High quality dispersions of hexabenzocoronene in organic solvents

We have studied the exfoliation and dispersion of hexabenzocoronene (HBC) in 28 different solvents. We see a wide range of dispersed concentrations and aggregation states, all of which can be related to the solvent properties. To a first approximation, the dispersed concentration is maximized for solvents with Hildebrand solubility parameter close to 21 MPa(1/2), similar to graphitic materials such as nanotubes and graphene. We have also studied the concentration dependence of the absorbance and photoluminescence of HBC for both a good solvent, cyclohexyl pyrrolidone (CHP), and a poor solvent, tetrahydrofuran (THF). In both cases, we observe features that can be associated with either individual molecules or aggregates, allowing us to establish metrics both for aggregate and individual molecule content. While the aggregate content always increases with concentration, good solvents disperse individual molecules at relatively high concentrations while poor solvents display aggregation even at low concentrations. Using these metrics, we determine that large populations of individual molecules are present at low concentrations in certain solvents with Hildebrand solubility parameters close to 21 MPa(1/2). However, the aggregation state of HBC is considerably more sensitive to solvent Hildebrand parameter for halogenated solvents than for amide solvents. We find a combination of high overall concentrations and large populations of individual molecules in four solvents: cyclohexyl pyrrolidone, 1-chloronaphthalene, 1-bromonaphthalene, and 1,2,4-trichlorobenzene. Scanning tunnelling microscopy (STM) measurements show the formation of self-assembled monolayers at the interface between a HBC-solvent dispersion and a highly oriented pyrolytic graphite (HOPG) substrate. Similar structures were observed on ultrathin supports by aberration-corrected transmission electron microscopy (TEM). Also observed were graphitic objects of size ~1 nm consistent with monomers or aggregated stacks of very few monomers. We believe this is strong evidence of the presence of individual molecules in dispersions prepared with appropriate solvents.     

Synthesis, magnetic and dielectric properties of Er-Ni doped Sr-hexaferrite nanomaterials for applications in High density recording media and microwave devices

A sol-gel combustion method has been successfully employed for the synthesis of Sr-hexaferrite nanomaterials doped with Er³⁺ and Ni²⁺ at strontium and iron sites, respectively. The X-ray diffraction analysis confirmed the single magnetoplumbite phase and the crystallite size was found to be in the range of 14-16 nm, suitable for obtaining signal-to-noise ratio in the high density recording media. The magnetic properties such as saturation magnetization (M_s), remanence (M_r) and coercivity (H_c) were calculated from hysteresis loops. M_s, M_r and H_c are observed to increase with the Er-Ni content. The dielectric constant (ε´) and dielectric loss (tan δ) is found to decrease with the increase in frequency and is explained on the basis of Maxwell-Wagner and Koops theory. The decrease in dielectric constant and dielectric loss but increase in saturation magnetization and remanence with Er-Ni content suggests that the materials are suitable for applications in microwave devices and high density recording media .     

Simple tensor products

Let ℱ be the category of finite-dimensional representations of an arbitrary quantum affine algebra. We prove that a tensor product S ₁ ⊗⋅⋅⋅⊗ S _N of simple objects of ℱ is simple if and only S _i ⊗ S _j is simple for any i<j.     

Thin and thick diffraction gratings: Thin matrix decomposition method

A brief review of the properties of transmission diffraction gratings is presented. Two types of gratings will be analyzed: thin and volume gratings explaining how the efficiency of the different orders that propagate inside the gratings can be calculated in both cases. For thin diffraction gratings the so-called amplitude transmittance method is applied in order to get the amplitude of the different orders, whereas in the case of volume gratings more complex methods are needed, such as Coupled Wave or modal theories. We will comment on the thin matrix decomposition method (TMDM), firstly proposed by Alferness, which gives a very intuitive approach and connects the properties of thin gratings to the properties of volume ones. The thin matrix decomposition method consists in dividing the volume grating in a number of thin gratings and applying the amplitude transmittance method to each thin grating. In this way the output of a grating will be considered as the input of the next and any individual grating can be treated by the amplitude transmittance method. The novelty of this work is that a comparison is made between the analytical expressions obtained by Alferness using the TMDM with the numerical results obtained using the coupled wave (CW) and rigorous coupled wave (RCW) theories for the efficiencies of the zero, first and second order when a plane wave incides onto a sinusoidal diffracion grating at the second on-Bragg replay angular condition.     

The effect of water on the transport of oxygen through nylon-6 films

The effect of presence of water on the transport of oxygen through films of Nylon-6 was evaluated at 5, 23, and 40°C by permeation experiments. Through the oxygen permeability experiments it was found that the diffusion of oxygen through Nylon-6 is not a simple Fickian process and the total diffusion process can be expressed by a bimodal diffusion mechanism. Permeability, solubility, and diffusion coefficients were determined as a function of water activity for both mechanisms. The effect of sorbed water on the oxygen diffusion and solubility in the polymer is presented as a function of the state of water in the polymer and as a result of the molecular competition between water and oxygen. © 1994 John Wiley & Sons, Inc.     

Steric influence in the direction of elimination of gas-phase pyrolyses of several highly branched secondary acetates

The rate coefficients for the gas-phase pyrolyses of a series of structurally related secondary acetates have been measured in a static system over the temperature range of 289.1–359.5°C and the pressure range 50.0–203.0 torr. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for 3-hexyl acetate, log k₁ (s^?) = (12.12 ± 0.33) ? (176.1 ± 3.9)kJ/mol/2.203RT; for 5-methyl-3-hexyl acetate, log k₁ (s^?) = (13.17 ± 0.20) ? (186.2 ± 2.3)kJ/mol/2.303RT; and for 5,5-dimethyl-3-hexyl acetate, log k₁ (s^?) = (12.70 ± 0.19) ? (177.4 ± 2.2)kJ/mol/2.303RT. The direction of elimination of these esters has shown from the invariability of olefin distributions at different temperatures and percentages of decomposition that steric hindrance is a determining factor in the eclipsed cis conformation. Moreover, a more detailed analysis indicates that the greater the alkyl–alkyl interaction, the less favored the elimination tends to be. Otherwise, an increase of alkyl–hydrogen interaction caused steric acceleration to be the determining factor.     

Determination of thiram in water and soils by cathodic stripping voltammetry based on adsorptive accumulation

Summary A very sensitive electrochemical stripping procedure for thiram is reported. Accumulation is achieved by adsorption of the compound on the hanging mercury drop electrode. Optimal experimental parameters include an accumulation potential of –0.2 V vs. Ag/AgCl, a 0.075 mol/l ammonia solution as supporting electrolyte and a differential pulse stripping mode. The detection limit is 0.3 ng ml^–1 after 120 s accumulation and 0.03 ng ml^–1 after 600 s accumulation. Results are reported for water and soil samples.

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Bestimmung von Thiram in Wasser und Böden mit Hilfe der Cathodic-Stripping Voltammetrie aufgrund adsorptiver Anreicherung

     

Magneto immunofluorescence assay for diagnosis of celiac disease

A magneto immunofluorescence assay for the detection of anti-transglutaminase antibodies (ATG2) in celiac disease was developed. The ATG2 were recognized by transglutaminase enzyme immobilized on the magnetic beads and then the immunological reaction was revealed by antibodies labeled with peroxidase. The fluorescent response of the enzymatic reaction with o-phenylenediamine and H₂O₂ as substrates was correlated with anti-transglutaminase titer, showing EC50 and LOD values of 1:11,600 and 1:74,500 of antibody titers, respectively. A total number of 29 sera samples from clinically confirmed cases of celiac disease and 19 negative control samples were tested by the novel magneto immunofluorescence assay. The data were submitted to the receiver-operating characteristic plot (ROC) analysis which indicated that 8.1 U was the most effective cut-off value to discriminate correctly between celiac and non-celiac patients. The immunofluorescence assay exhibited a sensitivity of 96.6%, a specificity of 89.5% and an efficiency 93.8% compared with the commercial optical ELISA kit.     

Syntesis and Crystal Structure of Cs2Nb6Br5F12: A Nb6 Cluster Compound with a One-Dimensional Nb6Br5F7F6 Unit Connection

The synthesis and the crystal structure of Cs₂Nb₆Br₅F₁₂ containing octahedral niobium clusters are presented in this work. This bromofluoride is based on a Nb₆Lⁱ₁₂F^a₆ (L=Br and F) unit and crystallizes in the orthorhombic system (space group, Cccm; Z=4; a=9.2446(2) Å, b=13.6256(3) Å, and c=17.1665(4) Å; R=0.0241). Fluorine and bromine are randomly distributed on the inner ligand positions, Lⁱ, that edge-bridge the Nb₆ cluster whereas fluorine fully occupies the apical positions (L^a). The units are linked to each other by apical ligands leading to an original one-dimensional unit connection. The cesium atoms are statistically distributed on several sites that describe parallel channels along the [1 0 0] direction. The influence of fluorine ligands upon the stabilization of this structure type as well as the structural relationships with Ba₂Zr₆Cl₁₇(B), Nb₆F₁₅, and NaMo₆Cl₁₃ will be evidenced and discussed.     

Determination of mercury in industrial workplaces by collection on activated carbon,solvent extraction and cold-vapour atomic-absorption spectrometry

A method for the determination of mercury in workplace environments in a chlor-alkali plant was optimized. Mercury was collected on activated carbon with a personal sampling pump at a flow-rate of 0.5 l/min, then the carbon was mineralized by heating with potassium permanganate and sulphuric acid. The mercuric ion was next chelated with ammonium pyrrolidinedithiocarbamate and extracted with methyl isobutyl ketone at acidic pH (1.6–5). The chelate in the extract was decomposed with sulphuric acid and the mercuric ion reduced to the metal with stannous chloride. The mercury was determined by coldvapour atomic-absorption spectrometry (CVAAS). The calibration graph was linear up to 5 ng/ml Hg in the initial solution, the relative standard deviation was 4.5% (for 2 ng/ml) and the detection limit was 0.14 ng/ml. All the figures of merit are referred to the initial sample. The proposed method gave good accordance with CVAAS without extraction.