates' mean diameter and distribution) in water. The results showed that MWNT aggregates of 2 ± 0.5 μm can be obtained by optimizing sonicator parameters to a sonication time of 89 s, amplitude of 144 μm, and pulse-on/off cycle of 44/30 s. These process settings for 100 mg/L MWNTs in a 30 mL aliquot of deionized water would consume 863 J/mL of sonication energy. Contrary to the popular belief, "sonication time" and/or "sonication energy input" were not found to be proportional to the degree of dispersion of MWNTs in water. This might be the reason for the frequent disparity and nonreproducibility of sonication results reported in scientific literature, especially for dispersing nanomaterials in a number of different systems. The amplitude of vibration was noted to be the most sensitive parameter affecting MWNT aggregates' diameter and distribution in water. The characterization of MWNTs was performed using electron microscopy, surface area analyzer, thermogravimetric analyzer, and zeta potential analyzer. This study can be helpful in evaluating sonication dispersion of particulate matter in other incompressible fluids such as graphene dispersion in organic solvents.

van der Waals Graphene Kirigami Heterostructure for Strain-Controlled Thermal Transparency

Programming thermal transport across interfaces by engineering strain is of critical importance for exploring mechanical controllable and thermal manageable devices with multifunctionalities. Here, we report a van der Waals heterostructure that is composed of bilayer graphene kirigami with diverse layer cut patterns and assembly organizations and show that the thermal flow intensity across the van der Waals interfaces, named as thermal transparency, could be continuously regulated by applying an external in-plane tensile strain. The density of atomic interactions across the interfaces and the distribution of delocalized phonon modes in each graphene kirigami are elucidated to understand the underlying thermal transport mechanism and are also incorporated into a theoretical model for quantitative predictions of thermal conductance under mechanical strain. A proof-of-conceptual van der Waals graphene kirigami heterostructure by design is proposed and validated through extensive full-scale atomistic simulations on the feasibility and reliability of regulating the transparency ratio of thermal transport by mechanical strain, demonstrating its potential applications in thermal and electronic devices.

Role of Molecular Polarity in Thermal Transport of Boron Nitride-Organic Molecule Composites

Understanding the role of fillers in the thermal transport of composite materials is of great importance to engineering better materials. The filler induces material interfaces within the composite, which influence the thermal transport between the matrix and themselves. The filler can also alter the molecular arrangement of the matrix in its vicinity, which may also impact the thermal transport ability. In this paper, molecular dynamics simulations are performed to study the thermal transport across the matrix-filler interfaces in hexagonal boron nitride (h-BN)-organic molecule composites. Four different organic molecules are studied as the matrixes. They include hexane (C₆H₁₄), hexanamine (C₆H₁₃NH₂), hexanol (C₆H₁₃OH), and hexanoic acid (C₅H₁₁COOH), which feature the same molecular backbone but increasingly different polar functional groups. The nominal local thermal conductivities of the hexane matrix with varying distances to the interface are calculated to demonstrate the influence of the filler on the thermal transport properties of the matrix. It is found that a more polar matrix exhibits a higher density in the near-interface region and a higher nominal local thermal conductivity, suggesting that the interfacial interaction can impact the local heat transfer ability of the matrix. In addition, the more polar matrix also leads to a larger interfacial thermal conductance with h-BN (hexane: 90.47 ± 14.49 MW/m² K, hexanamine: 113.38 ± 17.72 MW/m² K, hexanol: 136.16 ± 25.12 MW/m² K, and hexanoic acid: 155.17 ± 24.89 MW/m² K) because of the higher matrix density near the interface and thus more atoms exchanging energy with the filler. The results of this study may provide useful information for designing composite materials for heat transfer applications.

Adsorption mechanism of an antimicrobial peptide on carbonaceous surfaces: A molecular dynamics study

Peptides are versatile molecules with applications spanning from biotechnology to nanomedicine. They exhibit a good capability to unbundle carbon nanotubes (CNT) by improving their solubility in water. Furthermore, they are a powerful drug delivery system since they can easily be uptaken by living cells, and their high surface-to-volume ratio facilitates the adsorption of molecules of different natures. Therefore, understanding the interaction mechanism between peptides and CNT is important for designing novel therapeutical agents. In this paper, the mechanisms of the adsorption of antimicrobial peptide Cecropin A-Magainin 2 (CA-MA) on a graphene nanosheet (GNS) and on an ultra-short single-walled CNT are characterized using molecular dynamics simulations. The results show that the peptide coats both GNS and CNT surfaces through preferential contacts with aromatic side chains. The peptide packs compactly on the carbon surfaces where the polar and functionalizable Lys side chains protrude into the bulk solvent. It is shown that the adsorption is strongly correlated to the loss of the peptide helical structure. In the case of the CNT, the outer surface is significantly more accessible for adsorption. Nevertheless when the outer surface is already covered by other peptides, a spontaneous diffusion, via the amidated C-terminus into the interior of the CNT, was observed within 150 ns of simulation time. We found that this spontaneous insertion into the CNT interior can be controlled by the polarity of the entrance rim. For the positively charged CA-MA peptide studied, hydrogenated and fluorinated rims, respectively, hinder and promote the insertion.

Multilevel Molecular Modeling Approach for a Rational Design of Ionic Current Sensors for Nanofluidics

The complexity displayed by nanofluidic-based systems involves electronic and dynamic aspects occurring across different size and time scales. To properly model such kind of system, we introduced a top-down multilevel approach, combining molecular dynamics simulations (MD) with first-principles electronic transport calculations. The potential of this technique was demonstrated by investigating how the water and ionic flow through a (6,6) carbon nanotube (CNT) influences its electronic transport properties. We showed that the confinement on the CNT favors the partially hydrated Na, Cl, and Li ions to exchange charge with the nanotube. This leads to a change in the electronic transmittance, allowing for the distinguishing of cations from anions. Such an ionic trace may handle an indirect measurement of the ionic current that is recorded as a sensing output. With this case study, we are able to show the potential of this top-down multilevel approach, to be applied on the design of novel nanofluidic devices.

Multiscale Simulation Method for Quantitative Prediction of Surface Wettability at the Atomistic Level

The solid-liquid interface is of great interest because of its highly heterogeneous character and its ubiquity in various applications. The most fundamental physical variable determining the strength of the solid-liquid interface is the solid-liquid interfacial tension, which is usually measured according to the contact angle. However, an accurate experimental measurement and a reliable theoretical prediction of the contact angle remain lacking because of many practical issues. Here, we propose a first-principles-based simulation approach to quantitatively predict the contact angle of an ideally clean surface using our recently developed multiscale simulation method of density functional theory in classical explicit solvents (DFT-CES). Using this approach, we simulate the surface wettability of a graphene and graphite surface, resulting in a reliable contact angle value that is comparable to the experimental data. From our simulation results, we find that the surface wettability is dominantly affected by the strength of the solid-liquid van der Waal's interaction. However, we further elucidate that there exists a secondary contribution from the change of water-water interaction, which is manifested by the change of liquid structure and dynamics of interfacial water layer. We expect that our proposed method can be used to quantitatively predict and understand the intriguing wetting phenomena at an atomistic level and can eventually be utilized to design a surface with a controlled hydrophobic(philic)ity.

Molecular investigation of the wettability of rough surfaces using molecular dynamics simulation

In the present study, a computational investigation on the effect of surface roughness on the wettability behavior of water nanodroplets has been performed via molecular dynamics simulation.

Effect of an external electric field on capillary filling of water in hydrophilic silica nanochannels

Development of functional nanofluidic devices requires understanding the fundamentals of capillary driven flow in nanochannels.

Water thermophoresis in carbon nanotubes: the interplay between thermophoretic and friction forces

MD simulations show that the thermophoretic force is not velocity dependent while the friction force increases with the droplet speed.

Double-layer carbon nanocapsules with radioiodine content and its interaction with calcium, phosphorus, and strontium

First principles calculations have been performed for C₆₀@C₁₈₀ carbon double-layer endofullerenes with up to: three diatomic radioiodine molecules (¹³¹I₂), two potassium radio-iodide (K¹³¹I), and three sodium radio-iodide (Na¹³¹I) inside. The plane-wave pseudopotential (PP) method within the general gradient approximation (GGA) in the framework of the density functional theory (DFT) and time-dependent DFT (TD-DFT) was used to perform geometric optimizations (GOs) and molecular dynamics (MD) at 310 K and atmospheric pressure. We found that the double-layer carbon nanocapsules formed by two concentric fullerenes (C₁₈₀ surrounding C₆₀) are very stable and may contain a radiodosis, without altering their configuration; that is, the 3(¹³¹I₂)@C₆₀@C₁₈₀, 2(K¹³¹I)@C₆₀@C₁₈₀, and 3(Na¹³¹I)@C₆₀@C₁₈₀ systems constitute stable nanocapsules. We analyzed the interaction of double-layer endofullerene with radioactive content with some calcium, phosphorus, and strontium atoms, [n(X¹³¹I)@C₆₀@C₁₈₀ + mY], for X = I, K, Na; Y = Ca, P, Sr; n = 1, 2, 3; m = 1, …, 20. Our calculations show that up to m = 20 calcium atoms can easily be physisorbed by the outer surface of the double-layer endofullerene, maintaining their integrity and shielding the radiodosis of any interaction that can proceed from the outside. It is thus concluded that these double-layer endofullerenes can be functionalized as vectors to deliver radiodosis with structural advantages over the single layer systems; as they are more robust, stable, and possess a larger surface to functionalize with some atoms serving as molecular recognizers. Graphical abstract Double-layer carbon nanocapsules with radioiodine content and its interaction with calcium, phosphorus and strontium.

Carbon Nanotubes as Thermally Induced Water Pumps

Thermal Brownian motors (TBMs) are nanoscale machines that exploit thermal fluctuations to provide useful work. We introduce a TBM-based nanopump which enables continuous water flow through a carbon nanotube (CNT) by imposing an axial thermal gradient along its surface. We impose spatial asymmetry along the CNT by immobilizing certain points on its surface. We study the performance of this molecular motor using molecular dynamics (MD) simulations. From the MD trajectories, we compute the net water flow and the induced velocity profiles for various imposed thermal gradients. We find that spatial asymmetry modifies the vibrational modes of the CNT induced by the thermal gradient, resulting in a net water flow against the thermal gradient. Moreover, the kinetic energy associated with the thermal oscillations rectifies the Brownian motion of the water molecules, driving the flow in a preferred direction. For imposed thermal gradients of 0.5-3.3 K/nm, we observe continuous net flow with average velocities up to 5 m/s inside CNTs with diameters of 0.94, 1.4, and 2.0 nm. The results indicate that the CNT-based asymmetric thermal motor can provide a controllable and robust system for delivery of continuous water flow with potential applications in integrated nanofluidic devices.

Self-assembled nanocapsules in water: a molecular mechanistic study

The self-assembly mechanism of one-end-open carbon nanotubes (CNTs) suspended in an aqueous solution was studied by molecular dynamics simulations. It was shown that two one-end-open CNTs with different diameters can coaxially self-assemble into a nanocapsule. The nanocapsules formed were stable in aqueous solution under ambient conditions, and the pressure inside the nanocapsule was much higher than the ambient pressure due to the van der Waals interactions between the two parts of the nanocapsule. The effects of the normalized radius difference, normalized inter-tube distance and aspect ratio of the CNT pairs were systematically explored. The electric field response of the nanocapsules was studied using ab initio molecular dynamics simulations, which shows that the nanocapsules can be opened by applying an external electric field, due to the polarization of carbon atoms. This discovery not only sheds light on a simple yet robust nanocapsule self-assembly mechanism, but also underpins potential innovations in drug delivery, nano-reactors, etc.

Rotating-Electric-Field-Induced Carbon-Nanotube-Based Nanomotor in Water: A Molecular Dynamics Study

Using molecular dynamics simulations, it is shown that a carbon nanotube (CNT) suspended in water and subjected to a rotating electric field of proper magnitude and angular speed can be rotated with the aid of water dipole orientations. Based on this principle, a rotational nanomotor structure is designed and the system is simulated in water. Use of the fast responsiveness of electric-field-induced CNT orientation in water is employed and its operation at ultrahigh-speed (over 10¹¹ r.p.m.) is shown. To explain the basic mechanism, the behavior of the rotational actuation, originated from the water dipole orientation, is also analyzed . The proposed nanomotor is capable of rotating an attached load (such as CNT) at a precise angle as well as nanogear-based complex structures. The findings suggest a potential way of using the electric-field-induced CNT rotation in polarizable fluids as a novel tool to operate nanodevices and systems.

A theoretical study on the stability of CNT encased cyclic peptide beyond hydrogen bond cut-off

The Carbon nanotubes (CNT) are potential candidate for many biomedical applications especially in targeted drug delivery for cancer diseases. However, the use of CNT has limitations due to its insolubility in aqueous media. The self-assembly of cyclic peptide encased on the CNT has enhanced its dispersion in aqueous medium which extend their applications as antibacterial and drug delivery agents. To understand this process, an attempt has been made to investigate the dynamics and stability of trimer cyclic peptide encasing with CNT using classical molecular dynamics. The model cyclic peptide monomer constitutes 14 series of amino acids viz.; (cyclo-[(D-ARG-L-VAL-D-ARG-L-THR-D-AGR-L-LYS-D-GLY-L-ARG-D-ARG-L-ILE-D-ARG-L-ILE-D-PRO-L-PRO)]). Each cyclic peptide in the assembly stacking far apart at approximately 15 Å each other beyond hydrogen bond cut-off distance. The trimer was observed to be stable only over 10 ns of entire MD trajectory. But when there is electrostatic interaction between cyclic peptides at 6.5 Å distance then assembly is stable for entire 50 ns. Our result reveals that for a stable assembly, beyond the hydrogen bond cut-off distance, the electrostatic interaction plays significant role.

Slip divergence of water flow in graphene nanochannels: the role of chirality

Graphene has attracted considerable attention due to its characteristics as a 2D material and its fascinating properties, providing a potential building block for fabrication of nanofluidic conduits.

Molecular dynamics simulations of electric field induced water flow inside a carbon nanotorus: a molecular cyclotron

A nano-flow is induced by applying gigahertz rotating electric fields (EFs) of different strengths and frequencies on a carbon nanotorus filled with water molecules, using molecular dynamics simulations.

Molecular dynamics simulations of solvent-exfoliation and stabilization of graphene with the assistance of compressed carbon dioxide and pyrene–polyethylene glycol

Understanding the solvent-exfoliation and stabilization of graphene with cpCO₂and pyrene–polyethylene glycol from molecular dynamics simulations.

Interactions of calcium with the external surfaces of fullerenes and endofullerenes doped with radioactive sodium iodide

We report first-principles calculations carried out to analyze the adsorption of calcium on the outer surface of the fullerene C₆₀, yielding [C₆₀ + mCa]. Geometric optimization (GO) and molecular dynamics (MD) simulation were performed using the plane-wave pseudopotential method within the framework of density functional theory (DFT) and time-dependent DFT (TD-DFT) to investigate the configurations, the associated energies in the ground state, and the stabilities of fullerenes and endofullerenes doped with radioactive sodium iodide when they interact with calcium atoms on the outer fullerene surface (i.e., [nNa¹³¹I@C₆₀ + mCa]). The reason for investigating these calcium-functionalized (endo)fullerene systems was to gauge their potential stability when used as vectors to deliver radioiodine to cancerous tissue in the human body. In the simulations, we found that the geometric limit on the number of calcium atoms that can be physisorbed on the outer surface of an empty fullerene while maintaining its structural stability is 28 calcium atoms, which also takes into account the proportional expansion of the fullerene as the number of absorbed calcium atoms increases. However, the stability of a fullerene system during calcium adsorption also strongly depends on whether any atoms or molecules are being encapsulated by the fullerene, as these encapsulated atoms/molecules can also interact with the fullerene and influence its stability. A Mulliken electronegativity analysis revealed that, when atoms inside and/or outside the fullerene donate charge (electrons) to the fullerene, the fullerene expands. The excess charge on the carbon atoms of the fullerene weakens some of the carbon-carbon bonds, potentially causing them to break, in which case the fullerene loses its ability to encapsulate molecules and releases them. Graphical Abstract DFT simulation of a endo fullerene doped with radioactive sodium iodide interacting with 28 calcium atoms in a geometric arrangement.