Donnan equilibrium and the effective charge of sodium polyacrylate

Osmometry using an external stressor is a very useful method to measure the equilibrium osmotic pressure for dilute solutions of polyelectrolyte. By taking into account the contribution of the ideal gas law, the excluded volume, the solvency effect, and the Donnan equilibrium effect on the measured pressure it is possible to estimate the effective charge of sodium polyacrylate 35 kgmol⁻¹ as a function of the polymer concentration, the pH, the ionic strength, and the presence of Ca²⁺ ion. The numerical resolution of state equations has shown that the effective charge increases with the ionic strength or with the decreasing polymer concentration, in agreement with recent theoretical models. On the other hand, the effective charge is pH-independent. This statement remains valid as long as the degree of neutralization of the polyacrylate is over 0.5. Above this degree of neutralization, any further neutralization promoted by NaOH addition leads to the condensation of the Na⁺ counterion, in agreement with the general concept of ionic condensation. The effective charge represents only 10–20% of the total number of monomer units for pH within 6 and 9 and ionic strength below 0.1 M. The polymer can tolerate the presence of Ca²⁺ at least up to a molar ratio Ca²⁺/–COOH = 0.222 without any influence on the effective charge. Received: 11 July 2000/Accepted: 23 October 2000     

导电聚吡咯膜电极的现场电化学-ESR研究

本文报导聚吡咯(PP)膜电极在水溶液中的电化学─ESR行为, 定性地探讨了电位,电量对PP膜中极化学形成与转化过程的影响, 并在PP膜/溶液界面存在电化学反应的情形下, 考察了PP膜中可能的电荷传输机理。     

Rates of transport through a capsule membrane to attain Donnan equilibrium

The swelling of a capsule consisting of salt solution and polyelectrolyte, surrounded by a membrane, is studied. The membrane allows salt and water to pass, but is impermeable to polyelectrolyte molecules. Equilibrium swelling of the capsule is governed by Donnan equilibrium. Transport rates of a salt and water through the membrane are expressed in terms of a Darcy permeability and a salt diffusivity. The governing equations predict that the rate at which equilibrium is attained as the external salt concentration varies is controlled by the timescale for diffusion of salt, rather than by that for Darcy flow. Experiments were performed using capsules with membranes made of covalently linked HSA and alginate. The capsule volume varied with a single relaxation rate when the external salt concentration was changed, as predicted by theory. This constitutes the first step toward a simple method for determining the membrane properties of capsules by measuring rates of change of capsule volume.     

Donnan equilibrium of ionic drugs in pH-dependent fixed charge membranes: theoretical modeling

We have studied theoretically the partition equilibrium of a cationic drug between an electrolyte solution and a membrane with pH-dependent fixed charges using an extended Donnan formalism. The aqueous solution within the fixed charge membrane is assumed to be in equilibrium with an external aqueous solution containing six ionic species: the cationic drug (DH(+)), the salt cations (Na(+) and Ca(2+)), the salt anion (Cl(-)), and the hydrogen and hydroxide ions. In addition to these mobile species, the membrane solution may also contain four fixed species attached to the membrane chains: strongly acid sulfonic groups (SO(3)(-)), weakly acid carboxylic groups in dissociated (COO(-)) and neutral (COOH) forms, and positively charged groups (COO...Ca(+)) resulting from Ca(2+) binding to dissociated weakly acid groups. The ionization state of the weak electrolyte groups attached to the membrane chains is analyzed as a function of the local pH, salt concentration, and drug concentration in the membrane solution, and particular attention is paid to the effects of the Ca(2+) binding to the negatively charged membrane fixed groups. The lipophilicity of the drug is simulated by the chemical partition coefficient between the membrane and external solutions giving the tendency of the drug to enter the membrane solution due to hydrophobic interactions. Comparison of the theoretical results with available experimental data allows us to explain qualitatively the effects that the pH, salt concentration, drug concentration, membrane fixed charge concentration, and Ca(2+) binding exert on the ionic drug equilibrium. The role of the interfacial (Donnan) electric potential difference between the membrane and the external solutions on this ionic drug equilibrium is emphasized throughout the paper.     

Activation energy for polypyrrole oxidation: film thickness influence

The oxidation rates of polypyrrole films at different temperatures fit Arrhenius plots, allowing the obtention of the activation energy for the reaction. The activation energy increases for rising thicknesses, up to 4 μm, of the polymer film and decreases for rising film thicknesses. Those values include the constant chemical activation energy and the energy required to relax the polymeric structure allowing the entrance of anions from the solution. The existence of a maximum on the polymeric relaxation energy points to a parallel change on the film molecular structure during the electropolymerization time. The variation of the diffusion coefficient per degree of temperature for the counterions, as a function of the film thickness, is similar to that obtained for the activation energy. Diffusion coefficients were obtained from the electrochemical stretched exponential describing a range of relaxation behaviors in disordered and non-equilibrium systems.     

Donnan Exclusion Chromatography with Nonionic Adsorption

Abstract

The Donnan exclusion chromatography associated with nonionic adsorption has been studied on nucleotides (AMP, ADP and ATP) and aromatic acids (phthalic acid, benzole acid, salicylic acid and phenol) using crosslinked dextran cation-exchangers, SP-Sephadex and CM-Sephadex. It has been clarified experimentally based on a theoretical prediction that the combination of ionic exclusion and adsorption effects can make the separation in the Donnan exclusion range more effective in some cases.     

Labeled magnetic nanoparticles assembly on polypyrrole film for biosensor applications

In recent years, conducting polymers combined with metallic nanoparticles have been paid more attention due to their potential applications in microelectronics, microsystems, optical sensors and photoelectronic chemistry. The work presented in this paper describes the preparation and characterization of a nanocomposite composed by a thin polypyrrole (PPy) film covered with an assembly of magnetic nanoparticles (NPs). The magnetic particles were immobilized on PPy films under appropriate magnetic field in order to control their organization on the PPy film and finally to improve the sensitivity of the system in potential sensing applications. The electrical properties and morphology of the resulting PPy film and the PPy film/NPs composite were characterized with cyclic voltammetry, impedance spectroscopy (IS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and infra-red spectroscopy (IR). By using streptavidin labeled magnetic particles it was possible to functionalize the NPs assembly with biotin-Fab fragment K47 antibody. The designed biosensor had been successfully applied in rapid, simple, and accurate measurements of atrazine concentrations, with a significantly low detection limit of 5 ng/ml.     

Deposition of U(VI) at polypyrrole film electrodes modified with chlorophosphon azoIII

A modified Gr/ppy, chlorophosphon azoIII electrode was produced by the electrochemical polymerization of pyrrole and utilized for the deposition of uranyl ions. The polymerization rate of 1.43×10⁻¹ s⁻¹ was observed with the use of NBR, a precoating agent, which was 1.3 times faster than that without NBR. The amount of deposited uranyl ions determined by QCM was 1.12×10¹⁸ molecule cm⁻² when the polymerized ppy was 1.70 C cm⁻². The value for deposited uranyl ion was 2.68 times larger than that determined by the area of oxidation wave in CV, meaning that QCM might be the method of the choice for the investigation of uranyl ion deposition of a polymer modified electrode. The impedance of the ppy modified electrode increased with the consecutive modification of Gr/ppy,AzoIII,UO⁺ and the resistance for the electron transfer was 1.32 kΩ for the ppy-only electrode, 9.43 kΩ for AsoIII modification, and 11.82 kΩ for the deposition of the uranyl ion. The conductive process for Gr/ppy was diffusion controlled, however, for Gr/ppy,azoIII,UO⁺, it was thought to be the combination of ion doping process (low frequency region) and the electron transfer of the ppy film (high frequency region).     

Electrochemical synthesis of a polypyrrole thin film with supercritical carbon dioxide as a solvent

A conductive polypyrrole (PPy) film was successfully synthesized in a homogeneous supercritical carbon dioxide (scCO2)/acetonitrile (AN) system. The occurrence of a homogeneous supercritical state was confirmed by observations of the phase behavior of the system through a high-pressure cell with a viewing window. The concentration of a supporting electrolyte, tetrabutylammonium hexafluorophosphate (TBAPF6), significantly changed the phase behavior of the scCO2/AN system. The polymerization rate of the film in that system decreased with further addition of CO2. This result suggested that the low viscosity of scCO2 did not play an important role in improving the growth rate of the PPy film. The low polymerization rate might have been due to the electron-transfer resistance arising from the low dielectric constant of scCO2/AN mixture. The roughness of the film prepared in the homogeneous scCO2/AN system was 1/10 that synthesized in AN itself as a solvent. The slow growth of film and the high diffusion rate of the monomer seemed to account for the smooth flat film formation.     

Novel chemical synthesis of polypyrrole thin film electrodes for supercapacitor application

Present investigation describes the cost-effective, novel and simple chemical synthesis of polypyrrole (PPy) thin films for supercapacitor application. These PPy films are characterized by different techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The XRD pattern reveals the amorphous nature of PPy thin film, which is highly feasible for supercapacitors. Further, FTIR study confirms the formation of PPy. The surface morphological study exhibit the coverage of uniform and smooth morphology on thin film. The electrochemical supercapacitive properties of PPy thin films are evaluated using cyclic voltammetry (CV) in 0.5 M H₂SO₄ electrolyte, which exhibits the maximum specific capacitance of 329 Fg⁻¹ at the scan rate of 5 mV s⁻¹. Additionally, an equivalent series resistance (ESR) of PPy thin films is found to be 1.08 Ω using electrochemical impedance measurement.     

Interference-free glucose sensor based on glucose-oxidase immobilized in an overoxidized non-conducting polypyrrole film

Summary An amperometric glucose sensor is described; it is based on glucose oxidase immobilized in an overoxidized non-conducting polypyrrole membrane. The overoxidation process of polypyrrole produces a permselective, antifouling membrane capable of rejecting ascorbate, urate, acetaminophen and cysteine, as well as proteins and other surface active components typically present in serum. The amperometric assay of glucose in serum correlates well with an established routine procedure based on an enzymatic-colorimetric method.     

Electrochemical detection of Arachis hypogaea (peanut) agglutinin binding to monovalent and clustered lactosyl motifs immobilized on a polypyrrole film

Direct detection of peanut agglutinin/lactose interactions was realized by an electrochemical approach based on a polypyrrole coated electrode displaying pendant carbohydrates.     

Alkali‐treated semiconducting polypyrrole for infrared detection

A simple treatment of a polypyrrole (PPy) film with sodium hydroxide (NaOH) solutions reduces the conductivity (i.e., increased the resistance) and makes the film heat sensitive in the infrared (IR) spectrum with wavelength from 2.5 to 15.4 μm. The IR sensitivity increases with increasing treatment time in NaOH solutions and increasing exposed area to the radiation. The temperature coefficient of resistance, α, quantifying the sensitivity to IR, of the NaOH‐treated PPy is comparable with that of silicon and other conventional materials. The reduction and recovery of resistance due to IR on/off exposure exhibit a t^β dependence, where t is the time and β is determined to be around 0.5. This implies that the response of the material is controlled by diffusion of heat flux into and out of the sample. It is also observed that thermal ageing of the NaOH‐treated PPy film enhances the IR sensitivity. The sensing response of the treated films is observed beyond the thermal IR region to ultraviolet–visible wavelengths. Thus, the NaOH‐treated PPy appears to be a broadband sensing material. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Selective detection of extracellular glutamate in brain tissue using microelectrode arrays coated with over-oxidized polypyrrole

We describe the selective measurement of extracellular glutamate concentration in rodent brain using ceramic-based platinum microelectrode arrays (MEAs) coated with electropolymerized, over-oxidized polypyrrole (OPP) as a permselective barrier to interference from dopamine and ascorbate. OPP-coated MEAs displayed similar sensitivity (97 +/- 9 nA microM(-1) cm(-2)) and response time (ca. 1 s) to glutamate as previously described Nafion-coated MEAs but, unlike Nafion-coated MEAs, were unresponsive to 5 microM dopamine. Such OPP-coated MEAs afforded selective detection of evoked glutamate release in the dopamine-rich striatum of the mouse brain.     

Controlled electrochemical synthesis of polypyrrole nanoparticle thin film and its redox transition to a highly conductive and stable polypyrrole variant

We demonstrated here a unique method to produce a highly stable and conductive polypyrrole (PPY) nanoparticle film. The procedure entails controlling the redox switching and the electrochemical synthesis of PPY. PPY was synthesized at a very low forming potential or reaction rate in nonaqueous CH2Cl2 solvent to promote the PPY nanoparticle formation. Then its property was further optimized by first electrochemically reducing it at a hydrogen evolution potential in a neutral 0.1 M NaClO4, then in a slightly acidic 0.05 M asparagine electrolyte. The PPY nanoparticle thin film was characterized by AFM, UV-vis and EQCM. The procedures described here have proven to be reproducible. The data provided by the EQCM shows a reversible doping and undoping mechanism of asparagine indicating the presence of a highly conductive PPY variant. Both UV-vis and electrochemical characterization suggest that the PPY film made using our approach has excellent redox activity as well as high stability when characterized in asparagine solution. The reversible doping and undoping of asparagine during redox switching shows great potential of these PPY nanoparticle films as biological membranes for a broad range of biological applications.     

Charge regulation in protein ion-exchange chromatography: development and experimental evaluation of a theory based on hydrogen ion Donnan equilibrium

An extension of the stoichiometric displacement (SD) model for the ion-exchange adsorption of dilute proteins is developed which accounts for the effects of hydrogen ion Donnan equilibrium on the protein charge. The ability of the new model to fit retention data when the fluid phase pH is near the protein pI and the effects of hydrogen ion Donnan equilibrium are important is examined using four different proteins and four different column packings. The results indicate that the model is able to fit retention data using values for the protein pI and the change in protein charge with pH at the pI, i.e., (dz/dpH)pI, that are significantly closer to the values of these parameters determined by isoelectric focusing and acid-base titrametry in free solution, respectively, as compared to the values obtained by determining the characteristic binding change as a function of pH using the traditional stoichiometric displacement model. This suggests that when the fluid phase pH is near the protein pI, charge regulation is an important cause of the discrepancy between the electrical charge of a protein in free solution and the characteristic binding charge from the stoichiometric displacement model. The results also indicate that for the case where the fluid phase pH is near the protein pI, the new model accounts for the effect of charge regulation during protein ion-exchange adsorption more accurately than previous models in the literature.     

Hydrogen peroxide sensor based on horseradish peroxidase immobilized on an electrode modified with DNA-L-cysteine-gold-platinum nanoparticles in polypyrrole film

A third-generation mediator-free hydrogen peroxide biosensor was fabricated by combining electrodeposition and self-assembly techniques. In this strategy, Au-Pt hybrid nanoparticles were electrodeposited on a film of polypyrrole, and L-cysteine was assembled to their surface by exploiting the strong affinity between mercapto groups and nanoparticles. Then, DNA and horseradish peroxidase were respectively self-assembled on the surface of the electrode. The resulting electrode was characterized by impedance spectroscopy, and the electrode surface (without enzyme) was characterized by scanning electron microscopy. The response of the sensor towards hydrogen peroxide, as investigated by cyclic voltammetry and chronoamperometry, is linear between 4.9 µM to 4.8 mM, with a detection limit of 1.3 µM (at an S/N of 3). The apparent Michaelis-Menten constant $ \left( {K_M^{app}} \right) $ is 0.69 mM.     

Electrochemical fabrication of nanoporous polypyrrole film on HOPG using nanobubbles as templates

A versatile method for preparing nanoporous polypyrrole (PPy) film using electrogenerated nanobubbles as templates on highly oriented pyrolytic graphite (HOPG) is presented using in situ electrochemical atomic force microscopy (EC-AFM).