Electrokinetics in extremely bimodal suspensions

Prompted by the results obtained by Mantegazza et al. [Nature Physics 1 (2005) 103], where the electric birefringence of suspensions of elongated particles was strikingly affected by the presence of a sea of very small (size ratio lower than 10:1) colloidal spheres, we have undertaken an investigation of other electrokinetic phenomena in suspensions containing various relative concentrations of large (Teflon or polystyrene latex) and small (nanometer-sized silica spheres) colloids. We have determined the quantities that might be greatly affected by the size distribution of the particles, mainly in the presence of ac electric fields, since the response of the suspensions will show very characteristic relaxations, dominated in principle by the size of the particles. In this work, we report on measurements of the dielectric dispersion of mixed particles as a function of the concentration, ionic strength, and field frequency. The results indicate that the response is not just a simple combination of those obtained with suspensions of the individual particles, and in fact the presence of even small amounts of the small particles affects considerably the frequency response of the suspensions.     

Electrokinetics of soft particles

Theories of electrokinetics of soft particles, which are particles covered with an ion-penetrable surface layer of polyelectrolytes, are reviewed. Approximate analytic expressions are given, which describe various electrokinetics of soft particles both in dilute and concentrated suspensions, that is, electrophoretic mobility, electrical conductivity, sedimentation velocity and potential, dynamic electrophoretic mobility, colloid vibration potential, and electrophoretic mobility under salt-free condition.     

Deformation effects on ionic transport in polymeric films

When a periodical deformation is applied to a cellophane film through which an ion flux is established by a concentration gradient, an electrical signal, synchronous with the deformation, can be detected on electrodes placed on both sides of the membrane. This signal was studied as a function of the amplitude and frequency of the deformation and the ionic concentration. Ionic resistance, permeability, water content and ionic concentration measurements were carried out in order to explain the particular dependence of the signal with the ionic concentration. © 1994 John Wiley & Sons, Inc.     

Electrokinetics in microfluidic channels containing a floating electrode

Electrokinetic transport within a buffer-filled microchannel incorporating a flat bipolar electrode is investigated. The key finding is that the presence of the electrode disrupts the passage of electrical current through the microchannel and thereby alters the uniformity of the local electric field. Electroosmotic flow further modulates the local field gradient. These dynamics are demonstrated experimentally by utilizing the field gradient for concentration enrichment of negatively charged tracer molecules, and a set of computer simulations is presented to interpret the underlying electrokinetics.     

Gas transport through homogeneous and asymmetric polyethersulfone membranes

Both homogeneous and asymmetric polyethersulfone (PES) membranes were prepared by solvent casting. The sorption and permeation behavior of CO₂, O₂, and N₂ using these two kinds of cast PES membranes and commercially available homogeneous PES film was investigated to extract the pressure dependence of gas permeability and the permselectivity for CO₂ relative to N₂, and to confirm the validity of the working assumption that a skin layer in an asymmetric membrane can be essentially replaced by a thick homogeneous dense membrane. The pressure dependence of the mean permeability coefficient to CO₂ in homogeneous membranes obeys the dual-mode mobility model. The ideal separation factor for CO₂ relative to N₂ at an upstream pressure of 0.5 MPa attains ca. 40, while the permeability to CO₂ is about 2.7 Barrer at the same upstream pressure. The same separation factor in asymmetric membranes amounts to 35. The diffusion behavior for the skin layer in an asymmetric membrane with a thin skin layer can be simulated approximately by that in a homogeneous dense membrane. © 1993 John Wiley & Sons, Inc.     

Functional polymers. 6. Unusual catalysis of polymeric cinchona alkaloids in asymmetric reaction

Acrylonitrile-cinchona alkaloid copolymers catalyzed the asymmetric addition of benzyl mercaptan to 2-nitrostyrene to give always (+)-enantiomer in excess, indicating the participation of C(3)-chirality in the enantioface differentiating step.     

Defect induced asymmetric pit formation on hydroxyapatite

Defect sites on bone minerals play a critical role in bone remodeling processes. We investigated single crystal hydroxyapatite (100) surfaces bearing crystal defects under acidic dissolution conditions using real-time in situ atomic force microscopy. At defect sites, surface structure-dependent asymmetric hexagonal etch pits were formed, which dominated the overall dissolution rate. Meanwhile, dissolution from the flat terraces proceeded by stochastic formation of flat bottom etch pits. The resulting pit shapes were intrinsically dictated by the HAP crystal structure. Computational modeling also predicted different step energies associated with different facets of the asymmetric etch pits. Our microscopic observations of HAP dissolution are significant for understanding the effects of local surface structure on the bone mineral remodeling process and provide useful insights for the design of novel therapies for treating osteoporosis and dental caries.     

Alkaloid induced asymmetric electrocarboxylation of 4-methylpropiophenone

The alkaloid-induced electrocarboxylation of 4-methylpropiophenone is examined in mild conditions. Comparative studies with several inductors indicate that the efficient enantiodiscrimination of the electrocarboxylation depends on the nucleophilic quinuclidine nitrogen atom and the OH group of the inductors.     

2-Chlorotetrafluoroethanesulfinamide induced asymmetric vinylogous Mannich reaction

The generality of (S)-2-chlorotetrafluoroethanesulfinamide (CTFSA) induced asymmetric vinylogous Mannich (AVM) addition reaction has been investigated. The reaction of aldimines derived from (S)-CTFSA with 2-(tert-butyldimethylsilyloxy) furan (TBSOF) took place regioselectively at the γ-site at room temperature in DMSO under air atmosphere to give the desired addition products in syn-configuration with high diastereoselectivities (up to 98:2 dr). However, anti-configuration product as major isomer was obtained in the presence of tetra-butyl ammonium fluoride (TBAF) at −78 °C. Facile removal of the auxiliary group without epimerization demonstrated their synthetic potential for piperidone derivatives.     

Polyelectrolyte entry and transport through an asymmetric alpha-hemolysin channel

We study the entry and transport of a polyelectrolyte, dextran sulfate (DS), through an asymmetric alpha-hemolysin protein channel inserted into a planar lipid bilayer. We compare the dynamics of the DS chains as they enter the channel at the opposite stem or vestibule sides. Experiments are performed at the single-molecule level by using an electrical method. The frequency of current blockades varies exponentially as a function of applied voltage. This frequency is smaller for the stem entrance than for the vestibule one, due to a smaller coupling with the electric field and a larger activation energy for entry. The value of the activation energy is quantitatively interpreted as an entropic effect of chain confinement. The translocation time decreases when the applied voltage increases and displays an exponential variation which is independent of the stem or vestibule sides.     

Selective transport of cesium ion in polymeric CTA membrane containing calixcrown ethers

A series of calixcrown ethers for which the cavity size of the crown ring is varied from crown-6 to crown-7 to crown-8 were examined for the transport abilities toward alkali metal ions. These ligands were incorporated into supported liquid membranes (SLMs) and into polymer inclusion membranes (PIMs) composed of cellulose triacetate (CTA) as a support and 2-nitrophenyl octyl ether (NPOE) and tris(2-butoxyethyl) phosphate (TBEP) as a plasticizer. In both membrane systems, calixcrown-6 showed the best selectivity toward a cesium ion over other alkali metal ions. The polymeric CTA membrane showed more rapid transport rate as well as higher durability than did the SLMs.     

The transport of gases in synthetic polymeric membranes — an historic perspective

The early history of gas transport studies is briefly reviewed. These studies were almost entirely concerned with natural rubber membranes. By 1880 the simple form of the solution-diffusion model had been derived and checked and by 1920 the time lag method for determining diffusion coefficients had been developed. The work which followed, for the next thirty years, was mainly concerned with natural and synthetic elastomers all well above their glass transition temperature, mainly hydrocarbon in nature, and non-crystalline. p]A number of reasonably successful correlations were shown between the gas transport parameters and the physical and chemical properties of the polymers. It is clear now that these correlations break down with the polar polymers. p]Functional relationships which have stood the test of time are those between the force parameters of the gas and the logarithms of the solubility coefficients. The linear relationship between the pre-exponential factors (or the entropies) of the diffusion coefficients and the activation energies also holds for most polymers. The remarkable constancy of the ratios of the permeability coefficients for the atmospheric gases through a large number of different polymers also has continued to be valid. p]The arrival of numerous new polymers introduced the dual complications of crystallinity (and morphology) and the glassy state. The concept of dual mode sorption cast doubt on many of the results and conclusions reached with glassy polymers. Recent work, however, shows great promise that these difficulties can be overcome and attention can again be focused on the mechanism of such transport. Finally, the behavior of the gas transport parameters above and below the glass transition temperatures is discussed.     

Preparation of polymeric nanocapsules by radiation induced miniemulsion polymerization

In this research, polystyrene (PSt) nanocapsules with liquid cores were prepared by ⁶⁰Co γ-ray radiation induced miniemulsion polymerization, in which N-vinyl pyrrolidone (NVP) was used as the polar monomer. The characterization of polymer was carried out by ¹H NMR. It was verified that during polymerization, graft copolymerization between poly (pyrrolidone) (PVP) and PSt had taken place instead of random copolymerization. The interfacial tension between polymer and water was reduced because of the grafting reaction that had occurred, which was helpful to form nanocapsules. The influence of the ratio of St to NVP, the type and amount of the surfactant and the monomer/dodocane ratio on the particle morphology was studied by TEM. Finally, the releasing process of the synthesized nanoparticles was monitored by UV-vis measurement.     

手性助剂诱导催化不对称Strecker反应研究进展

亚胺的不对称Strecker反应是合成α-氨基酸化合物的重要途径之一,一般在手性助剂的诱导下进行.近年来,用于诱导不对称Strecker反应的各类手性助剂的研究受到广泛关注.本文综述了α-苯乙胺、α-氨基酸、α-氨基酸衍生的酰胺、α-苯甘氨醇、亚磺酰基亚胺、糖胺、肼等七类化合物作为手性助剂在不对称Strecker反应中的应用研究现状,就该研究领域存在的问题进行了分析,并对其前景进行了展望.     

Fluid transport in nanochannels induced by temperature gradients

We investigate the mechanisms of fluid transport driven by temperature gradients in nanochannels through molecular dynamics simulations. It is found that the fluid-wall interaction is critical in determining the flow direction. In channels of very low surface energy, where the fluid-wall binding energy ε(fw) is small, the fluid moves from high to low temperature and the flow is induced by a potential ratchet near the wall. In high surface energy channels, however, the fluid is pumped from low to high temperature and the pressure drop caused by the temperature gradient is the major driving force. In addition, as the fluid-wall interaction is strengthened, the flow flux assumes a maximum, where ε(fw) is close to the lower temperature T(L) of the channel and ε(fw)/kT(L) ≈ 1 is roughly satisfied.     

Cycloisomerization of 1,6-enynes: asymmetric multistep preparation of a hydrindane framework in water with polymeric catalysts

[Reaction: see text] Cycloisomerization of 1,6-enynes proceeded smoothly in water under heterogeneous conditions in the presence of a palladium complex supported on polystyrene-poly(ethylene glycol) copolymer resin to give the corresponding cyclopentanes with a high level of chemical greenness. Multistep asymmetric synthesis of a hydrindane framework was achieved via palladium-catalyzed asymmetric pi-allylic alkylation, propargylation, and cycloisomerization of 1,6-enynes, where all three steps were performed in water with recyclable polymeric catalysts.     

Sorption and permeation in elastic solids: applicability to gas transport in glassy polymeric materials

A thermodynamic analysis of sorption and transport in elastic solids is presented. The penetrant's chemical potential in the solid is calculated from the free energy changes that accompany deformation of the solid to accommodate the penetrant and mixing of the deformed solid and penetrant. Sorption isotherms are obtained by equating this chemical potential to that of that of the gas phase. The penetrant diffusivity is determined from a statistical mechanical analysis described in the literature. Both pure and mixed gas solubility, diffusivity and permeability are predicted. Mixed gas behavior is predicted using material parameters obtained from pure gas data only. The results suggest certain relationships between the dual mode model parameters that are found experimentally. Comparison with several experimental gas–polymer systems indicate the elastic solid analysis can reproduce pure gas data and predict mixed gas data well with physically realistic material parameters. The results provide a basis for investigating the relationships between the mechanical, volumetric, gravimetric and thermodynamic processes that give rise to transport.