Electronic Properties of Carbon Nanotubes Intercalated with Li⁺ and Mg²⁺: Effects of Ion Charge and Ion Solvation

The influence of bare and solvated cations imbedded inside single-walled carbon nanotubes (SWCNTs) on the SWCNT electronic properties is studied by <i>ab initio</i> electronic structure calculations. The roles of ion charge and ion solvation are investigated by comparing Li⁺ vs Mg²⁺ and Li⁺ vs its solvatocomplex with two ethylene carbonate (EC) molecules, [Li(EC)₂]⁺. Two achiral nanotubes with similar radii but different electronic structure are considered, namely, the metallic, (15,15) armchair, and semiconducting, (26,0) zigzag, SWCNTs. The intercalation process is energetically favorable for both CNT topologies, with all bare cations and the solvatocomplex under investigation, with the doubly charged Mg²⁺ ion exhibiting the highest energy gain. Insertion of the bare ions into the SWCNTs increases the electronic entropy. The electronic entropy changes because the ions introduce new energy levels near the Fermi level. Those initially empty levels of the cations accept electron density and generate electronic holes in the valence band of both SWCNT topologies. As a consequence, the semiconducting (26,0) zigzag SWCNT becomes metallic, exhibiting hole conductivity. Solvation of the bare Li⁺ ion by EC molecules completely screens the influence of the ion charge on the SWCNT electronic properties, independent of the topology. The last fact validates the common practice of employing standard, nonpolarizable force field models in classical molecular dynamics simulations of electrolyte solutions interacting with CNTs. The strong dependence of the nanotube electronic properties on the presence of bare ions can be used for development of novel cation sensors for mass spectroscopy applications

A new potential model for acetonitrile: Insight into the local structure organization

International audienceThorough understanding of the microscopic organization and dynamics of individual constituents is a crucial step in the description and the prediction the properties of electrolyte solutions based on dipolar aprotic solvents such as acetonitrile. For this aim, a new potential (force field) model for acetonitrile was developed on the basis of comprehensive approach comprising quantum chemical calculations, ab initio molecular dynamics simulations and empirical parameterization. The developed potential model is able to reproduce the experimental thermodynamic and dynamic properties of neat acetonitrile in the range of temperatures between 228.15 and 348.15 K. The local structure of neat liquid acetonitrile then was analyzed in a framework of the nearest neighbor approach. It was shown that the distance standard deviations relative to the average distance between the nearest neighbors have a non-linear behavior that was traced back to the changes in the mutual orientation between acetonitrile molecules. The closest neighbors have a dominant antiparallel dipoles orientation with respect to a reference acetonitrile molecule, while for the further nearest neighbors perpendicular and parallel mutual orientation is observed. The nearest neighbors approach in combination with angular distribution functions was used for the estimation of the Kirkwood factor. Our results show that in order to reproduce the corresponding experimental values derived in the framework of the Onsager-Kirkwood-Fröhlich theory, it is necessary to take into account the mutual orientation of the 5–6 nearest neighbors. Although the atomic charges, on N and the methyl group hydrogen atoms, are negative, the values of the N ⋯ H distance and the N ⋯ H–C (methyl group), are compatible with a weak hydrogen bond between the two atoms

Atomistic Simulations of Coating of Silver Nanoparticles with Poly(vinylpyrrolidone) Oligomers: Effect of Oligomer Chain Length

Silver nanoparticles (AgNPs) possess unique physicochemical properties, which are different from those of matter of the same chemical composition on a larger scale. These features open up the opportunity for their use in many promising chemical and biomedical applications. In this study we have developed an atomistic model for molecular dynamics (MD) simulations of AgNP coated by poly­(<i>N</i>-vinyl-2-pyrrolidone) (PVP) oligomers. We focus on identifying the relative length of PVP oligomers, enabling effective protecting of a crystalline silver core of 4.5 nm diameter from water contacts. Three different PVP-coated AgNP systems have been compared: (i) a nanoparticle coated by a mixture of short-chain PVP oligomers of the varying size and (ii,iii) the silver core wrapped by a single, long-chain PVP polymer with the number of monomers equal to 816 and 1440, respectively. We have validated the MD models of the PVP–AgNPs using a series of MD simulations reproducing adsorption, wrapping, and coating of PVP around a silver core either as short PVP oligomers or as a single-chain, long polymer of a varying length. Our simulations predict that the saturated coating of PVP around the silver core of the given diameter can occur when the polymer chain length approaches 2600–2800 units