Vladimír Mareček

Microelectrodes. definitions, characterization, and applications: (Technical report)

Theory, preparation,and applications of microelectrodes and microelectrode arrays are critically reviewed, and future trends in the field are outlined. An operational definition of a microelectrode is also recommended.

Double layers at liquid/liquid interfaces

The electrical double layer at the interface between two immiscible electrolyte solutions (ITIES) has been studied by the fast-galvanostatic-pulse method for the system consisting of aqueous NaBr and a solution of tetrabutylammonium tetraphenylborate in nitrobenzene. The double-layer capacity has been evaluated as a function of the potential difference across the interface. The modified Verwey–Niessen model, in which a layer of oriented solvent molecules (the inner layer) separates two space-charge regions (the diffuse double layer), seems to provide a reasonable framework to interpret the experimental data, assuming (i) that the approximations to the Poisson–Boltzmann equation by Gouy and Chapman are removed and (ii) that the boundary between the space-charge region and the inner layer is considered to be diffuse rather than sharp. The use of the tetrabutylammonium cation as the reference ion in voltammetric studies of the water/nitrobenzene interface is discussed.

The dependence of the electrochemical charge-transfer coefficient on the electrode potential: Study of the Fe(CN)63−/Fe(CN)64− redox reaction on polycrystalline Au electrode in KF solutions

Abstract The kinetics of the Fe(CN) 6 3− /Fe(CN) 6 4− redox reaction on a polycrystalline gold electrode in KF solutions was studied by the potential step method. It was found that the apparent rate constant of this reaction changes approximately linearly with the KF concentration and that the apparent charge-transfer coefficient is a linear function of the electrode potential. The experimental results, which were analyzed considering the formation of ion pairs between the reactant and potassium cations and also considering the effect of the double layer can be interpreted on the basis of the quantum theory of the electron-transfer reaction through a bridge activated complex. The value of the reorganization energy of the classic subsystem λ=(0.47±0.06) eV calculated from our results is comparable with the λ value derived from the results of other authors.

Ac impedance analysis of tetraethylammonium ion transfer at liquid/liquid microinterfaces

The kinetics of ion transfer reactions at the microhole-supported water/nitrobenzene interface have been studied by an ac impedance technique. A thin layer approximation of diffusional impedance has been derived for the geometry of the hole. In contrast to the large-area interface, the charge transfer resistance at the liquid microinterface is comparable in magnitude with the electrolyte resistance, which makes its determination more reliable. The apparent rate constant of tetraethylammonium ion transfer, kapp= 0.22 cm s–1, as determined at the liquid microinterface with an area ranging from 100 to 1000 µm2, has been found to be about twice as high as that previously reported.

The double layer at the interface between two immiscible electrolyte solutions—IV. Solvent effect

Differential capacitance of the electrical double layer at the water/nitrobenzene and water/1,2-dichloroethane interfaces was measured by an ac impedance technique. Potential difference across the diffuse layer in water, nitrobenzene and 1,2-dichloroethane and the corresponding capacitances, were calculated by using the modified Poisson-Boltzmann theory (version MPB4), which accounts for both the finite ion size and image forces. The two diffuse layers, one in water and the other in organic solvent, were assumed to be independent, ie they were associated only through an equal and opposite surface charge density. As compared with the Gouy-Chapman theory, the MPB theory provided a more reasonable description of the interface over the whole range of electrolyte concentrations and surface charge densities even in the system comprising the solvent of a low dielectric permittivity, such as 1,2-dichloroethane. By using the MPB theory, the interfacial potential difference and the inverse capacitance for both the systems studied were shown to split into two contributions, of which one is essentially independent of the electrolyte concentration and can be attributed to the presence of an ion-free inner (or compact) layer. The effect of the surface charge is to reduce the thickness of this layer, eg through the liquid phase interpenetration.

Selective complexation of biogenic amines by macrocyclic polyethers at a liquid/liquid interface

Abstract Electrochemical results show that selective complexation of β-phenylethylammonium ions by di-benzo-18-crown-6 at the water/nitrobenzene interface arises both from discrimination in binding these ions to the crown-ether macrocycles and from resolvation effects, which modify the energetics of the ion transfer. Since these effects are closely related to the structure of the transferred ion, chemical substitution (e.g. the introduction of OH or CH3 groups into the substrate ion molecule) can eventually reverse the ion exchange equilibrium at the interface. The results throw some light on the regulation of the passage of polar molecules through the lipidic barriers of biological systems.

Surface complex formation at the water/1,2-dichloroethane interface

Abstract Surface tension measurement at an aqueous drop/1,2-dichloroethane interface was used to study adsorption and surface reactions of calix[4]arene ligands. The found difference in surface tension in acidic and alkaline medium was ascribed to a difference in charge of the adsorbed ligand. Formation of a surface complex with Ba 2+ and Ca 2+ cations reduces the change in surface tension as compared to an adsorbed dissociated ligand with higher charge. A mechanism of the surface reaction is proposed.

Interfacial tension and impedance measurements of interfaces between two immiscible electrolyte solutions

Abstract Interfacial tension and impedance measurements were used to study the electrochemical properties of the interface between 0.1 M LiCl in water and 0.05 M tetrabutylammonium tetraphenylborate in nitrobenzene in the absence and the presence of various tetraalkylammonium ions. While the presence of these ions has no effect on the interfacial tension, the interfacial capacity can be considerably enhanced. This enhancement appears to be an artefact arising from the inadequate representation of the interface or the electrochemical cell by the Randles equivalent circuit employed for impedance data analysis. Modifications to this equivalent circuit are discussed.

Fluctuation analysis of liquid/liquid and gel/liquid interfaces

Abstract Voltage fluctuations resulting from the transfer of ions at liquid/liquid and gel/liquid interfaces were analyzed. Water/nitrobenzene, agar gel/nitrobenzene and water/PVC gel-nitrobenzene interfaces were investigated both in the absence and in the presence of picrate ions, dissolved in both phases. For frequencies higher than the range 6–60 Hz the power spectra were not influenced by the presence of picrate ions in the solution to a major extent. At lower frequencies, the real components of the equivalent impedance increased significantly less in the presence of picrate than in its absence. In this range, the resistivity was found to be approximately proportional to the inverse frequency squared, both in the absence and in the presence of picrate. By using an equivalent circuit approach, the effect of the cell geometry on the resulting power spectra has been examined.

Electrochemically controlled formation of a silicate membrane at a liquid|liquid interface

Abstract A new method is described for the preparation of a silicate membrane. Its principal idea lies in the separation of the template and precursor ions participating in sol–gel processes, by the interface between two immiscible electrolyte solutions (ITIES). The sol–gel process is initiated by potential-controlled adsorption of the template ions at the interface and their transfer from the organic to the aqueous phase. Association of the positive template ions with the silicate ions at the aqueous side of the interface produces neutral species which condense and form a compact layer at the interface. The thickness of the layer can be controlled by the magnitude of the electrical charge passed.