Preferential ion and water intake using charged carbon nanotubes

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.

Aromatic Polyamide Reverse-Osmosis Membrane: An Atomistic Molecular Dynamics Simulation

Polyamide (PA) membrane-based reverse-osmosis (RO) serves as one of the most important techniques for water desalination and purification. Fundamental understanding of PA RO membranes at the atomistic level is critical to enhance their separation capabilities, leading to significant societal and commercial benefits. In this paper, a fully atomistic molecular dynamics simulation was performed to investigate PA membrane. Our simulated cross-linked membrane exhibits structural properties similar to those reported in experiments. Our results also reveal the presence of small local two-layer slip structures in PA membrane with 70% cross-linking, primarily due to short-range anisotropic interactions among aromatic benzene rings. Inside the inhomogeneous polymeric structure of the membrane, water molecules show heterogeneous diffusivities and converge adjacent to polar groups. Increased diffusion of water molecules is observed through the less cross-linked pathways. The existence of the fast pathways for water permeation has no effect on membrane's salt rejections.

Porous carbon nanotubes: molecular absorption, transport, and separation

We use classical molecular dynamics simulations to study nanofluidic properties of porous carbon nanotubes. We show that saturated water vapor condenses on the porous nanotubes, can be absorbed by them and transported in their interior. When these nanotubes are charged and placed in ionic solutions, they can selectively absorb ions in their interior and transport them. Porous carbon nanotubes can also be used as selective molecular sieves, as illustrated on a room temperature separation of benzene and ethanol.

Assessment of Common Simulation Protocols for Simulations of Nanopores, Membrane Proteins, and Channels

Molecular dynamics (MD) simulation has become a common technique to study biological systems. Transport of small molecules through carbon nanotubes and membrane proteins has been an intensely studied topic, and MD simulations have been able to provide valuable predictions, many of which have later been experimentally proven. Simulations of such systems pose challenges, and unexpected problems in commonly used protocols and methods have been found in the past few years. The two main reasons why some were not found before are that most of these newly discovered errors do not lead to unstable simulations. Furthermore, some of them manifest themselves only after relatively long simulation times. We assessed the reliability of the most common simulations protocols by MD and stochastic dynamics (SD) or Langevin dynamics, simulations of an alpha hemolysin nanochannel embedded in a palmitoyloleoylphosphatidylcholine (POPC) lipid bilayer. Our findings are that (a) reaction field electrostatics should not be used in simulations of such systems, (b) local thermostats should be preferred over global ones since the latter may lead to an unphysical temperature distribution,

Atomistic insight into adsorption, mobility, and vibration of water in ion-exchanged zeolite-like metal-organic frameworks

The adsorption, mobility, and vibration of water in ion-exchanged rho-zeolite-like metal-organic frameworks (ZMOFs) are investigated using atomistic simulations. Because of the high affinity for the ionic framework and nonframework ions, water is strongly adsorbed in rho-ZMOFs with a three-step adsorption mechanism. At low pressures, water is preferentially adsorbed onto Na(+) ions, particularly at site II; with increasing pressure, adsorption occurs near the framework and finally in the large cage. Upon water adsorption, Na(+) ions are observed to redistribute from site I to site II and gradually hydrated with increasing pressure. In Li-, Na-, and Cs-exchanged rho-ZMOFs, the adsorption capacity and isosteric heat decrease with increasing ionic radius attributed to the reduced electrostatic interaction and free volume. The mobility of water in Na-rho-ZMOF increases at low pressures but decreases upon approaching saturation. With sufficient amount of water present, the mobility of Na(+) ions is promoted. The vibrational spectra of water in Na-rho-ZMOF exhibit distinct bands for librational motion, bending, and stretching. The librational motion has a frequency higher than bulk water due to confinement. With increasing loading and hence stronger coordinative attraction, the bending frequency shows a blue shift. Symmetric and asymmetric modes are observed in the stretching as a consequence of the strong water-ion interaction. This study provides a fundamental microscopic insight into the static and dynamic properties of water in charged ZMOFs and reveals the subtle interplay between water and nonframework ions.

Enhancement in hydrogen storage in carbon nanotubes under modified conditions

We investigate the hydrogen adsorbing characteristics of single-walled carbon nanotubes (CNTs) through fundamental molecular dynamics simulations that characterize the role of ambient pressure and temperature, the presence of surface charges on the CNTs, inclusion of metal ion interconnects, and nanocapillary effects. While the literature suggests that hydrogen spillover due to the presence of metallic contaminants enhances storage on and inside the nanotubes, we find this to be significant for alkali and not transition metals. Charging the CNT surfaces does not significantly enhance hydrogen storage. We find that the bulk of the hydrogen storage occurs inside CNTs due to their nanocapillarity effect. Storage is much more dependent on external thermodynamic conditions such as the temperature and the pressure than on these facets of the CNT structure. The dependence of storage on the external thermodynamic conditions is analyzed and the optimal range of operating conditions is identified.