Perchlorate interchange during the redox process of PPy/PVS films in an acetonitrile medium. a voltammetric and EDX study

Polypyrrole/poly(vinyl sulfonate) (PPy/PVS) films in acetonitrile containing 0.1 M LiClO4 were studied by cyclic voltammetry. Consecutive voltammograms pointed to a continuous increase in the charge involved in the process, suggesting a rise in the number of the electroactive participants involved in the redox process. However, voltammograms obtained for the PPy/ClO4 films in analogous conditions pointed to a steady-state behavior from the very early cycles. Theoretical studies based on the Nernst and Butler-Volmer equations indicated that perchlorate ions are involved during the oxidation/reduction process of the PPy/PVS films when the steady state is reached. This result was confirmed by "ex situ" energy-dispersive X-ray analysis of the films. In this regard, the electrochemical behavior of PPy/PVS polymers was similar to that of PPy/ClO4 films when a high number of cycles were carried out. The exchange of ClO4- during the redox reaction of the PPy/PVS films made it necessary to incorporate Li+ cations inside the polymer during the initial voltammetric cycles to compensate for the negative charges of PVS polyanions. Li+ cations are mainly stabilized inside the polymer by the ion pairs formed with the sulfonated groups of the PVS. An increase and shift of the voltammetric cycles indicated a restructuring of the polymeric chains with consecutive scans.     

In situ FTIR spectroscopy study of the break-in phenomenon observed for PPy/PVS films in acetonitrile

The in situ Fourier transform infrared (in situ FTIR) technique was used for the first time to investigate the break-in phenomenon observed for polypyrrole/poly(vinyl sulfonate) (PPy/PVS) films in acetonitrile containing 0.1 M LiClO(4). Consecutive potential scans provided a continuous increase of the infrared band intensities, simultaneous to an increase observed in the charge involved in the voltammetric peaks, suggesting a rise in the number of the polymeric chains participating in the infrared signal at the same time as the electroactive participants increase in the redox process. Moreover, in situ FTIR spectra evidence that the new infrared-activated chains in each voltammetric cycle adopt the same polymeric structure achieved by the chains activated in the initial cycles. However, if we achieve a cathodic potential limit of -2.1 V (vs Ag/AgCl), a restructuring of the polymeric morphology is observed. In situ FTIR spectra obtained for PPy/ClO(4) films under the same conditions pointed to a steady-state behavior from the very early voltammetric scans. Moreover, the intensities of FTIR bands obtained for PPy/ClO(4) films in the early voltammetric cycles are much higher than those obtained for PPy/PVS films after several potential scans. Only when high cathodic and high anodic potential limits were used for the consecutive cycles did the FTIR band intensities from PPy/PVS become similar to those obtained from PPy/ClO(4), indicating that in both films a similar number of polymeric chains were infrared active. Polarization at a high anodic potential (+1.3 V vs Ag/AgCl) produced overoxidation of the polymer appearing characteristic 1725 cm(-1) band assigned to the formation of carbonyl groups. Furthermore, the approximately 1540 cm(-1) band shifted to higher wavenumbers, indicating that overoxidation reduced the length of conjugated chains in the polypyrrole.     

Electrokinetics of the amphifunctional metal/electrolyte solution interface in the presence of a redox couple

Double layers (DL) at amphifunctionally electrified interfaces, such as that of an oxidized metal in an aqueous electrolyte solution, arise from coupling between ionic and electronic surface-charging processes. The electronic component enters the double-layer formation in the well-known situation where a potential is externally applied. In that case, the DL is fully or partly polarized depending on the possibility of interfacial electron transfer, that is, a faradaic process. This paper reports on the conjunction of the chemical/electrochemical processes at the interface in the case where the solution contains a redox-active couple. This makes it possible to polarize/depolarize a DL without invoking any external circuit. Streaming potential data obtained for the gold/(Fe(CN)6(3-)/Fe(CN)6(4-), KNO3) electrolyte interface are analyzed in terms of a recently developed theory which takes into account reversible bipolar faradaic depolarization, the inherent nonlinearity of the lateral field, and the effects of flow on the rate of the faradaic reactions. It appears that the theory largely overestimates the bipolar currents, leading to physically unrealistic zeta-potentials. A careful analysis of monopolar voltammetric data reveals quasi-reversible behavior of the redox couple under the typical convective conditions and electrolyte compositions met in electrokinetic experiments. Inclusion of reduced reversibility (the extent of which is position-dependent under the streaming-potential measurement conditions) leads to a consistent set of zeta-potentials which compare well to the values for the background electrolyte.     

Resonance Raman observation of the allyl cation produced after ultraviolet photodissociation of cyclopropyl bromide in acetonitrile solution

We have obtained transient resonance Raman spectra of the [CH₂CHCH₂]⁺ (allyl cation) produced following C-band excitation of cyclopropyl bromide. The experimental resonance Raman spectrum display an overtone progression in the nominal [CCC]⁺ stretch mode and its combination bands with the CH/CH₂ rocking modes. Density functional theory computations were performed to estimate the vibrational frequencies for the allyl cation, the allyl radical, the cyclopropyl radical, the cyclopropyl bromide molecule and the gauche-allyl bromide molecule and compared to the experimental vibrational frequencies. This comparison indicates that the allyl cation can be formed as a product of cyclopropyl bromide photodissociation in acetonitrile solution.     

氧化铝自配合溶液中吸附Zn^2^+及SCN^-的初探

研究了在不同PH、浓度、时间、温度、离子强度和螯合剂的条件下, 用不同浓度的硝酸预处理的氧化铝吸附剂自锌(II)-硫氰酸根配合水溶液中吸附Zn^2^+及SCN^-的特性, 并与自简单锌盐、硫氰酸盐水溶液中的吸附特性相比较, 结果表明配合物体系中的吸附作用不同于非配合物体系, 除了主要以离子交换机理进行外, 还可能在氧化物/水界面上发生配位吸附作用。     

Radiation-induced redox speciation of77Br in aqueous solution as determined by anion chromatography

⁷⁷Br is of potential use in diagnostic nuclear medicine as the parent of ultrashort-lived^77mSe in a radioisotopic generator system. The chemical form(s) of the radiobromine in aqueous solution, and their radiation-induced chemical transformations, are of importance for biomedical generator development. Approximately 150 mCi of radiobromine, initially present as Br^–, was studied over a period of three weeks and analyzed for BrO _x ^– (x=0–4) by anion exchange chromatography. Radiolysis of the solution gave rise to a small quantity of BrO ₃ ^– , but 99.5% of the activity remained as bromide.     

Kinetic model of quaternary ammonium cation transport through the human red blood cell membrane/electrolyte solution interface in vitro

Under the influence of quaternary ammonium cations the negative surface charge of human erythrocytes undergoes time-dependent changes, which can be detected microelectrophoretically. This transient response of the cell surface charge can be considered in terms of a current, reflecting both adsorption and permeation phenomena occurring at the mebrane/solution interface. Our paper presents a kinetic model of the observed behaviour and its electrical equivalent circuit, assuming adsorption and transport processes of the quaternary ammonium cations.     

The standard redox potential of the phenyl radical/anion couple

The voltammogram of aryldiazonium tetrafluoroborates in acetonitrile (ACN), at low concentration, shows a first one-electron wave followed at a more negative potential by a small second wave; this last one corresponds to the reduction of the radical formed at the level of the first wave. Simulation of the voltammogram permits one to determine the standard redox potential of the radical/anion couple Eo(Ph./Ph-) = 0.05 V/SCE and the reduction mechanism of the diazonium cation. An electron transfer concerted with the cleavage of the C-N bond furnishes the aryl radical; this radical undergoes two competitive reactions: reduction at the electrode and H-atom transfer.     

Polypyrrole (PPy) chemical synthesis with xylan in aqueous medium and production of highly conducting PPy/nanofibrillated cellulose films and coatings

Polypyrrole was chemically synthesised by using, for the first time, Birchwood xylan as additive, and ammonium peroxydisulfate (APS) as oxidant. The impact of additive concentration, polymerisation time and reagents concentration on PPy conductivity was studied. It was shown that, once fixed the pyrrole (Py)/APS and Py/xylan optimal ratios, the best conductivities (26 S/cm) were obtained for short polymerisation times (30 min) and increased reactants concentration. Morphological analysis of PPy particles, Py depletion kinetics and oxido-reduction potential measurements of the solutions provided interpretation elements on the impact of the polymerisation time on PPy pellet conductivity. Furthermore, optimised PPy particles obtained with xylan (PPy_x) were mixed with nanofibrillated cellulose (NFC) in order to obtain freestanding films. Their electrical and handling performances were evaluated at increasing PPy weight fraction in the samples. The conductivity mechanism of the most conductive sample (in comparison with a low performing sample) was investigated by measuring the conductivity as a function of temperature (4–350 K) and two transport regimes were identified. Selected formulations were finally used to produce conducting PPy/NFC coatings on non-absorbent (glass) and absorbent (copy paper) substrates. The impact of NFC in the percolation of PPy particles, then in the coating conductivity, was investigated.     

Influence of ammonium nitrate in phase transitions of Langmuir and Langmuir-Blodgett films at air/solution and solid/solution interfaces

The effect of ammonium nitrate on the phase transitions in Langmuir films of amphiphiles-stearic acid, stearyl amine (STAM), stearyl alcohol, dihexadecylphosphate, and the quarternized ammonium salt dioctadecyldimethylammonium bromide have been studied at air/water interface and in local ordering of their Langmuir-Blodgett films (LB films). The study shows that except for the stearyl amine (STAM) all other monolayers exhibit a liquid-expanded to liquid-condensed transition with slight expansion in area in the presence of ammonium nitrate. STAM monolayers show a new phase transition, which possibly arises due to the differently ionized amino groups, and change in solvation sheath due to an ion-dipole type interaction between the amino groups and the ammonium ion in the subphase. Mixed films of the amine with the acid and alcohol did not show such intermediate phases indicating that competing H-bonds between polar groups themselves and dipolar couplings between the polar groups and ammonium nitrate play a major role in the organization of the molecules at the interface. The above effect resulting in a change in the local order is borne out by Brewster angle micrographs (BAM) of the Langmuir films of STAM at air/solution interface. Such behavior is also seen at solid/liquid interfaces where the polar component of surface energy undergoes a drastic change for the amine films transferred onto solid substrates from the air/ammonium nitrate solution interface.     

超氧负离基研究: 无水乙腈中超氧负离子基的稳定性及其捕获

考察了影响无水乙腈中超氧负离子基(O2^-)稳定性的因素,发现氧促使乙腈中O2^-浓度迅速减小,通过产物鉴定和UV动力学分析提出氧促O2^-与乙腈反应的可能机理。研究了O2^-与2,2,6,6-四甲基-4-羟基-N-溴哌啶(NB)的反应,表明NB可望成为非水溶剂中O2^-的专一捕获剂。     

Comparison of n-tetradecane/electrolyte emulsions properties stabilized by DPPC and DPPC vesicles in the electrolyte solution

The properties of n-tetradecane/electrolyte emulsions with DPPC or DPPC vesicles in the electrolyte solution were investigated. The DPPC molecules form different aggregates, which possess different surface affinity, size and structure, and therefore we assumed some differences in the adsorption at the oil droplet/water interface. The n-tetradecane emulsions in 1:1, 1:2 and 1:3 electrolytes were prepared by mechanical stirring in the presence of DPPC at natural pH. Electrokinetic properties of the systems were investigated taking into account the effective diameter and multimodal size distribution of the droplets as well as the zeta potentials using the dynamic light scattering technique. The zeta potential of the droplets was negative in all systems with NaCl. In the emulsions with CaCl(2) at a higher concentration of electrolyte and emulsions with LaCl(3) with all investigated concentrations, positive values were observed. Similar measurements were performed for DPPC vesicles in the electrolyte solution. The pH and ionic strength changes induce those in the electrical charge of DPPC layer or vesicle surface. This is due to the fact that the DPPC molecule contains -PO(-) and -N(CH(3))(3) groups, which are in equilibrium with H(+) and OH(-), as well as other ions present in the solution, i.e. Na(+), Ca(2+), La(3+) or Cl(-). In the n-tetradecane/electrolyte emulsion stabilized by DPPC or DPPC vesicles the zeta potential may be also related to acid-base interactions. The effect of the ions from the solution on the DPPC layer adsorbed on n-tetradecane droplets or DPPC vesicles is discussed.     

Chemical instability promotes apparent electrochemical irreversibility: Studies on the electrode kinetics of the one electron reduction of the 2,6-diphenylpyrylium cation in acetonitrile solution

Concentration profiles are obtained via numerical solution for the species involved in EC and EC₂ mechanisms in a tubular flow cell. Voltammetric waves are simulated and the effect of the kinetic parameters on the waveshape is studied by considering the mass transport corrected Tafel slope. It is demonstrated that, when coupled to a follow up homogeneous step with sufficiently fast kinetics, a fast electron transfer may appear electrochemically irreversible in terms of its Tafel gradient. The electrochemical reduction of 2,6-diphenylpyrylium fluoroborate in acetonitrile solution is studied at a gold flow cell. Simulation of the recorded data permits us to infer a value of 1–2.5 × 10⁻³ cm s⁻¹ for the standard electrochemical rate constant for the 2,6-diphenylpyrylium cation/radical couple.     

Electron-induced reactions in condensed films of acetonitrile and ethane

Reactions in pure and mixed films of C(2)H(6) and CD(3)CN deposited on a Au surface at 35 K have been induced by low-energy electrons and investigated by Thermal Desorption Spectrometry (TDS). The incident electron energy (E(0)) was varied between 5 and 16 eV and a number of different products were identified. Beside the main products, CD(4), CD(3)H, and C(2)D(6), molecules resulting from atom scrambling during radical chain reactions (C(2)H(5)D) and recombination products (CD(3)CD(2)CN and C(2)H(5)CD(3)) were identified while others were characteristically absent. The quantity of the different products varied with E(0). The observed electron-driven processes are in accord with previous findings from gas phase experiments on dissociative electron attachment and electron impact ionization. On this basis, reaction mechanisms leading to the formation of the observed products are suggested for different ranges of E(0).     

Influence of solution conditions on formation of amphiphilic hyperbranched polyanion/linear polycation multilayer films

Amphiphilic hyperbranched polyester (P2) consisting of a hydrophobic core, surrounded by aromatic carboxylic acids, is self-assembled into aggregates in aqueous solution at pH region of 3.8–4.7 and in THF–water mixed solution at THF/water volume ratio of 1/100–1/10. With P2 in both aqueous and THF–water mixed solution as polyanion and linear poly(diallydimethylammonium chloride) (PDAC) as polycation, self-assembled films were successfully formed by layer-by-layer dipping. The solution condition of P2, including the pH of aqueous solution and the THF/water volume ratio, affected not only the absorption behavior of P2 but also the surface morphology and hydrophilicity of the films with P2 as the outmost layer. At lower pH or higher THF/water volume ratio the aggregation of P2 in solution was enhanced, thus resulting in higher adsorption rate for P2, more rough and less hydrophilic surface for the films.     

Geometrical preferences of the crotyl anion,radical and cation

ab initio MO calculations have been performed on the cis and trans isomers of the crotyl cation, free radical and anion in each of two orientations of the Me rotor about the allylic framework. In agreement with available experimental data, both the crotyl cation and free radical prefer trans skeletal geometries. On the other hand, the cis isomer of the crotyl anion is found to be more stable than the trans, the same preference as has been noted for alkali metal allyl organometallics in solution, but opposite to that recently reported for the free (gas phase) anion. The Me groups are predicted to eclipse the partial double bond for the trans isomers of all three systems and for the cis cation. These results are rationalized with the aid of perturbation MO theory.     

Adsorption of procaine at the mercury/electrolyte solution interface

The adsorption of the local anaesthetic procaine hydrochloride at the mercury/electrolyte interface solution is followed using capacitance measurements. The adsorption is studied at various procaine concentrations, in potassium chloride, potassium bromide or potassium fluoride used as supporting electrolytes, and at various pH values and temperatures. Procaine has basic properties with two acidity constants K. The results indicate the way the procaine molecules orientate at the interface. In all cases studied no hemimicelles or condensed film are observed.     

Resonance effect in the allyl cation and anion: a revisit

The interest over the magnitude of the conjugation effect in the allyl cation (1) and anion (2) has been revived recently by Barbour and Karty (J. Org. Chem. 2004, 69, 648-654), who derived the resonance energies of 20-22 and 17-18 kcal/mol for 1 and 2, respectively, using an empirical extrapolation approximation. This paper revisits the case by explicitly calculating the Pauling-Wheland resonance energy, which measures the stabilization from the most stable resonance structure to the delocalized energy-minimum state of a conjugated system, using our newly developed block-localized wave function (BLW) method. This BLW method has the geometrical optimization capability. The computations result in adiabatic resonance energies of 37 kcal/mol for 1 and 38 kcal/mol for 2. The significant disagreement between these values and Barbour and Karty's results originates from the neglect of structural and electronic variations in their derivation which are energy costing.