Electrolyte Concentration Dependence Research Articles

Effects of Monolayer Density and Bulk Ionic Strength on Acid−Base Equilibria at the Air/Water Interface

The surface density and electrolyte concentration dependences of the pKa of CH3(CH2)21NH3+ at the air/water interface are obtained using the method of second harmonic generation. If it is assumed that the amphiphiles are uniformly distributed at the interface, the application of the Gouy−Chapman model yields pKa values ranging from 10.2 to 8.2 as the interface density varies from 100 to 22 A2/molecule. By accounting for the inhomogeneity of the interface, i.e., there are amphiphiles islands of “liquid phase” density immersed in a “gas phase” of very low-density amphiphiles, a single pKa of 10.1 ± 0.2 is obtained at all interface densities. The acidity of the long chain amine is enhanced by 3 times at the air/water interface compared with the bulk aqueous solution.

Voltammetric Measurement of Intermicellar Interaction Parameters; Correlation with Predicted Interaction Energies

We have used the rotating disk electrode (RDE) to measure micellar long-time self-diffusion coefficients (Ds) and intermicellar interaction parameters (kd) for cetyltrimethylammonium chloride (CTAC) micelles as a function of KCl concentration. Ds values were found to be a function of surfactant concentration and the observed behavior conformed to the linear interaction theory up to 1.00 mol dm-3 KCl. The electrolyte concentration dependence on Ds exhibited three regions of behavior. With increasing KCl concentration, Ds initially increased because of increasing Coulombic screening and then decreased linearly because of a linear spherical expansion of the micelles resulting from the increasing aggregation number, Nagg. Finally, Ds decreased precipitously because of the micellar structural transition from spherical to rod-shaped particles and associated electrolyte-dependent micellar elongation. Intermicellar interaction parameters were also found to be a function of electrolyte concentration and exhibited ...

Hole superexchange across a triazole bridged osmium monolayer/electrode interface†

The effects of electrolyte and temperature on the electrochemical response of spontaneously adsorbed monolayers of [Os(bpy)2bptCl], where bpy is 2,2′-bipyridyl and bpt is 3,5-bis(pyridin-4-yl)-1,2,4-triazole, on clean platinum microelectrodes are reported. While cyclic voltammetry of the Os2+/3+ redox reaction is nearly ideally reversible, the bipyridyl based reductions are reversible only for scan rates greater than approximately 5 V s–1, suggesting that the highly reduced species undergoes a subsequent chemical reaction. The electrolyte concentration dependence of the double layer capacitance, Cdl, was measured for monolayers containing only Os2+ centres at potentials on either side of the potential of zero charge, p.z.c. While the limiting interfacial capacitance observed at high electrolyte concentrations is approximately 25 µF cm–2 for potentials negative of the p.z.c., it decreases to only 12 µF cm–2 for potentials positive of the p.z.c. This observation suggests that at positive potentials the monolayers are relatively more perfect and contain less solvent and electrolyte ions. Oxidation of the monolayer to Os3+ causes Cdl to increase by less than 15%. These observations suggest that the physical location of charges within monolayers (bridge versus remote redox site) has a profound effect on the double layer structure. Chronoamperometry, conducted on a microsecond time-scale, was used to measure the heterogeneous electron transfer rate constant, k, for the Os2+/3+ redox reaction. For electrolyte concentrations above 0.1 M, redox switching is characterised by a single unimolecular rate constant (k/s–1). Tafel plots of the dependence of ln k on overpotential show that the rate constants for reduction of the Os3+ centres are approximately four times larger than those for oxidation of Os2+ sites within the monolayer for a wide range of overpotentials. This observation is interpreted in terms of a bridge mediated hole superexchange mechanism.

Analysis of charge transport impedance in the reduction of thin films of conducting polyaniline in aqueous trichloroacetic acid solutions

Abstract Some theoretical predictions of the charge transport properties of mixed electronic-ionic conductors are compared with experimental results for a thin-film polyaniline electrode in aqueous trichloroacetic acid solutions. The transport capacitance and characteristic time were determined from the impedance/admittance frequency dispersions. The supporting electrolyte concentration and film-thickness dependences were studied at several de potentials corresponding to various reduction levels of conducting polyaniline. The general form of the theoretical equation for the diffusion-migration impedance is found to give a good description of the system under study, i.e. a thin-film electrode in the first redox system of polyaniline. A very thin polymer layer that contacts the substrate electrode and so has different charge transport characteristics from the thicker polymer films is postulated. The enhanced ionic conductivity due to incorporation of excess supporting electrolyte in the porous conducting polyaniline is observed. The specific transport capacitances and apparent anion diffusion coefficients of thin polyaniline films in trichloroacetic acid solutions were found to be of the order of 102F cm−3 and 10−12−10−11 cm2 s−1 respectively.

Interfacial Field Effects on Reductive Chloride Elimination from Spontaneously Adsorbed Monolayers

For spontaneously adsorbed monolayers of [Os(bpy) 2 Cl(p2p)] + , where p2p is 1,2-bis(4-pyridyl)ethane and bpy is 2,2'-dipyridyl, reduction induces rapid loss of the chloride ligand. Cyclic voltammetry and chronoamperometry in DMF reveal that two sequential, one-electron ligand-based reductions lead to exchange of chloride for solvent at the metal center. High speed chronoamperometry has been used to probe the kinetics ofthis coupled electron-transfer chemical reaction. The heterogeneous electron transfer rate constant for the second electron transfer (k), linking charge states 0 and 1-, has been evaluated using nanosecond time scale chronoamperometry. The rate constant depends exponentially on overpotential for absolute overpotentials less than 0.200 V, and transfer coefficients (α) of 0.500 ± 0.08 are observed. The standard heterogeneous rate constant increases with increasing supporting electrolyte concentration over the range 0.01-0.5 M, and a limiting value of 7.8 ± 0.6 x 10 4 s -1 is observed for high electrolyte concentrations. The structure of the monolayer and the potential distribution at the modified interface have been probed from the electrolyte concentration dependence of the interfacial capacitance. These data suggest that, for some electrolyte concentrations, the electric field decays linearly over at least part of the monolayer thickness,leading to a large interfacial electric field at the reaction site. The kinetics of chloride elimination have been probed by monitoring the charge recovered during monolayer reoxidation as the reducing potential triggering chloride ligand loss is held for various times. These studies revealed that chloride elimination is a first-order process that occurs on a microsecond time scale. The rate of chloride loss (k elim ) increases linearly with an increasingly more negative potential, and k elim also increases linearly with increasing square root of the supporting electrolyte concentration. Temperature-resolved measurements of the elimination rate reveal that not only does the activation energy depend on the field strength but so too does the pre-exponential factor.

Impedance spectroscopy on poly(3-methylthiophene) films in aqueous solutions

Electrochemical impedance data for poly(3-methyl-thiophene) films deposited on platinum electrodes are collected in aqueous solutions of KNO 3 and HNO 3. The impedance results show that dedoped poly(3-methyl-thiophene) may be described as an electroactive electrode film into which ions are inserted upon oxidation. A transitional behaviour between the dedoped states is seen which may be adequately described by a Randles equivalent circuit. In the doped state in KNO 3, the electrolyte concentration dependence of the impedance spectra indicates that poly(3-methyl-thiophene) is best described as a porous metal electrode. Results similar to those in KNO 3 were obtained in KCl, K 2SO 4, KClO 4, except that no transitional regime were observed upon doping, and that in KCl and K 2SO 4 the polymer film behaved as an electrode with apparent infinite length pores in the doped state.

Impedance analysis of the electrochemical doping of poly(3-methyl-thiophene) from aqueous nitrate solutions

Electrochemical impedance data for poly(3-methyl-thiophene) (P3MeT) films deposited on platinum electrodes were collected in aqueous solutions of KNO 3 and HNO 3. The impedance results show that undoped P3MeT (0–400 mV vs. Ag/AgCl/3.5 M KCl) can be described as an electroactive electrode film into which ions are inserted upon oxidation. A space-charge layer develops in the polymer, probably at the polymer-solution interface. This space-charge layer can be adequately described as a diffuse double layer consisting of holes and counter-ions. At 600 mV a transitional behaviour is seen which can be described by a Randles equivalent circuit, and the response is similar to that of an ion-insertion electrode. This transition reflects the onset of doping, which is also seen as a break in the potential versus doping charge curve recorded by electrochemical voltage spectroscopy. Apparent diffusion coefficients are found to decrease from ca. 10 −7 cm 2/s at 0 mV to ca. 10 −10 cm 2/s at 600 mV. This decrease may be due to an enhancement of ionic transport by electrostatic coupling of the moving charged species (counter-ions and holes) below the insulator-metal transition. At 800 mV the thickness and electrolyte concentration dependences of the impedance spectra indicate that P3MeT is best described as a porous metal electrode. The experimentally determined values of capacitance per surface area are not consistent with typical metal/electrolyte double layers, unless unreasonably large surface-to-volume ratios are assumed for the porous electrode. Therefore it is suggested that the capacitance of the porous polymer electrode is due to doping (redox capacity). Replacing K + ions by protons had only a negligible effect on the electrochemical response of the film. Polarization at 1 V deactivates the film, and a new equivalent circuit is required to describe the degraded polymer. P3MeT is found to exhibit a doping-dependent wettability.

Piezoelectric Admittance Measurements of Montmorillonite/Poly(vinyl alcohol) Composite Film-Coated Quartz Crystal Oscillators

Abstract The piezoelectric admittance measurements of montmorillonite/poly(vinyl alcohol) (PVA) composite film-coated quartz crystal oscillators were performed in air dried over silica gel, in an atmosphere of moisture with 100% relative humidity at 25 °C and in contact with Na2SO4 aqueous solutions. By use of the electrical equivalent circuit model conventionally proposed for a piezoelectric quartz crystal, the relevant circuit parameters; resistance R, inductance L, and capacitance C were evaluated as a function of the Na2SO4 concentration (0.01–1.0 M (1 M = 1 mol dm−3)). R was almost constant above ca. 0.35 M and increased with decreasing the concentration in the lower concentrations, while L and C were almost independent of the concentration. On the basis of the electrolyte concentration dependence of R, the structural change of the clay/PVA composite film due to swelling with the electrolyte is discussed in comparison with the results for the clay film.

Charge formation in microporous membranes by single-acid site dissociation

The electrolyte concentration and pH dependence of the effective charge density of weak ion exchange membranes have been studied by combining solutions of the Poisson—Boltzmann equation in cylindrical pores with a simple dissociation equilibrium of weakly acid groups attached to the pore walls. Analytical expressions for the effective charge density and wall potential are presented which describe these quantities in terms of pH, electrolyte (1 : 1) concentration, acid constant, density of acid groups and the pore size. The concentration dependence of the effective fixed charge density experimentally observed for cellulose membranes and NaCl-solutions agrees quantitatively with the theoretical predictions. For track-etched mica membranes and KCl-solutions the influence of pH and electrolyte concentration on the effective charge density can be qualitatively explained. Also an interpretation of electro-osmotic findings obtained with an asymmetric cellulose acetate membrane and NaCl-solutions is given.

Oxygen Reduction on Pt in Aqueous K 2 CO 3 and KOH

The electrolyte concentration dependences of platinum oxidation and oxygen reduction (OR) on Pt were investigated in and electrolytes. Cyclic voltammetry was used to characterize the oxidation processes on the Pt electrode, and the rotating ring‐disk electrode (RRDE) technique was used to evaluate the kinetic processes for OR in and , at room temperature. The charge due to Pt oxidation reached a maximum at a concentration of about 2–3M for a range of electrode potentials. This is consistent with changes in the rates and the relative importances of the reversible and aging processes with changes in hydroxide‐ion activity. No maxima were observed with carbonate electrolytes at comparable ionic strengths. At high overpotentials, in both electrolytes, current densities were limited by the rate of adsorption onto Pt. The Tafel slope for this process was essentially infinite for the more‐oxidized surfaces. At lower overpotentials, the observed Tafel slope decreased and the fraction of current passing through the 2‐electron pathway increased with increasing hydroxide‐ion activity. In carbonate electrolytes, the Tafel slope was indicative of the direct 4‐electron pathway for OR with negligible amounts of detectable peroxide ion at all electrolyte concentrations. Comparisons of mass‐transfer‐corrected current densities for the two electrolytes at the same concentration showed higher currents in the electrolytes, due to the much lower solubility in . When normalized to the same concentration, the activity of Pt for OR is about two times greater in the carbonate electrolyte.

Dependence of bi-ionic potential across charged membranes on salt concentration.

Measurements of bi-ionic potential were carried out across anion-exchange membranes for KCl/KIO3 system and across cation-exchange membranes for KCl/NaCl system. The transport numbers of counterions in the membranes were nearly unity under the experimental conditions. The increase of bi-ionic potential was observed with an increase in the external electrolyte concentration. The electrolyte concentration dependence of bi-ionic potential would be due to both the changes in the mobility ratio of two counterions in the membrane and in the difference of standard chemical potentials between the membrane and the external solution.

Extraction of 1:1 electrolytes from aqueous solutions by resins containing dibenzo-18-crown-6 as anchor groups

The electrolyte concentration dependence of the distribution ratio of 1:1 electrolytes between resins containing dibenzo-18-crown-6 and water at 298 K was studied. The apparent equilibrium constants for the process taking place when the resin is equilibrated with aqueous solutions of different electrolytes are reported. The results obtained clearly reflect the influence of the anion on the extraction of cations by the resin containing the macrocyclic ligand.

Electrolyte concentration dependence of diffusiophoresis in liquids

The electrolyte concentration dependence of diffusiophoresis in liquids is studied in a diaphragm cell. A comparison is made between diffusiophoretic diffusivities calculated by ignoring the explicit concentration dependence and assuming the calculated values to be constant at average conditions and those calculated including the explicit electrolyte concentration dependence. The latter results are obtained from a mathematical solution involving Picard iterations. Results suggest that if the ratio between the endpoints of the range of electrolyte concentrations present is less than two, that the electrolyte concentration dependence is not very important. Further results suggest that the agreement between experiment and theory is best at electrolyte concentrations above 10 −3 M, in agreement with the infinitely thin double layer and low surface potential assumptions assumed in deriving the theory.

Multi-ionic Potential and Membrane Permeability Matrix. I. NaCl–CsCl Bi-ionic System with a Cation Exchange Membrane

Abstract This paper is concerned with analysis of membrane transport processes on the basis of nonequilibrium thermodynamics. Inter-ionic correlations between permeating ions are quantitatively estimated by applying the membrane permeability matrix theory to the system in which the NaCl and CsCl solution phases are divided by a highly selective cation exchange membrane. The membrane permeability matrix of the system may be expressed by four ionic correlation terms concerning with Na+ and Cs+. The electroconductive and diffusional membrane permeabilities are expressed as a function of the matrix elements having the dimension of permeating speed of ions. The conductive and diffusional permeabilities as well as the matrix elements are estimated from membrane potential, conductance, and ionic flux data. The external electrolyte concentration dependence of these permeability parameters is discussed. The conductive and diffusional permeabilities considerably differ from each other and their ratio changes with the external electrolyte concentration, whereas the permeability ratio of Na+ to Cs+ is constant. Electroconductive and diffusional characteristics of membrane transport processes are consistently analyzed in terms of the memberane permeability matrix theory.

Thermoosmosis through charged membranes. Theoretical analysis of concentration dependence.

A theoretical equation for thermoosmosis through charged membranes in electrolyte solutions is derived from nonequilibrium thermodynamics. The theory shows that the volume flux through the membrane is proportional to the temperature difference across the membrane. The proportionality constant, i.e., the thermoosmotic coefficient is a function of electrolyte concentration. The electrolyte concentration dependence of the thermoosmotic coefficient calculated is compared with our previous experimental results. Agreement between theory and experiments is satisfactory.