Petr Vanýsek

INTERFACE BETWEEN TWO IMMISCIBLE LIQUID ELECTROLYTES: A REVIEW

An overview on electrochemistry on the interfaces between two immiscible electrolyte solutions is given.

Tuning ion correlations at an electrified soft interface

Ion distributions play a central role in various settings—from biology, where they mediate the electrostatic interactions between charged biomolecules in solution, to energy storage devices, where they influence the charging properties of supercapacitors. These distributions are determined by interactions dictated by the chemical properties of the ions and their environment as well as the long-range nature of the electrostatic force. Recent theoretical and computational studies have explored the role of correlations between ions, which have been suggested to underlie a number of counterintuitive results, such as like-charge attraction. However, the interdependency between ion correlations and other interactions that ions experience in solution complicates the connection between physical models of ion correlations and the experimental investigation of ion distributions. We exploit the properties of the liquid/liquid interface to vary the coupling strength of ion–ion correlations from weak to strong while monitoring their influence on ion distributions at the nanometer scale with X-ray reflectivity and the macroscopic scale with interfacial tension measurements. These data are in agreement with the predictions of a parameter-free density functional theory that includes ion–ion correlations and ion–solvent interactions over the entire range of experimentally tunable correlation coupling strengths (from 0.8 to 3.7). This study provides evidence for a sharply defined electrical double layer for large coupling strengths in contrast to the diffuse distributions predicted by mean field theory, thereby confirming a common prediction of many ion correlation models. The reported findings represent a significant advance in elucidating the nature and role of ion correlations in charged soft matter.

The Effect of Film Thickness and Growth Method on Polyaniline Film Properties

The properties of polyaniline (PANI) films, grown electrochemically on Au using the potential sweep method to two different upper potential limits (type I and II) to various thicknesses in sulfuric acid solution, were studied systematically using cyclic voltammetry, ac impedance, and for the first time, by tracking the rate of the hydrogen evolution reaction (HER). The HER results show that both films follow the nucleation and growth mechanism initially and that continuing growth occurs primarily at the outer PANI surface. Because type II PANI films are formed at a much more rapid rate than type I films, a greater amount of anodic degradation products are incorporated into type II films. Impedance and HER measurements show that type II films, compared for the first time in this work with type I films, have a more open PANI film/Au interfacial structure. A new observation is that the cyclic voltammograms for type I films show a prepeak which is absent for type II films. Furthermore, constant PANI redox peak potentials are observed for type I films, whereas these shift with film thickness for type II films. This unique observation implies that type II films undergo structural changes during their growth, while type I films remain more uniform.

Charge transfer processes on liquid/liquid interfaces: The first century

Abstract Electrochemistry dealing with interfaces between two immiscible and ionically conductive solutions (ITIES) has been documented in literature already a hundred years ago. The work has only recently attracted a wider attention, with more modern treatment founded by the late Professor Jiři Koryta. The principle of the work, its past and new trends in experimental electrochemistry, optical studies of the interface and recently emerging theoretical work are reviewed.

Interfacial potential differences at mixed conductor interfaces : Nernst, Nernst-Donnan, Nernst distribution and generalizations

Abstract The lesser known equations of Nernst that have been crucial in the theoretical basis of membrane electrochemistry, prove to have important applications in modern cells using ion conductors and mixed conductors as electrode modifiers. This paper elaborates the multi-ion equilibrium potential differences, especially emphasizing redox ions partitioned between electrolytes and various ion conductor films and solids. Methods for calculation of interfacial potential differences independent of whole cell potential differences are developed using the electrochemical potential concept for electrons and ions. Principal new results include use of the Fermi level of electrons in redox systems. Applications to non-classical interfaces: electrolyte/pure ionic and mixed, give general interfacial potential difference expressions that reduce in limits to the Nemst-Donnan, Nemst Distribution potential difference, and cover intermediate Donnan exclusion failure. New formulations appropriate to simultaneous ion exchanging and electron exchanging interfaces are introduced. Errors in the theory of redox pds at liquid/liquid interfaces are corrected. The prevalence of steady state as well as equilibrium membrane cells in modern symmetric (ion selective electrode) and asymmetric (modified electrode) cells is noted.

Characterization of porous aluminum oxide films by metal electrodeposition

Abstract It is well known that porous (with underlying compact oxide) aluminum oxide films of varying pore size and diameter can be formed electrochemically on the surface of aluminum, depending on the oxidation conditions employed. In the present work, the properties of various porous Al oxide films were compared in terms of their electrochemical responses during silver electrodeposition. The cyclic voltammetric and current-time responses during silver deposition show an increased rate in the following sequence: sulfuric acid grown films (smallest pores), phosphoric acid grown (larger pores) and barrier oxide films (no pores), under identical applied voltages. Thicker porous oxide films formed after longer times of anodizing show lower deposition currents, while films formed at higher voltages (expected to yield larger diameter pores) result in higher silver deposition currents.

An electrochemical study of the composition of thin, compact Pd oxide films

An electrochemical study of the properties of thin, compact oxide films (Pd α-oxides) formed at polycrystalline Pd wire electrodes and at Pd-sputter-coated quartz crystals (to enable in situ mass measurements) in alkaline and neutral sulfate solutions has been carried out. In alkaline solutions, the mass of the film is consistent with the formation and reduction of Pd(OH)2· H2O, in agreement with the ca.–58 mV pH dependence observed over a narrow alkaline pH range using solutions that do not lead to competitive anion adsorption effects. This film can be transformed, without a change of oxidation state or significant thickening, by a particular continuous potential cycling regime, to one which reduces at ca. 270 mV more negatively, consistent with the development of a deprotonated, charge compensated, hydrated oxide film, i.e., Pd(OH)2·yH2O ·xOH–·xNa+. In neutral sulfate solutions, evidence for sulfate adsorption on the reduced Pd surface and in the early stages of α-oxide formation is seen. Nevertheless, the mass of the α-oxide film could be measured, indicating it to be PdO · H2O in this medium.

Ion Transport across a Microscopic Interface between two Immiscible Electrolytes

Voltammetric characteristics of a small interface (microinterface) formed between two immiscible solutions of electrolytes was examined. Comparison was made with the results obtained with a larger size liqui/liquid interface on which the charge-transfer behavior has been studied. The transport across the water-nitrobenzene interface has been investigated by addition of semihydrophobic ions such as tetramethylammonium, tetraethylammonium, picrate, choline, and dodecylsulfate

Multi-ion Nernst distribution potential equations : interfacial potentials at equilibrium liquid/liquid and membrane interfaces

Abstract This paper presents analyses of practical cases of multi-ion Nernst distribution potential equations, and gives tables of interfacial potential difference (pd) expressions. The form of pd expressions for mixtures of partitioning ions within the window is emphasized. The topic of blocking is investigated and the whole range of behavior, from simple salt distribution Nernst distribution potentials to blocked interfaces that obey the more familiar Nernst-Donnan equation, is covered. The relation between the two limits, through Donnan exclusion and Donnan failure, is shown. Examples of model systems are calculated and illustrated. The notion of a “potential window” and conditions for establishment of the window using strongly and weakly partitioning salts are indicated. The problem of defining reference partitioning species to establish a reference pd and an ideally non-polarizable interface is emphasized. Likewise, the conditions for an ideally polarizable liquid/liquid interface are mentioned. A new concept of “repartitioning” is explained to interpret effects of foreign, “inert” salt partitioning on a reference pd. Equilibrium, not steady state, pds at single non-classical interfaces are discussed here, but single interface systems are inevitably part of larger two-interface or three-interface systems. Factors determining rates of development of pds and some connections between equilibrium single interface and steady state total membrane potential differences are considered to explain a result, viz., steady state membrane pds, comprising two interfacial pds and intervening diffusion-migration pd, often have the same mathematical form as single equilibrium interfacial pds, but contain mobilities or diffusion coefficients as multiplying factors.

Properties of thin, hydrous Pd oxide films

Abstract Thin, hydrous (dispersed) Pd oxide films (β-oxide) formed electrochemically at polycrystalline Pd electrodes in basic solutions have been studied using a range of electrochemical techniques as well as transmission electron microscopy (TEM). These films have been formed by multi-cycling potential methods to maximum estimated thicknesses of ca. 100 nm. The hydrous oxide film forms only when a potential of 2 V is exceeded, and electrochemical evidence indicates that it exists in the form of islands or strands lying above the compact Pd α-oxide film. Based on its −90 mV pH dependence during reduction, the β-oxide film is suggested to have the following composition: PdO 2 (OH − ) 2 ·(Na + ) 2 ·( n + 2)H 2 O. Impedance studies have indicated that the film is non-conducting, and an equivalent circuit very similar to that for the reduced non-conducting forms of Ir oxide and polyaniline films applies to the Pd β-oxide film material. The TEM examination of cross-sections of the oxide films confirms that it is highly porous and dispersed in nature, with pore diameters up to 2 to 3 nm and with a very low density, estimated on the basis of its measured thickness and charge density.