M. A. Choba

Peculiarities of the electrical double layer structure on renewed electrodes of eutectic alloys

Abstract Differential capacitance curves are measured on renewable electrodes of SnPb and SnCd eutectic alloys in the region of their ideal polarizability. It is shown that, upon the renewal of the electrode surface, the transients of the double layer capacitance are observed which indicate anomalously high surface-phase changes on electrodes. A model of the temporal effects observed is proposed and substantiated. According to this model, the surface enrichment with the surface-active alloy component proceeds via the mechanism of surface diffusion of this component from the areas enriched with it (boundaries of Sn crystal grains).

Self‐Assembled Amphiphilic Bilayers of Surfactant Brij‐52 on Gold Electrodes

The self-assembling of the uncharged nonionic surfractant Brij-52 on gold electrodes is reported. The resulting amphiphilic layers are characterized by means of impedance spectroscopy. Self-assembled Brij-52 layers have uniform structure and contain only a minor number of defects. The capacitance of optimal Brij-52 membrane calculated from the whole impedance spectrum rather than at certain frequency, is 1.0±0.4 μΦ cm–2, which is close to the capacitance of the lipid bilayer which separates the two aqueous phases, indicating that the self-assembling of Brij-52 resulted in formation of its bilayer on the gold electrode surface. The uniform bilayers of Brij-52 on gold support are stable, in contrast to lipids, which provide the opportunity of their application for analytical purposes. The possibility of using supported Brij-52 layers in affinity sensors is discussed.

Kinetics of surface segregation of bismuth atoms at the interface of a mechanically renewable Ag-Bi alloy electrode with a surface-inactive electrolyte solution

The time effects observed on mechanically renewed electrodes of eutectic-type Ag-Bi alloys in NaF solutions in the potential range of ideal polarizability are studied by the impedance method and cyclic voltammetry. The changes in the electric double layer (EDL) capacitance observed with the increase in the time of the electrode-solution contact from the moment of electrode renewal indicate that the process of surface enrichment with Bi atoms occurs. The analysis of obtained data leads to a conclusion that the surface segregation of Bi proceeds by the mechanism of surface diffusion, which provides the anomalously high diffusion rates as compared with the processes of volume diffusion in the solid phase. The results of studies of the surface segregation of bismuth on the Ag-Bi alloy/electrolyte interface are compared with the Auger-spectroscopic data for the interface of the same alloy with vacuum. Assumptions are put forward and substantiated that allow the differences in the kinetics of surface segregation processes observed on different interfaces to be consistently interpreted.

Using impedance to study surface segregation on restored electrodes of Ag-Sn and Au-Sn alloys

Results from investigating time dependences for the capacitance of double electric layers, observed from the time of the mechanical restoration of electrodes of Ag-Sn, Au-Sn binary alloys with low contents of the second component in solutions of surface-inactive electrolytes, are analyzed. It is shown that the observed time dependences are due to surface segregation of the second component of alloys at the boundary with a solution.

Surface segregation of cadmium atoms at the interface of mechanically renewable Ag–Cd alloy electrode with surface-inactive electrolyte solution

The surface segregation of cadmium atoms at the interface of mechanically renewed electrodes of the Ag–Cd alloy (containing 1 at % Cd; the region of cadmium solid solutions in silver) with a surface-inactive electrolyte solution is studied in the potential range of ideal polarizability by means of impedance spectroscopy and cyclic voltammetry. It is shown that method of mechanic renewal makes it possible to obtain well reproducible data on the kinetics of electrode surface enrichment with atoms of cadmium which is the surface-active component of this alloy. As follows from the analysis of dependences of the electric double layer (EDL) capacitance on the potential (C vs. E curves) pertaining to different times after the electrode renewal, these dependences can be well described in terms of the model of capacitors connected in series (“the common dense layer”). Using this model, the dependences of the alloy surface coverage with cadmium atoms on the time of exposure of the freshly renewed electrode surface to the electrolyte solution are calculated. The mechanism of surface segregation of cadmium atoms is proposed. It allows the kinetics of relaxation processes that occur in the alloy surface layer to be consistently interpreted. The electrochemical data are supplemented and compared with the results of analysis of the surface composition and the data of the profile analysis over the depth of the same alloy at its interface with vacuum, performed by means of Auger electron spectroscopy.

Salient features of the electrical double layer structure on mechanically renewed gold-silver electrodes in aqueous solutions of a surface-inactive electrolyte

The behavior of electrodes, which are made of binary Au-Ag alloys (Ag content 1–15 at %) and renewed by mechanical cutting in aqueous solutions of sodium fluoride, is studied with the aid of cyclic voltammetry and impedance methods. It is established that, in the region of potentials corresponding to ideal polarizability, the differential capacitance of the electrical double layer rapidly changes with time elapsed after the renewal of the surface of the electrodes. The change in the capacitance is brought about by the exit of silver atoms into a surface layer. The implication is that silver is the surface-active component in these alloys. The rate of the surface segregation of silver atoms depends on the electrode potential. The segregation rate substantially increases upon going into the region that corresponds to positive charges of the silver electrode surface and to the beginning of adsorption of atomic oxygen on the electrode. Based on phenomenological models, a method for processing capacitance curves is realized, which links experimentally observed time effects to variations that occur in the surface composition, and assumptions concerning the mechanism of relaxation processes that are responsible for the observed time effects are put forth. Explicit data on the effect, which is exerted by mechanical renewal on the composition of the surface layer of Au-Ag alloys at different distances from the interface with a vacuum, are obtained with the aid of an x-ray photoelectron spectroscopy method. It is established that the surface layer (∼0.5 nm) is enriched by silver atoms as compared with the alloy’s bulk.

Modeling of Surface Segregation Effects Observed on Renewed Electrodes of Eutectic Alloys

Abstract Experimental data on the transients of the electric double layer (EDL) capacitance measured after the mechanical renewal of the surface of electrodes of eutectic Sn-Pb and Ag-Bi alloys with small concentrations of second components at the interface with sodium fluoride solutions are analyzed and generalized. The observed transients are associated with the surface segregation of second components of alloys at the boundary with the solution. Phenomenological models of the electrode/solution interface which make it possible to calculate the surface coverage of the metal phase with different alloy components in different times after the renewal are proposed and substantiated. It is shown that the kinetics of surface segregation of lead and bismuth atoms at the alloy electrode interfaces with electrolyte solutions can be described in the first approximation by a model of nonsteady-state surface diffusion.

Peculiarities of the adsorption behavior of adamantanol-1 on a mechanically renewable silver electrode

Using the method of electrochemical measurements on electrodes with mechanically renewable surface, the adsorption behavior of adamantanol-1 (AdOH) on the Ag electrode interface with solutions of a surface-inactive electrolyte (NaF) is studied. Based on the results of impedance and voltammetric measurements, it is shown that the kinetics of AdOH adsorption in the ideal polarizability potential range (near the zero charge potential) is described within the framework of the mechanism of the quasichemical reaction Ag(H2O)ads + AdOH = Ag(AdOH)ads + H2O on metal surfaces with energy uniformity. A phenomenological model is proposed that makes it possible to consistently describe the temporal effects on renewable Ag electrodes in the potential range of its initial oxidation in solutions containing AdOH.

Kinetics of Electroreduction of S2O82- Anions on Mechanically Renewable Silver Electrode

The kinetics of electroreduction of peroxodisulfate anions on a mechanically renewed silver electrode is studied by voltammetric and impedance methods. Impedance diagrams obtained in the region of negatively charged metal surface are successfully modeled by the equivalent circuit formed by the solution resistance and also the reaction resistance connected in parallel with the constant-phase element substituted for the double layer capacitance. The analysis of experimental data carried out in terms of this circuit and their comparison with the literature data makes it possible to assume that the reduction kinetics of S2O82− anions on this electrode can be adequately described in the framework of the phenomenological theory of slow discharge. The reasons for deviation of the literature data on the kinetics of this reaction on polycrystalline Ag electrode from the relationships following from the slow discharge theory are put forward and substantiated.

Specific features of the structure of electrical double layer on mechanically renewed Ag-Sn electrodes in aqueous surface-inactive electrolytes

The behavior of an Ag–Sn binary alloy (3–9 at. % Sn; region of solid solutions of Sn in Ag) in aqueous NaF solutions is studied with the aid of an impedance method. After the electrode surface renewal in potentiostatic conditions in the region of ideal polarizability, the EDL differential capacitance rapidly changes with time. These changes testify that the content of tin atoms in a surface layer increases with the duration of contact between a renewed surface and the solution, i.e. tin is a surface-active component of the alloy. On the basis of an analysis of the observed time effects in the framework of available phenomenological models for the electrode/solution interface and the experimental data obtained earlier for an Ag3Sn electrode in aqueous NaF solutions, a mechanism is suggested, which allows one to qualitatively interpret the formation kinetics of an equilibrium interface between alloys under study and the electrolyte, which occur in the metallic phase. The mechanism includes two stages: (1) a fast escape of atoms of the surface-active component of the alloy (Sn) into a surface layer, and (2) a slower stage of the formation of intermetallic compound Ag3Sn in a surface layer.