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.

Transport of hydrated nitrate and nitrite ions through graphene nanopores in aqueous medium

Nitrate ( NO 3 - ) and nitrite ( NO 2 - ) ions are naturally occurring inorganic ions that are part of the nitrogen cycle. High doses of these ions in drinking water impose a potential risk to public health. In this work, molecular dynamics simulations are carried out to study the passage of nitrate and nitrite ions from water through graphene nanosheets (GNS) with hydrogen-functionalized narrow pores in presence of an external electric field. The passage of ions through the pores is investigated through calculations of ion flux, and the results are analyzed through calculations of various structural and thermodynamic properties such as the density of ions and water, ion-water radial distribution functions, two-dimensional density distribution functions, and the potentials of mean force of the ions. Current simulations show that the nitrite ions can pass more in numbers than the nitrate ions in a given time through GNS hydrogen-functionalized pore of different geometry. It is found that the nitrite ions can permeate faster than the nitrate ions despite the former having higher hydration energy in the bulk. This can be explained in terms of the competition between the number density of the ions along the pore axis and the free energy barrier calculated from the potential of mean force. Also, an externally applied electric field is found to be important for faster permeation of the nitrite over the nitrate ions. The current study suggests that graphene nanosheets with carefully created pores can be effective in achieving selective passage of ions from aqueous solutions.

Analysing thermophoretic transport of water for designing nanoscale-pumps

We propose a new design for thermally induced water pumping through carbon nanotubes by imposing a thermal gradient along the length of a carbon nanotube (CNT), which connects two water-filled reservoirs. We analyse the flow parameters by varying the imposed thermal gradient (4.62 to 20.98 K nm-1), the radius (0.81 to 1.89 nm) and the length (5 to 50 nm) of the CNT. Using molecular dynamics simulations, we compute the volumetric flow rate of the pump, velocity profiles of flow and thermophoretic forces acting on water molecules for various thermal gradients. The directed motion of water molecules is induced by the spatial variations of CNT-water energy interactions at the interface and the variations in the oscillation of the carbon atoms from hot to cold ends. The net flow and average velocity of water molecules are found to increase linearly with the applied thermal gradient, as well as with an increase in the radius and length of the CNT. We observe that nano-pumps with an increase in the radius and length of the CNT connecting the reservoirs perform better and also achieved higher efficiency levels. The analysis of the results indicates that the present design leads to a realistic system capable of providing continuous transportation of water leading to interesting practical applications in nanoscale devices.

The Influence of Synthesis Parameters on Vertically Aligned CNT Sheets: Empirical Modeling and Process Optimization Using Response Surface Methodology

In the present work, vertically aligned carbon nanotube (VA-CNT) sheets were synthesized via pyrolysis of polybenzimidazole (PBI)-Kapton inside the pores of anodized aluminum oxide (AAO). The synthesized VA-CNT sheets were then evaluated for the desalination of salty water. The results indicated that the VA-CNT sheets were effective for application as an adsorbent for desalination of salty water due to their high adsorption capacity, with no loss of CNTs in the treated water. This study explored the impact of operating time and temperature on liquid adsorption performance through optimization and modeling methods. An empirical model was developed through the evolution of a full factorial design process which considered two significant factors for enhanced antibacterial efficiency and adsorption uptake. The highest antibacterial efficiency was achieved with carbon precursors synthesized at a higher temperature. However, optimal values were obtained for both antibacterial efficiency and adsorption uptake (NaCl) with a combination of CNT membranes. The best conditions for such a membrane were 800 °C and 18 min. Under these conditions, antibacterial efficiency, contact angle, carbon content, adsorption uptake (NaCl = 10,000) and adsorption uptake (NaCl = 20,000) were 90.079, 1.69256, 75.213, 76.2352 and 0.997, respectively.

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.

Significant curvature effects of partially charged carbon nanotubes on electrolyte behavior investigated using Monte Carlo simulations

Carbon nanotubes and graphene are among the major nanomaterials in nanoscience and technology. Despite having π electrons, these nanocarbon allotropes have been simply considered as neutral in classical calculations. In this study, the effects of partial charges on graphene and curved interfaces on molecular adsorption were investigated using Monte Carlo simulations of N2 and NaCl aqueous solutions on graphene and carbon nanotubes. The simulated N2 adsorption behavior and adsorption potential on partially charged and non-charged graphene coincided with each other. The adsorption potentials suggested that partially charged graphene attracted Na ions and repelled Cl ions. However, those tendencies were not present in NaCl aqueous solutions on graphene. Conversely, in partially charged carbon nanotube models, a preference for Na ions and repulsion of Cl ions in the internal nanospaces were observed in the adsorption potentials using Monte Carlo simulations. Curved interfaces in the internal nanospaces of carbon nanotubes enhanced these properties, suggesting significant electrostatic interactions in a curved π-conjugated system.

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.

Molecular dynamics simulation of the material removal process and gap phenomenon of nano EDM in deionized water

This simulation shows the melting, vaporizing, removing and solidifying processes of a material as well as the bubble forming process during nano EDM.