Electrochemical determination of serotonin and the competitive adsorption with dopamine at 5,5-ditetradecyl-2-(2-trimethylammonioethyl)-1,3-dioxane bromide lipid film modified by glassy carbon electrode.

A novel and effective approach to sensitively determine serotonin, known as 5-hydroxytryptamine (5-HT), has been proposed based on a 5,5-ditetradecyl-2-(2-trimethylammonioethyl)-1,3-dioxane bromide (DTDB) self-assembled lipid bilayer membrane modified glassy carbon electrode (DTDB/GCE). A DTDB/GCE shows the strong electrocatalysis for the oxidation of 5-HT, with the peak potential shifted to less positive value of 0.376 V vs. SCE, and effectively eliminates the interference from ascorbic acid (AA), even in the presence of 100-fold concentration of AA. Differential pulse voltammetry (DPV) gave a linear current for 5-HT from 2.0 x 10(-7) to 1.0 x 10(-5) M. At the DTDB/GCE, the oxidation of 5-HT was controlled by the adsorption process; for 5-HT coexisting with DA, the competitive adsorption was observed.     

Electrochemical reduction of 1-([(4-halophenyl)imino]methyl)-2-naphthols in aprotic media.

The electrochemical reduction of 1-([(4-halophenyl)imino]methyl)-2-naphthols on graphite electrodes was studied using cyclic voltammetry, chronoamperometry, constant-potential coulometry and preparative constant-potential electrolysis techniques. The data revealed that the reduction on graphite was irreversible and followed an EC mechanism. The diffusion coefficients and the number of electrons transferred were determined using the chronoamperometric Cottrell slope and the ultramicro disc Pt-electrode steady-state current. The number of electrons was also determined by bulk electrolysis. The compounds were subjected to constant-potential preparative electrolysis and the electrolysis products were purified and identified by spectroscopic methods. Based on these findings, a mechanism for the electro-reduction process is proposed.     

Selective extraction of iron(III) from aqueous nitrate solution in the presence of cobalt(II), copper(II) and nickel(II) ions using bis(delta2-2-imidazolinyl)-5,5'-dioxime.

The feasibility of using bis(delta2-2-imidazolinyl)-5,5'-dioxime (H2L) for the selective extraction of iron(III) from aqueous solutions was investigated by employing an solvent-extraction technique. The extraction of iron(III) from an aqueous nitrate solution in the presence of metal ions, such as cobalt(II), copper(II) and nickel(II), was carried out using H2L in binary and multicomponent mixtures. Iron(III) extraction has been studied as a function of the pH, equilibrium time and extractant concentration. From the extracted complex species in the organic phase, iron(III) was stripped with 2 M HNO3, and later determined using atomic-absorption spectrometry. The extraction was found to significantly depend on the aqueous solution pH. The extraction of iron(III) with H2L increases with the pH value, reaching a maximum in the zone of pH 2.0, remaining constant between 2 and 3.5 and subsequently decreasing. The quantitative extraction of iron(III) with 5 x 10(-30 M H2L in toluene is observed at pH 2.0. H2L was found to react with iron(III) to form ligand complex having a composition of 1:2 (Fe:H2L).     

Oxidation of isoniazid by quinolinium dichromate in an aqueous acid medium and kinetic determination of isoniazid in pure and pharmaceutical formulations.

The kinetics of oxidation of isoniazid in acidic medium was studied spectrophotometrically. The reaction between QDC and isoniazid in acid medium exhibits (4:1) stoichiometry (QDC:isoniazid). The reaction showed first order kinetics in quinolinium dichromate (QDC) concentration and an order of less than unity in isoniazid (INH) and acid concentrations. The oxidation reaction proceeds via a protonated QDC species, which forms a complex with isoniazid. The latter decomposes in a slow step to give a free radical derived from isoniazid and an intermediate chromium(V), which is followed, by subsequent fast steps to give the products. The reaction constants involved in the mechanism are evaluated. Isoniazid was analyzed by kinetic methods in pure and pharmaceutical formulations.     

Influence of surfactant and lipid chain length on the solubilisation of phosphatidylcholine vesicles by micelles comprised of polyoxyethylene sorbitan monoesters

Photon correlation spectroscopy and freeze-fracture electron microscopy have been used to determine the ability of a range of micelle-forming, polyoxyethylene (20) sorbitan monoesters (Tweens) to solubilise vesicles prepared from phosphatidylcholines of different acyl chain lengths and degrees of saturation with a view to rationalising (in terms of their membrane toxicity) which of the micelle-forming surfactants to use as drug delivery vehicles. The phosphatidylcholines used were dimyristoyl-, dipalmitoyl-, distearoyl- and dioleoylphosphatidylcholine (DMPC, DPPC, DSPC and DOPC, respectively) while the nonionic polyoxyethylene sorbitan monoesters studied were polyoxyethylene (20) sorbitan monolaurate (Tween 20), a 9:1 weight ratio mixture of polyoxyethylene (20) sorbitan monopalmitate and monostearate (Tween 40), a 1:1 weight ratio mixture of polyoxyethylene (20) sorbitan monopalmitate and monostearate (Tween 60), and polyoxyethylene (20) sorbitan monooleate (Tween 80). The ability of the Tween micelles to solubilise phospholipid vesicles was found to depend both upon the length of the surfactant acyl chain and the length of the acyl chains of the phospholipid comprising the vesicle. Vesicles composed of long saturated diacyl chain phospholipids, namely DSPC and DPPC, were the most resistant to solubilisation, while those prepared from the shorter acyl chained DMPC were more readily solubilised. In terms of their solubilisation behaviour, vesicles made from phospholipids containing long, unsaturated acyl chains, namely DOPC behaved more akin to those vesicles prepared from DMPC. None of the Tween surfactants were effective at solubilising vesicles prepared from DPPC or DSPC. In contrast, there were clear differences in the ability of the various surfactants to solubilise vesicles prepared from DMPC and DOPC, in that micelles formed from Tween 20 were the most effective solubilising agent while those formed by Tween 60 were the least effective. As a consequence of these observations it was considered that Tween 60 was the surfactant least likely to cause membrane damage in vivo and, therefore, is the most suitable surfactant for use as a micellar drug delivery vehicle.     

Fingering phenomena during spreading of surfactant solutions

Fingering instabilities are observed at the contact line of drops of surfactant solutions spreading spontaneously on solid surfaces coated by a film of solvent. The occurrences of instabilities, and the characteristics of the instability pattern, are controlled by the surfactant concentration and the thickness of the film adsorbed or deposited on the substrate. This work provides experimental data as a basis for forthcoming theoretical analyses.     

Novel fixed‐site–carrier polyvinylamine membrane for carbon dioxide capture

Fixed‐site–carrier membranes were prepared for the facilitated transport of CO₂ by casting polyvinylamine (PVAm) on various supports, such as poly(ether sulfone) (PES), polyacrylonitrile (PAN), cellulose acetate (CA), and polysulfone (PSO). The cast PVAm on the support was crosslinked by various methods with glutaraldehyde, hydrochloric acid, sulfuric acid, and ammonium fluoride. Among the membranes tested, the PVAm cast on polysulfone and crosslinked by ammonium fluoride showed the highest selectivity of CO₂ over CH₄ (>1000). The permeance of CO₂ was then measured to be 0.014 m³ (STP)/(m² bar h) for a 20 μm thick membrane. The effect of the molecular weight of PVAm and feed pressure on the permeance was also investigated. The selectivity increased remarkably with increasing molecular weight and decreased slightly with increased pressure in the range of 1 to 4 bar. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4326–4336, 2004     

Rheooptical investigation of the crystallization of poly(ethylene terephthalate) under tensile strain

The crystallization of poly(ethylene terephthalate) under uniaxial tensile strain at different extension rates was investigated with optical polarimetry in a temperature range between the glass-transition temperature and the quiescent crystallization temperature. The evolution of the optical properties of the polymer, including the turbidity, birefringence, and dichroism, were monitored simultaneously with the mechanical parameters. To complete the semicrystalline microstructure characterization of the polymer under strain, an online wide-angle X-ray diffraction (WAXD) technique was used in separate experiments, which were performed under the same thermomechanical conditions. For real-time measurements, a high-energy synchrotron radiation source was used. The optical properties provided information about both the crystalline and amorphous phases, whereas the WAXD patterns essentially gave information about the crystalline phase. The two experimental techniques were then used in a complementary way to characterize the semicrystalline microstructure. Significant deviations from the stress-optical rule were found. This was attributed to both transient effects and the appearance of crystallites, which consisted of highly oriented molecular segments that could contribute to the optical anisotropy but not necessarily to the stress. The behavior of the optical dichroism was found to be qualitatively different from that of the birefringence. The latter monotonically increased with the strain, whereas the former first increased with the strain, passed through a maximum, and then decreased to a steady-state value. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1915–1927, 2004     

Pressure and temperature dependence of the gas‐transport properties of dense poly[2,6‐toluene‐2,2‐bis(3,4‐dicarboxylphenyl)hexafluoropropane diimide] membranes

The gas‐transport properties of poly[2,6‐toluene‐2,2‐bis(3,4‐dicarboxylphenyl)hexafluoropropane diimide] (6FDA‐2,6‐DAT) have been investigated. The sorption behavior of dense 6FDA‐2,6‐DAT membranes is well described by the dual‐mode sorption model and has certain relationships with the critical temperatures of the penetrants. The solubility coefficient decreases with an increase in either the pressure or temperature. The temperature dependence of the diffusivity coefficient increases with an increase in the penetrant size, as the order of the activation energy for the diffusion jump is CH₄ > N₂ > O₂ > CO₂. Also, the average diffusion coefficient increases with increasing pressure for all the gases tested. As a combined contribution from sorption and diffusion, permeability decreases with increases in the pressure and the kinetic diameter of the penetrant molecules. Even up to 32.7 atm, no plasticization phenomenon can be observed on flat dense 6FDA‐2,6‐DAT membranes from their permeability–pressure curves. However, just as for other gases, the absolute value of the heat of sorption of CO₂ decreases with increasing pressure at a low‐pressure range, but the trend changes when the feed pressure is greater than 10 atm. This implies that CO₂‐induced plasticization may occur and reduce the positive enthalpy required to create a site into which a penetrant can be sorbed. Therefore, a better diagnosis of the inherent threshold pressure for the plasticization of a glassy polymer membrane may involve examining the absolute value of the heat of sorption as a function of pressure and identifying the turning point at which the gradient of the absolute value of the heat of sorption against pressure turns from a negative value to a positive one. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 354–364, 2004     

Relationship between the dynamic softening transition and wet sliding friction of elastomer compounds

For properly chosen elastomer compounds, thermorheological characterization is combined with an examination of the variation of the wet sliding friction with temperature. A conceptual argument leads to the assumption that the wet sliding friction should maximize at the energy dissipation peak associated with the dynamic softening transition at a characteristic frequency determined by the sliding speed and the effective smallest surface asperity scale. The dynamic softening transition is characterized with the peak in tan δ/G′ⁿ, where tan δ is the loss tangent, G′ is the elastic modulus, and n is a constant between 0 and 1. The William–Landel–Ferry transform is uncritically applied for extrapolating the position of the peak in tan δ/G′ⁿ at high frequencies. Even based on the criterion of tan δ, the results obtained on a concrete surface indicate that the effective smallest asperity scale is of order of 100 μm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2467–2478, 2004