Molecular dynamics study of carbon nanotube oscillators revisited

We performed molecular dynamics simulation of double walled carbon nanotube (DWCNT) oscillators under constant energy and constant temperatures with various commensurations and nanotube lengths. We clarify and resolve questions and differences raised by previous simulation results of similar systems. At constant energy, sustained oscillation is available for a wide range of initial temperatures. But low initial temperature is advantageous for DWCNTs to sustain oscillation under constant energy. We observed sustained oscillation at constant energy for both commensurate and incommensurate DWCNTs. On the other hand, under constant temperatures, both high and low temperatures are disadvantageous to sustain DWCNT oscillations. At constant low temperature, neither commensurate nor incommensurate DWCNTs can maintain oscillation. At appropriate constant temperatures, the oscillatory behavior of incommensurate nanotubes is much more sustained than that of commensurate tubes. The oscillatory frequency of DWCNTs depends significantly on the length of tubes. The initial oscillatory frequency is inversely proportional to the DWCNT lengths. The oscillation frequency of DWCNTs is insensitive to the initial temperatures at constant energy, but slightly dependent on the temperature at constant temperatures.     

Polymer-nanoparticle interfacial behavior revisited: a molecular dynamics study

By tuning the polymer-filler interaction, filler size and filler loading, we use a coarse-grained model-based molecular dynamics simulation to study the polymer-filler interfacial structural (the orientations at the bond, segment and chain length scales, chain size and conformation), dynamic and stress-strain properties. Simulated results indicate that the interfacial region is composed of partial segments of different polymer chains, which is consistent with the experimental results presented by Chen et al. (Macromolecules, 2010, 43, 1076). Moreover, it is found that the interfacial region is within one single chain size (R(g)) range, irrespective of the polymer-filler interaction and the filler size, beyond which the bulk behavior appears. In the interfacial region, the orientation and dynamic behaviors are induced by the interfacial enthalpy, while the size and conformation of polymer chains near the filler are controlled by the configurational entropy. In the case of strong polymer-filler interaction (equivalent to the hydrogen bond), the innerest adsorbed polymer segments still undergo adsorption-desorption process, the transport of chain mass center in the interfacial region exhibits away from the glassy behavior, and no plastic-like yielding point appears in the stress-strain curve, which indicates that although the mobility of interfacial polymer chains is restricted, there exist no "polymer glassy layers" surrounding the filler. In addition, it is evidenced that the filler particle prefers selectively adsorbing the long polymer chains for attractive polymer-filler interaction, validating the experimental explanation of the change of the bound rubber (BR). In short, this work provides important information for further experimental and simulation studies of polymer-nanoparticle interfacial behavior.     

Hydration of alkali ions from first principles molecular dynamics revisited

Structural and dynamical properties of the hydration of Li(+), Na(+), and K(+) in liquid water at ambient conditions were studied by first principles molecular dynamics. Our simulations successfully captured the different hydration behavior shown by the three alkali ions as observed in experiments. The present analyses of the dependence of the self-diffusion coefficient and rotational correlation time of water on the ion concentration suggest that Li(+) (K(+)) is certainly categorized as a structure maker (breaker), whereas Na(+) acts as a weak structure breaker. An analysis of the relevant electronic structures, based on maximally localized Wannier functions, revealed that the dipole moment of H(2)O molecules in the first solvation shell of Na(+) and K(+) decreases by about 0.1 D compared to that in the bulk, due to a contraction of the oxygen lone pair orbital pointing toward the metal ion.     

The mixed alkali effect in ionically conducting glasses revisited: a study by molecular dynamics simulation

When more than two kinds of mobile ions are mixed in ionic conducting glasses and crystals, there is a non-linear decrease of the transport coefficients of either type of ion. This phenomenon is known as the mixed mobile ion effect or Mixed Alkali Effect (MAE), and remains an unsolved problem. We use molecular dynamics simulation to study the complex ion dynamics in ionically conducting glasses including the MAE. In the mixed alkali lithium-potassium silicate glasses and related systems, a distinct part of the van Hove functions reveals that jumps from one kind of site to another are suppressed. Although, consensus for the existence of preferential jump paths for each kind of mobile ions seems to have been reached amongst researchers, the role of network formers and the number of unoccupied ion sites remain controversial in explaining the MAE. In principle, these factors when incorporated into a theory can generate the MAE, but in reality they are not essential for a viable explanation of the ion dynamics and the MAE. Instead, dynamical heterogeneity and "cooperativity blockage" originating from ion-ion interaction and correlation are fundamental for the observed ion dynamics and the MAE. Suppression of long range motion with increased back-correlated motions is shown to be a cause of the large decrease of the diffusivity especially in dilute foreign alkali regions. Support for our conclusion also comes from the fact that these features of ion dynamics are common to other ionic conductors, which have no glassy networks, and yet they all exhibit the MAE.     

Nuclear dynamics in the vicinity of a crossing seam: Vibrational spectrum of HD2 revisited

So far, most studies of the geometric phase effect have presumed that the phase is path‐independent; hence, one must supply another restriction concerning the boundary condition on the nuclear wave functions when dealing with nonsymmetric isotopomers of X₃ systems. We report calculations of the vibrational spectra of HD₂ using a recently proposed generalized Born–Oppenheimer (GBO) formalism. The calculations demonstrate that there are significant differences between the results calculated from the present GBO method and those based on the preceding presumption. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 83: 279–285, 2001     

Mendeleyev revisited

Despite the periodic table having been discovered by chemists half a century before the discovery of electronic structure, modern designs are invariably based on physicists’ definition of periods. This table is a chemists’ table, reverting to the phenomenal periods that led to the table’s discovery. In doing so, the position of hydrogen is clarified.

     

Recovery revisited

During the last decade, various IUPAC documents have presented concepts related to the important ability of a measurement procedure to provide correct quantity values for amount, concentration, or content of a component in a system. The measurement procedure often, but not necessarily, includes one or more separation steps prior to the final instrumental reading. The vocabularies present concepts with some definitions and terms that disagree or are ambiguous, mostly revolving around the term recovery and its derivatives. The present proposal consists in defining three concepts with the terms actual quantity, initially estimated quantity, and recovered quantity ratio, where quantity is used in the metrological sense of the international vocabulary of basic and general terms in metrology (BIPM, IEC, IFCC, ILAC, ISO, IUPAC, IUPAP, OIML International vocabulary of basic and general terms in metrology, draft, 3rd edn. BIPM, Sèvres, 2005), not as a synonym of amount.

Several routes to the glassy states in the one component soft core system: revisited by molecular dynamics

Molecular dynamics simulations have been performed to study the glass transition for the soft core system with a pair potential φ(n)(r) = ε(σ∕r)(n) of n = 12. Using the compressibility factor, PV/Nk(B)T=P?(ρ*), its phase diagram can be represented as a function of a reduced density, ρ? = ρ(ε∕k(B)T)(3∕n), where ρ = Nσ(3)∕V. In the present work, NVE relaxations to the glassy or crystalline states starting from the unstable states in the phase diagram have been revisited in details and compared with other processes. Relaxation processes can be characterized by the time dependence of the dynamical compressibility factor (PV/Nk(B)T)(t)?(≡g(ρ(t)*)) on the phase diagram. In some cases, g(ρ(t)*) reached a crystal branch in the phase diagram; however, metastable states are found in many cases. With connecting points for the metastable states in the phase diagram, we can define a glass branch where the dynamics of particles are almost frozen. The structures observed there have common properties characterized as glasses. Although overlaps of glass forming process and nanocrystallization process are observed in some cases, these behaviors are distinguishable to each other by the characteristics of structures. There are several routes to the glass branch and we suggest that all of them are the glass transition.     

Hewitt reaction revisited

A range of alkyl phenylphosphonites are prepared from the reaction of phenylphosphinic acid with the corresponding alkyl chloroformates. A mechanism for this reaction is proposed.     

Liquid-crystalline phthalocyanines revisited

A Commentary on the paper “Homologous series of liquid-crystalline metal free and copper octa-n-alkoxyphthalocyanines„, by J. F. van der Pol, E. Neeleman, J. W. Zwikker, R. J. M. Nolte, W. Drenth, J. Aerts, R. Visser and S. J. Picken. First published in Liquid Crystals, 6, 577-592 (1989).     

Covalent radii revisited

A new set of covalent atomic radii has been deduced from crystallographic data for most of the elements with atomic numbers up to 96. The proposed radii show a well behaved periodic dependence that allows us to interpolate a few radii for elements for which structural data is lacking, notably the noble gases. The proposed set of radii therefore fills most of the gaps and solves some inconsistencies in currently used covalent radii. The transition metal and lanthanide contractions as well as the differences in covalent atomic radii between low spin and high spin configurations in transition metals are illustrated by the proposed radii set.     

Trimethylsilylcellulose synthesis revisited

A simplification of the overall method of producing trimethylsilylcellulose solutions in common organic volatile solvents is reported. Microcrystalline cellulose was derivatized aiming its solubilization in tetrahydrofuran and toluene to obtain ultra-thin films of cellulose by spin-coating. The main simplifications are: 1) elimination of a swelling step before dissolution in the binary solvent N,N-dimethylacetamide/LiCl by using microcrystalline cellulose; 2) derivatization of cellulose –OH groups with –Si(CH₃)₃ groups was successful with no additional catalyst, like trimethylsilyl chloride, the degree of substitution (DS) being 2.2 ± 0.1. The DS at the extreme surface of the films was computed from the X-ray Photoelectron Spectroscopy data: using solely C 1s fitted component areas and, alternatively, from both C 1s and Si 2p areas. The DS estimated with a technique specific of the surface is close to the bulky one obtained from gravimetric measurements reported in the literature for materials synthesized with catalyst. The topographic uniformity is shown by Atomic Force Microscopy.     

Zirconium tetrachloride revisited

Zirconium tetrachloride, ZrCl₄, is a strategic material with wide‐ranging applications. Until now, only one crystallographic study on ZrCl₄ has been reported [Krebs (1970). Z. Anorg. Allg. Chem. 378 , 263–272] and that was more than 40 years ago. The compound used for the previous determination was prepared from ZrO₂ and Cl₂–CCl₄, and single‐crystal X‐ray diffraction (SCXRD) studies on ZrCl₄ obtained from Zr metal have not yet been reported. In this context, we prepared ZrCl₄ from the reaction of Zr metal and Cl₂ gas in a sealed tube and investigated its structure at 100, 150, 200, 250, and 300 K. At 300 K, the SCXRD analysis indicates that ZrCl₄ crystallizes in the orthorhombic space group Pca2₁ [a = 6.262 (9), b = 7.402 (11), c = 12.039 (17) Å, and V = 558.0 (14) ų] and consists of infinite zigzag chains of edge‐sharing ZrCl₆ octahedra. This chain motif is similar to that observed previously in ZrCl₄, but the structural parameters and space group differ. In the temperature range 100–300 K, no phase transformation was identified, while elongation of intra‐chain Zr…Zr [3.950 (1) Å at 100 K and 3.968 (5) Å at 300 K] and inter‐chain Cl…Cl [3.630 (3) Å at 100 K and 3.687 (9) Å at 300 K] distances occurred.     

Fournier polarography revisited

A theory for Fournier polarography and higher order harmonics is presented. This is valid for reversible systems under semi-infinite diffusion to stationary and expanding plane electrodes. The algorithm is simple, accurate and exploits the identities holding for the interfacial concentrations. The computations — minimal in nature — can be carried out easily and the results given here were evaluated taking into account the presence of harmonics to, at least, the twenty-fifth order.