The porous structure of pulp fibres with different yields and its influence on paper strength

The porous structure of the interior of papermaking fibres is a well-known important property of the fibres. Changes of this structure will influence tensile and burst strength of paper formed from the fibres and a change in pore size of the pores within the fibre wall is also important for the ability of molecules to diffuse in and out of the fibre wall. Relevant examples of this latter effect are the removal of lignin during cooking and the addition of performance chemicals during papermaking. In this paper, pore sizes and the pore size distribution of unbleached softwood fibres have been studied. A well-characterised fibre material consisting of laboratory cooked spruce and pine pulp of various lignin contents was used. Pore size and pore size distribution were measured by studies of the relaxation behaviour of ²H in fibres saturated with ²H₂O. Beside this the total and surface charge of the fibres were also measured together with strength properties of papers from unbeaten fibres. For both pulps, there is a maximum in pore radius at a yield around 46%. Calculations of fibre wall volume from water retention values and yield levels show that there is a discontinuity in pore radius as a function of the fibre wall volume around a yield of 51%. It is suggested that this discontinuity is caused by the breakdown of the hemicellulose/lignin matrix within the fibre wall at this yield level. The strength of the papers formed from the fibres shows a correlation with the surface charge of the fibres. Based on the change in surface charge with yield and the change in total charge with yield, this correlation is suggested to be due to an opening up of the external part of the fibre wall. This stresses the importance of the chemical composition and physical structure of the outer layer of the fibre wall.     

Flow potentiometric and constant-current stripping analysis for silver(I) with carbon- and platinum-fibre electrodes

At concentrations above 50 μg l^?1, silver(I) is determined in nitric acid medium by means of potentiostatic deposition onto a platinum-fibre electrode and subsequent constant-current stripping in the sample or potentiometric stripping in a potassium permanganate medium. Interference from copper(II) is reduced by a pulsed potential procedure whereby copper deposited onto the fibre electrode is reoxidized intermittently. At concentrations below 50 μg l^?1, silver(I) is determined by using a mercury-coated carbon-fibre electrode and constant-current stripping in acetonitrile containing 0.20 M perchloric acid. Potentiostatic deposition for 30 min yielded a detection limit of 0.24 μg l^?1 silver(I) at the 3σ level.     

TiCl4 Dissolved in Ionic Liquid Mixtures from Аb Initio Molecular Dynamics Simulations

To gain a deeper understanding of the TiCl4 solvation effects in multi-component ionic liquids, we performed ab initio molecular dynamics simulations of 1-butyl-3-methylimidazolium [C4C1Im]+, tetrafluoroborate [BF4]−, chloride [Cl]− both with and without water and titanium tetrachloride TiCl4. Complex interactions between cations and anions are observed in all investigated systems. By further addition of water and TiCl4 this complex interaction network is extended. Observations of the radial distribution functions and number integrals show that water and TiCl4 not only compete with each other to interact mainly with [Cl]−, which strongly influences the cation-[BF4]− interaction, but also interact with each other, which leads to the fact that in certain systems the cation-anion interaction is enhanced. Further investigations of the Voronoi polyhedra analysis have demonstrated that water has a greater impact on the nanosegregated system than TiCl4 which is also due to the fact of the shear amount of water relative to all other components and its higher mobility compared to TiCl4. Overall, the polar network of the IL mixture collapses by including water and TiCl4. In the case of [Cl]− chloride enters the water continuum, while [BF4]− remains largely unaffected, which deeply affects the interaction of the ionic liquid (IL) network.     

Multiresidue method for the gas-chromatographic analysis of some polychlorinated and polybrominated pollutants in biological samples

Summary A multiresidue method for the analysis of a number of chlorinated and brominated organic compounds in biological samples has been developed. The method includes gas-chromatographic analysis of hexachlorinated and hexabrominated benzenes (HCB and HBB), polychlorinated and polybrominated biphenyls (PCB and PBB), polychlorinated phenols (PCP), guaiacols (PCG), paraffins (CP) and naphthalenes (PCN), polybrominated diphenyl ethers (PBDE) and the pesticides DDT, polychlorinated terpenes (PCC or toxaphene), chlordanes, hexachlorocyclohexanes (HCH), aldrin and dieldrin. The use of charcoal column in the clean-up procedure makes it also possible to analyze the planar PCB congeners (pPCB). Cod muscle samples were spiked with 10 and 50 times the detection levels of the compounds to be analyzed. The mean recoveries, relative to internal standards, for 40 compounds or compound groups were between 51 and 120%.     

Quantum mechanical/molecular mechanical free energy simulations of the glutathione S-transferase (M1-1) reaction with phenanthrene 9,10-oxide

Glutathione S-transferases (GSTs) play an important role in the detoxification of xenobiotics in mammals. They catalyze the conjugation of glutathione to a wide range of electrophilic compounds. Phenanthrene 9,10-oxide is a model substrate for GSTs, representing an important group of epoxide substrates. In the present study, combined quantum mechanical/molecular mechanical (QM/MM) simulations of the conjugation of glutathione to phenanthrene 9,10-oxide, catalyzed by the M1-1 isoenzyme from rat, have been carried out to obtain insight into details of the reaction mechanism and the role of solvent present in the highly solvent accessible active site. Reaction-specific AM1 parameters for sulfur have been developed to obtain an accurate modeling of the reaction, and QM/MM solvent interactions in the model have been calibrated. Free energy profiles for the formation of two diastereomeric products were obtained from molecular dynamics simulations of the enzyme, using umbrella sampling and weighted histogram analysis techniques. The barriers (20 kcal/mol) are in good agreement with the overall experimental rate constant and with the formation of equal amounts of the two diastereomeric products, as experimentally observed. Along the reaction pathway, desolvation of the thiolate sulfur of glutathione is observed, in agreement with solvent isotope experiments, as well as increased solvation of the epoxide oxygen of phenanthrene 9,10-oxide, illustrating an important stabilizing role for active site solvent molecules. Important active site interactions have been identified and analyzed. The catalytic effect of Tyr115 through a direct hydrogen bond with the epoxide oxygen of the substrate, which was proposed on the basis of the crystal structure of the (9S,10S) product complex, is supported by the simulations. The indirect interaction through a mediating water molecule, observed in the crystal structure of the (9R,10R) product complex, cannot be confirmed to play a role in the conjugation step. A selection of mutations is modeled. The Asn8Asp mutation, representing one of the differences between the M1-1 and M2-2 isoenzymes, is identified as a possible factor contributing to the difference in the ratio of product formation by these two isoenzymes. The QM/MM reaction pathway simulations provide new and detailed insight into the reaction mechanism of this important class of detoxifying enzymes and illustrate the potential of QM/MM modeling to complement experimental data on enzyme reaction mechanisms.     

Effects of pH on the kinetic reaction

In most cases, kinetic unfolding reactions of proteins follow a simple one-step mechanism that does not involve any detectable intermediates. One example for a more complicated unfolding reaction is the acid-induced denaturation of holo-myoglobin (hMb). This reaction proceeds through a transient intermediate and can be described by a sequential two-step mechanism (Konermann et al. Biochemistry 1997, 36, 6448-6454). Time-resolved electrospray ionization mass spectrometry (ESI MS) is a new technique for monitoring the kinetics of protein folding and unfolding in solution. Different protein conformations can be distinguished by the different charge state distributions that they generate during ESI. At the same time this technique allows monitoring the loss or binding of noncovalent protein ligands. In this work, time-resolved ESI MS is used to study the dependence of the kinetic unfolding mechanism of hMb on the specific solvent conditions used in the experiment. It is shown that hMb unfolds through a short-lived intermediate only at acidic pH. Under basic conditions no intermediate is observed. These findings are confirmed by the results of optical stopped-flow absorption experiments. This appears to be the first time that a dependence of the kinetic mechanism for protein unfolding on external conditions such as pH has been observed.     

Barrier and mechanical properties of modified starches

The study addressed starch-based coatings on paper and fabrics. Coated materials and free starch films containing different amounts of a well-established plasticizer (glycerol) or potential plasticizer (mainly polyols) were tested with respect to water vapour permeance (WVPe), water vapour permeability (WVP), glass transition temperature (T_g), and mechanical strength (tensile tests). Both normal and high- amylose potato starch were used. These starches were modified by (a) oxidation, (b) oxidation and hydroxypropylation or (c) oxidation and hydrophobically modified by reaction with octenyl- or alkenyl-substituted succinic acid anhydride. Free films of hydroxypropylated high-amylose potato starch showed a lower WVP than did the corresponding starches based on regular potato starch. The WVP of the hydrophobically modified regular potato starches was substantially higher than that of films of the corresponding hydroxypropylated starches. The expected hydrophobic effect of the succinic acid anhydrides in terms of a reduced WVP could not be observed. When glycerol was used as a plasticizer, about 30 parts (by wt.) per hundred parts of starch were needed in order to reduce the T_g and to cause observable changes in the mechanical properties of the free films.     

An enzyme reactor electrode for determination of amino acids

L-Leucine can be determined with an enzyme reactor electrode containing L-amino acid oxidase immobilized with glutaraldehyde to glass. The reactor also contains immobilized catalase which splits the hydrogen peroxide formed. Oxygen for the reaction is also supplied by adding hydrogen peroxide to the samples. The electrode is an ammonia gas sensor. The calibration curve is strictly linear with Nernstian slope between 3·10^-5 and 10^-3 M leucine.     

A Long Cycle-Life High-Voltage Spinel Lithium-Ion Battery Electrode Achieved by Site-Selective Doping

Spinel LiNi_0.5Mn_1.5O₄ (LNMO) is a promising cathode candidate for the next-generation high energy-density lithium-ion batteries (LIBs). Unfortunately, the application of LNMO is hindered by its poor cycle stability. Now, site-selectively doped LNMO electrode is prepared with exceptional durability. In this work, Mg is selectively doped onto both tetrahedral (8a) and octahedral (16c) sites in the Fd m structure. This site-selective doping not only suppresses unfavorable two-phase reactions and stabilizes the LNMO structure against structural deformation, but also mitigates the dissolution of Mn during cycling. Mg-doped LNMOs exhibit extraordinarily stable electrochemical performance in both half-cells and prototype full-batteries with novel TiNb₂O₇ counter-electrodes. This work pioneers an atomic-doping engineering strategy for electrode materials that could be extended to other energy materials to create high-performance devices.     

Reductive potentiometric stripping analysis for elements forming sparingly soluble mercury compounds with amalgamated metal as the reducing agent

The suitability of amalgamated metals as reducing agents in reductive potentiometric stripping analysis is demonstrated. The amalgams are generated in a mercury pool by electrolysis of dissolved metals. During the stripping process, the reducing agent, which is stored inside the working electrode, reacts with sparingly soluble mercury compounds of the analytes preconcentrated on the electrode surface. The importance of the timing of the processes involving amalgam formation and analyte preconcentration is discussed. With amalgamated sodium, the technique is suitable for the determination of selenium and sulphur at the 10^-7 M level with 1–2 min preconcentration. Halides may be determined at the 10^-6 M level with a few seconds of preconcentration; a less powerful reducing agent such as amalgamated zinc is then suitable.