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961.
Matejícek P Uchman M Lokajova J Stepanek M Prochazka K Spírkova M 《The journal of physical chemistry. B》2007,111(29):8394-8401
The size and structural changes of nanoparticles formed after the addition of poly(2-vinylpyridine), PVP, to block copolymer micelles of polystyrene-block-poly(methacrylic acid), PS-PMA, were studied by light scattering and atomic force microscopy. Due to the strong hydrogen bonding between PVP and PMA segments, complex structures based on the core/shell micelles form in mixed selective solvents. As proven by a combination of light scattering and atomic force microscopy, individual PS-PMA micelles are "glued" together by PVP chains. The dialysis against solvents with a high content of water results in transient increase in polydispersity and turbidity of originally clear solutions. However, the precipitated polymer material dissolves in basic buffers and stable soluble nanoparticles reform in aqueous media. The behavior of their solutions was studied in a broad pH range by light scattering, atomic force microscopy and capillary zone electrophoresis. 相似文献
962.
The rate of oxidation of benzhydrol and benzhydrol-d to benzophenone by alkaline permanganate has been measured over a pH range 10 to 12.5 and pressures from 1 to 1000 bar. The pressure effect on the rate is small, Δ≠ = ?6 to ?8 cm3mol?1. This is held to support a hydrogen atom transfer in the rate-determining step. 相似文献
963.
Given an edge-weighted graph and an integer k, the generalized graph coloring problem is the problem of partitioning the vertex set into k subsets so as to minimize the total weight of the edges that are included in a single subset. We recall a result on the equivalence between Karush-Kuhn-Tucker points for a quadratic programming formulation and local optima for the simple flip-neighborhood. We also show that the quality of local optima with respect to a large class of neighborhoods may be arbitrarily bad and that some local optima may be hard to find. 相似文献
964.
Klemens Schachl Hailemichael Alemu K. Kalcher Helmut Moderegger Ivan Svancara Karel Vytras 《Fresenius' Journal of Analytical Chemistry》1998,362(2):194-200
A carbon thick film electrode modified with an MnO2-film is investigated as an amperometric detector for hydrogen peroxide in flow-injection analysis (FIA). At an operating
potential of +0.48 V vs. Ag/AgCl catalytic oxidation of the analyte is exploited for amperometric monitoring. Experimental
parameters, such as pH of the carrier, working potential, flow rate and injection volume, are optimized. The amperometric
signals are linearly proportional to the concentration of H2O2 in the range from 0.005 to 10 mg/L, showing a detection limit (3σ) of 2.3 μg/L. The method is applied to the determination
of H2O2 in rain water and to a simple assay to quantify glucose in human plasma.
Received: 29 January 1998 / Revised: 4 May 1998 / Accepted: 13 May 1998 相似文献
965.
Karel Goossens Kathleen Lava Peter Nockemann Dr. Kristof Van Hecke Dr. Luc Van Meervelt Prof. Dr. Kris Driesen Dr. Christiane Görller‐Walrand Prof. Dr. Koen Binnemans Prof. Dr. Thomas Cardinaels Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(3):656-674
N‐Alkyl‐N‐methylpyrrolidinium cations have been used for the design of ionic liquid crystals, including a new type of uranium‐containing metallomesogen. Pyrrolidinium salts with bromide, bis(trifluoromethylsulfonyl)imide, tetrafluoroborate, hexafluorophosphate, thiocyanate, tetrakis(2‐ thenoyltrifluoroacetonato)europate(III) and tetrabromouranyl counteranions were prepared. For the bromide salts and tetrabromouranyl compounds, the chain length of the alkyl group CnH2n+1 was varied from eight to twenty carbon atoms (n=8, 10–20). The compounds show rich mesomorphic behaviour: highly ordered smectic phases (the crystal smectic E phase and the uncommon crystal smectic T phase), smectic A phases, and hexagonal columnar phases were observed, depending on chain length and anion. This work gives better insight into the nature and formation of the crystal smectic T phase, and the molecular requirements for the appearance of this highly ordered phase. This uncommon tetragonal mesophase is thoroughly discussed on the basis of detailed powder X‐ray diffraction experiments and in relation to the existing literature. Structural models are proposed for self‐assembly of the molecules within the smectic layers. In addition, the photophysical properties of the compounds containing a metal complex anion were investigated. For the uranium‐containing mesogens, luminescence can be induced by dissolving them in an ionic liquid matrix. The europium‐containing compound shows intense red photoluminescence with high colour purity. 相似文献
966.
Josef Pola Miroslav Maryko Vladimír Vorlí
ek Zdenk Bastl Anna Galíkov Karel Vacek Rodica Alexandrescu Florian Dumitrache Ion Morjan Lavinia Albu Gabriel Prodan 《应用有机金属化学》2005,19(9):1015-1021
Nano‐magnetic, thermally stable iron‐based composites were obtained by a one‐step procedure consisting of continuous‐wave infrared laser‐induced and ethylene‐sensitized co‐pyrolysis of gaseous iron pentacarbonyl and hexamethyldisiloxane in argon. The simultaneously occurring formation of iron from iron pentacarbonyl and that of organosilicon polymer from hexamethyldisiloxane yield iron nanoparticles surrounded by an organosilicon polymer shell. The particles were characterized by spectral analyses, electron microscopy, thermal gravimetry and magnetic measurements. They become superficially oxidized in the atmosphere. Their composition, thermal behaviour and magnetic properties depend on the flow rates of the precursors and the total pressure of the procedure. Magnetization curves, exchange bias Hex at T = 5 K and AC susceptibility were studied in the temperature range 5–400 K. The values of Hex verified the observed degree of the particle surface oxidation. The system of the iron nanoparticles is in a ferromagnetic blocked state and the temperature dependence of the coercivity and susceptibility is in accord with the transmission electron microscopy data. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
967.
There is a relation between the irreversibility of thermodynamic processes as expressed by the breaking of time-reversal symmetry, and the entropy production in such processes. We explain on an elementary mathematical level the relations between entropy production, phase-space contraction and time-reversal starting from a deterministic dynamics. Both closed and open systems, in the transient and in the steady regime, are considered. The main result identifies under general conditions the statistical mechanical entropy production as the source term of time-reversal breaking in the path space measure for the evolution of reduced variables. This provides a general algorithm for computing the entropy production and to understand in a unified way a number of useful (in)equalities. We also discuss the Markov approximation. Important are a number of old theoretical ideas for connecting the microscopic dynamics with thermodynamic behavior. 相似文献
968.
Binary potassium and rubidium silicate glasses were irradiated with high-energy electrons. The changes induced in the glasses were observed as a decay of alkali X-ray signals with time. The decay curves displayed incubation periods, connected with changes of the transport mechanism. The electron exposures were interrupted relative to the incubation periods, next the glasses were annealed to enable easier relaxations, and successively irradiated again to finish the decay curve. Relaxation, which depends on the point of the decay curve at which exposure is interrupted, is calculated for the glasses. Relaxation was found to depend strongly on the interruption point with regard to the incubation period. The longer irradiation causes worse recovery of the structure during annealing, especially if the irradiation lasts longer than the incubation time. The larger ions and lower concentration also inhibit relaxation. 相似文献
969.
Modulating the Binding of Polycyclic Aromatic Hydrocarbons Inside a Hexacationic Cage by Anion–π Interactions 下载免费PDF全文
Dr. Nema Hafezi Dr. James M. Holcroft Dr. Karel J. Hartlieb Edward J. Dale Dr. Nicolaas A. Vermeulen Charlotte L. Stern Dr. Amy A. Sarjeant Prof. J. Fraser Stoddart 《Angewandte Chemie (International ed. in English)》2015,54(2):456-461
We report the template‐directed synthesis of BlueCage6+, a macrobicyclic cyclophane composed of six pyridinium rings fused with two central triazines and bridged by three paraxylylene units. These moieties endow the cage with a remarkably electron‐poor cavity, which makes it a powerful receptor for polycyclic aromatic hydrocarbons (PAHs). Upon forming a 1:1 complex with pyrene in acetonitrile, however, BlueCage?6 PF6 exhibits a lower association constant Ka than its progenitor ExCage?6 PF6. A close inspection reveals that the six PF6? counterions of BlueCage6+ occupy the cavity in a fleeting manner as a consequence of anion–π interactions and, as a result, compete with the PAH guests. This conclusion is supported by a one order of magnitude increase in the Ka value for pyrene in BlueCage6+ when the PF6? counterions are replaced by much bulkier anions. The presence of anion–π interactions is supported by X‐ray crystallography, and confirms the presence of a PF6? counterion inside its cavity. 相似文献
970.