Sonic analog of gravitational black holes in bose-einstein condensates

It is shown that, in dilute-gas Bose-Einstein condensates, there exist both dynamically stable and unstable configurations which, in the hydrodynamic limit, exhibit a behavior resembling that of gravitational black holes. The dynamical instabilities involve creation of quasiparticle pairs in positive and negative energy states, as in the well-known suggested mechanism for black-hole evaporation. We propose a scheme to generate a stable sonic black hole in a ring trap.     

A Regularization Technique for a Class of Singular Integral Problems

We discuss here a method for the extraction of the singularparts of a variety of problems involving singular integrands.The method is based on the systematic use of a partial fractionidentity; we give here applications to numerical quadratureand to the solution of singular integral equations of variouskinds.     

Black Holes in Bose–Einstein Condensates

It is shown that there exist both dynamically stable and unstable dilute-gas Bose–Einstein condensates that, in the hydrodynamic limit, exhibit a behavior completely analogous to that of gravitational black holes. The dynamical instabilities involve creation of quasiparticle pairs in positive and negative energy states. We illustrate these features in two qualitatively different one-dimensional models. We have also simulated the creation of a stable sonic black hole by solving the Gross–Pitaevskii equation numerically for a condensate subject to a trapping potential that is adiabatically deformed. A sonic black hole could in this way be created experimentally with state-of-the-art or planned technology.     

Segmentation and additive approach: A reliable technique to study noncovalent interactions of large molecules at the surface of single‐wall carbon nanotubes

This investigation explores a new protocol, named Segmentation and Additive approach (SAA), to study exohedral noncovalent functionalization of single‐walled carbon nanotubes with large molecules, such as polymers and biomolecules, by segmenting the entire system into smaller units to reduce computational cost. A key criterion of the segmentation process is the preservation of the molecular structure responsible for stabilization of the entire system in smaller segments. Noncovalent interaction of linoleic acid (LA, C₁₈H₃₂O₂), a fatty acid, at the surface of a (10,0) zigzag nanotube is considered for test purposes. Three smaller segmented models have been created from the full (10,0)‐LA system and interaction energies were calculated for these models and compared with the full system at different levels of theory, namely ωB97XD, LDA. The success of this SAA is confirmed as the sum of the interaction energies is in very good agreement with the total interaction energy. Besides reducing computational cost, another merit of SAA is an estimation of the contributions from different sections of the large system to the total interaction energy which can be studied in‐depth using a higher level of theory to estimate several properties of each segment. On the negative side, bulk properties, such as HOMO‐LUMO (highest occupied molecular orbital ‐ lowest occupied molecular orbital) gap, of the entire system cannot be estimated by adding results from segment models. © 2016 Wiley Periodicals, Inc.     

Theoretical and experimental study of the catalytic cathodic stripping square-wave voltammetry of chromium species

The electrocatalytic mechanism of Cr(III) reduction in the presence of diethylenetriaminepentaacetic acid (DTPA) and nitrate ions is studied theoretically and experimentally by using stripping square-wave voltammetry (SWV). Experimental curves are in excellent agreement with theoretical profiles corresponding to a catalytic reaction of second kind. This type of mechanism is equivalent to a CE mechanism, where the chemical reaction produces the electroactive species. Accordingly, the reaction of Cr(III)–H₂O–DTPA and \( {\mathrm{NO}}_3^{-} \) would produce the electroactive species Cr(III)–NO₃–DTPA and this last species would release \( {\mathrm{NO}}_2^{-} \) to the solution during the electrochemical step. In this regard, the complex of Cr(III)–DTPA would work as the catalyzer that allows the reduction of \( {\mathrm{NO}}_3^{-} \) to \( {\mathrm{NO}}_2^{-} \). Furthermore, it was found that the electrochemical reaction is quite irreversible, with a constant of k _s?=?9.4?×?10^?5 cm s^?1, while the constant for the chemical step has been estimated to be k _chem?=?1.3?×?10⁴ s^?1. Considering that the equilibrium constant is K?=?0.01, it is possible to estimate the kinetic constants of the chemical reaction as k ₁?=?1?×?10² s^?1 and k _?1?=?1.29?×?10⁴ s^?1. These values of k ₁ and k _?1 indicate that the exchange of water molecules by nitrate is fast and that the equilibrium favors the complex with water. Also, a value for the formal potential E°’?≈??1.1 V was obtained. The model used for simulating experimental curves does not consider the adsorption of reactants yet. Accordingly, weak adsorption of reagents should be expected.     

Para-Xylylenes and analogues by base-induced elimination from 1,4-bis-(dialkylsulfoniomethyl)arene salts in poly(1,4-arylene vinylene) synthesis by the wessling soluble precursor method

para-Xylylenes are generated by treatment of various 1,4-bis(dialkylsulfoniomethyl)arene dihalides with base in water, methanol, and aqueous acetonitrile, as shown by UV-Vis spectroscopy. This procedure allows the monitoring of the transient xylylene monomers that yield polyelectrolyte precursor polymers for poly(arylene vinylene)s, formed by variations of the chemistry developed originally by Wessling and co-workers. Alkoxy, alkyl, and halogen ring substituents on the sulfonium salt precursors do not greatly affect the ability to generate and observe the growth and decay of steady-state concentrations of the para-xylylene intermediates. Use of strong resonance-acceptor substituents—such as cyano or nitro—on the ring reduces production of a strong para-xylylene absorption, possibly due to reluctance of the ylides initially formed from the bis-sulfonium salts to eliminate to the xylylenes. By variation of UV-Vis conditions, it was found that use of 20% aqueous acetonitrile rather than water allowed formation of low to modest molecular weight polyelectrolytes in cyano-substituted cases (M_w = 8000–37,000). Use of UV-Vis test experiments should be useful for screening of bis sulfonium salt precursors that may be expected to give high molecular weight polyelectrolytes—in cases where para-xylylene formation is easy—as well as for finding reaction conditions that will optimize polymer formation. © 1992 John Wiley & Sons, Inc.     

Some natural aqueous extracts of plants as green inhibitor for carbon steel corrosion in 0.5 M sulfuric acid

The inhibiting impact of natural aqueous extracts of some plants such as curcumin, parsley and cassia bark extracts for the corrosion of carbon steel (C-steel) in 0.5 M H₂SO₄ solution was inspected utilizing some techniques such as galvanostatic and potentiodynamic anodic polarization and weight loss measurements. Outcomes indicated that the percentage inhibition efficiency increases with increasing the concentration of the extract due to its horizontal adsorption on the C-steel surface. The process of adsorption is followed by the Temkin isotherm. These natural extracts acted as pitting corrosion inhibitors by shifting the pitting potential to more noble values. The sequence of inhibition efficiency of the natural extracts decreases in the following order: cassia bark extract > parsley extract > curcumin extract. This arrangement is related to the molecular size of the major components of the three natural extracts used.     

Thermotropic copolyesters of a series of aromatic diols with phenylterephthalic acid and 4,4′-oxybisbenzoic acid

Wholly aromatic, liquid crystalline, main chain copolyesters derived from various linearly substituted aromatic diols with mixtures of 2-phenylterephthalic acid and a nonlinear aromatic dicarboxylic acid, 4,4′-oxybisbenzoic acid, were prepared by acidolysis condensation polymerization reactions and characterized for their liquid crystalline properties. The formation of a liquid crystalline phase at elevated temperatures was not prevented by the introduction of up to 50 mol % of the nonlinear diacid in the copolymers, and all of those copolyesters exhibited nematic liquid crystalline phases. Furthermore, the inclusion of a nonlinear monomer was not as effective as was the presence of a phenyl substituent in decreasing the melting transition of these copolymers. All of the copolymers had high glass transition temperatures and high thermal stabilities. © 1993 John Wiley & Sons, Inc.