Structure of Surfaces and Interfaces of Poly(N,N-dimethylacrylamide) Hydrogels

We investigated the surface structure of hydrogels of poly(N,N-dimethylacrylamide) (PDMA) hydrogels synthesized and cross-linked simultaneously by redox free radical polymerization. We demonstrate the existence of a less cross-linked layer at the surface of the gel at least at two different length scales characterized by shear rheology and by neutron reflectivity, suggesting the existence of a gradient in cross-linking. The composition of the layer is shown to depend on the degree of hydrophobicity of the mold surface and is weaker for more hydrophobic molds. While the macroscopic tests proved the existence of a relatively thick under-cross-linked layer, we also demonstrated by neutron reflectivity that the gel surface at the submicrometric scale (500 nm) was also affected by the surface treatment of the mold. These results should have important implications for the measurement of macroscopic surface properties of these hydrogels such as friction or adhesion.     

Self-induced pressure shift and temperature dependence measurements of CO2 at 2.05μm with a tunable diode laser spectrometer

By using a high resolution tunable diode laser absorption spectrometer combined with a cryogenically cooled optical multi-pass cell, we have measured the self-induced pressure shift coefficients for 8 transitions in the R branch of the (20(0)1)(III)←(00(0)0)(I) band of carbon dioxide around 2.05μm. This spectral region is of particular interest for the monitoring of atmospheric CO(2) with Differential Absorption Lidars (DiAL). The measurement of these shift coefficients was realized at five different temperatures ranging from 218 to 292K in order to determine their temperature dependence. The results are thoroughly compared to previous values reported in the literature for the (20(0)1)(III)←(00(0)0)(I) band of CO(2). The temperature dependence of the self-induced pressure shifts are reported experimentally for the first time for this specific CO(2) band.     

Probing simultaneously the volume and surface structure of nanospheres adsorbed at a solid-liquid interface by GISANS

We demonstrate in this paper the possibilities offered by Grazing Incidence Small Angle Neutron Scattering (GISANS) for the study of solid/liquid interfaces. We present experimental results obtained by Specular Neutron Reflectivity (SNR) and GISANS on a model system made of silica nanospheres adsorbed on a silicon wafer by electrostatic interactions both at solid/air interface and solid/liquid interfaces. At the solid/liquid interface, we demonstrate that grazing incidence scattering enables to discriminate the surface and the bulk scattering. The surface structure factor derived from GISANS shows that the nanospheres are organized as a repulsive liquid system, with a surface fraction occupation consistent with values obtained by SNR. This original setup highlights a direct correlation between the structure of the silica nanospheres in solution and their organization on the surface: due to the strong electrostatic repulsions between spheres, their organization at the surface is close to the projection in 2D of the 3D organization of the nanospheres in solution.     

Coupling of Simultaneous Fluorescence and Electrophysiology: Historical Survey, Contributions, and Prospects

The coupling of simultaneous fluorescence and eleclrophysiological measurements paved the way for the development of potentiometric and intracellular free ion-sensitive probes, although the basic and initial aim of these combined studies remains to record fast conformational changes during ion channel activation. Recent high-resolution studies of some channels put back on the agenda challenging methods that combine spectroscopy and electrophysiology. Fluorescence is certainly the most versatile and sensitive spectroscopy in this kind of experiment, and we have recently witnessed significant breakthroughs, at the level of whole cells with intact or mutated channels or with planar lipid bilayers doped with channels or their peptide models. After our initial study of membrane dynamics associated with excitability, following transient pyrene excimer signals during action potentials or voltage-clamp of nerve fibers, we tested the feasability of FRAP (fluorescence recovery after photobleaching) experiments on planar lipid bilayers. This technique was later applied to lateral diffusion measurements of channel forming peptides (alamethicin labeled with fluorescein and the voltage-sensitive segment S4 of the Shaker potassium channel labeled with NBD) under applied voltage. We provide independent evidence for voltage-dependent partitioning and transmembrane insertion and propose renewed experimental avenues to reveal movements and conformational changes associated with ion channel gating and opening.     

Dynamic mechanical properties of ionomer/ionomeric microbead composites

Composites of sulfonated polystyrene (PS-SSA) (0-8 mol % sulfonation) mixed with submicronic styrene-4 vinylpyridine (PS-4VP) (31 mol % 4VP content) microspheres were investigated by differential scanning calorimetry, Fourier transform infrared spectroscopy, and dynamic mechanical spectroscopy at 1 Hz in the glass transition region. The resulting proton transfer reaction from the SSA to the 4VP units was confirmed by IR spectroscopy, and led to a significant increase in the post-T_g Young's modulus as well as a lengthening of the rubbery plateau. Surprisingly, the addition of sulfonated polystyrene microspheres to styrene-vinyl pyridine copolymers had no such effects, possibly because of steric factors. © 1995 John Wiley & Sons, Inc.     

Automatic microanalyzers: VII. Automatic analyzer for rapid microdetermination of sulfur

A commercially available analyzer for determination of sulfur (0.5–100%) in organic and some inorganic compounds is described. It involves combustion of the sample at high temperature (1050 °C) to form SO₂ and SO₃ in a vertical reactor. SO₃ is reduced to SO₂ and nitrogen oxides to N₂ on copper at 850–900 °C. At this temperature the chemical reactions of SO₂ and copper are minimized so that SO₂ is obtained quantitatively in this range.Use of a vertical reactor and an autosampler makes an easy and complete automation of the sulfur determination possible. With this automation, great improvements are noticed in accuracy and precision over manual methods. The average time for a single determination is about 8 min.     

Sur le microdosage de l'azote d'après le principe de Dumas

Résumé Les auteurs ont apporté diverses améliorations à la technique deShelberg etSchöniger de microdosage rapide de l'azote d'après le principe deDumas. Elles ont permis d'éliminer plusieurs causes d'erreurs systématiques, d'augmenter l'universalité de la méthode et de faciliter les manipulations.

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Summary The authors have made some improvements in theShelberg andSchöniger rapid micro method of determining nitrogen based on theDumas procedure. Several sources of systematic errors have been eliminated, the range of the method has been widened, and the manipulations have been made easier.

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Zusammenfassung Bei dem Verfahren vonShelberg bzw.Schöniger zur raschen Bestimmung des Stickstoffes nach dem Prinzip vonDumas wurden verschiedene Verbesserungen ausgearbeitet, womit mehrere Fehlerquellen ausgeschaltet, die Anwendbarkeit des Verfahrens erweitert und seine praktische Ausführung erleichtert wurden.

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Dédié à l'occasion du 100^me anniversaire de la naissance deFriedrich Emich.     

The mechanism of the stoichiometric phase transfer carbonylation of benzyl bromide by the cobalt carbonyl anion

The kinetics of the stoichiometric phase-transfer carbonylation of benzyl bromide by the cobalt carbonyl anion have been investigated. Pseudo first order constants vary linearly with the amount of tetrabutylammonium chloride present. The effect of the solvent is partly due to its ability to extract the ion pair Bu₄N⁺Co(CO)₄^? into the organic phase. This ion pair is barely soluble in water and the amount of Co(CO)₄^? anion in the organic phase is kept constant by a simple anion exchange mechanism without migration of the tetrabutylammonium cation.     

The holographic bubble chamber experiment and the determination of the effective charmed quark mass and theK-factor for hadronic charm production

In the first holographic bubble chamber experiment — the HOBC experiment — we have accumulated a total of 40000 holograms with particle interactions. We have determined the total charm pair cross section inpN collisions to be 23.3 _?7.7 ⁺¹⁰ μb and 3.6 _?1.7 ^+2.3 μb for 360 and 200 GeV/c incident protons respectively. We have assumed a linear dependence of the cross section on the atomic number of the target. This experiment has demonstrated the feasibility of holographic recording in small bubble chambers. Assuming that the charm cross section can be described by the standard QCD factorized expression with gluon fusion and quark-antiquark annihilation, we have used our measured charm cross sections with other measurements to determine the effective charmed quark mass to be 1.8 _?0.35 ^+0.25 GeV/c². TheK factor, which describes the importance of the higher order corrections, is calculated to be 9.8 _?6.9 ^+12.5 (See noted added in proof.)     

Competition between entropy and electrostatic interactions in a binary colloidal mixture of spheres and platelets

We describe the phase behavior of an aqueous mixture of discotic nanoparticles of laponite and spherical magnetic nanoparticles of maghemite. To obtain stable mixtures from a chemical point of view, the maghemite nanoparticles are first coated by a thin layer of silica in order to adapt their surface chemistry to that of laponite nanoparticles: this enables one to raise volume fractions of maghemite Phi mag in the laponite suspensions up to several percent. Although the system is out of equilibrium, a "fluid-solid" state diagram was established showing that the mixtures undergo a fluid-solid transition, similar to that of pure suspensions of laponite, over a given volume fraction of laponite Phi lap and over a given Phi mag. An increase in Phi mag shifts Phi lap toward the lower values. When a solid sample is just above Phi lap, the application of an external magnetic field gradient induces a solid-to-liquid transition if the sample is located not too far from Phi lap on the state diagram. The structure of the mixtures, determined either at small scale by small-angle neutron scattering (SANS) or at intermediate scales by optical microscopy, shows that the solid samples are phase separated at a local scale: they are made of densely connected domains of laponite nanoparticles surrounding liquid pockets of maghemite nanoparticles. The size of the pockets grows with time. The magnetic liquid pockets are responsible for the rupture of the solid samples when an external magnetic field gradient is applied since their deformation induces local mechanical stress, internally damaging the network formed by the solid domains of laponite. The microscopic phase separation is the result of two opposite effects: (i) entropic effects that tend to phase separate the system macroscopically when the packing entropy overcomes the orientational entropy and (ii) long-range electrostatic repulsions that freeze the system.