Z. Samec

Galvani potential scales for water-nitrobenzene and water-1,2-dichloroethane interfaces

Abstract A practical definition of the absolute Galvani potential scale for a liquid—liquid interface can be based on a set of single-ion transfer Gibbs energies. The consistency of transfer energies for picrate, perchlorate, tetramethylammonium, tetraethylammonium, tetrabutylammonium and tetraphenylarsonium has been examined by using cyclic voltammetry at the interface betwen two immiscible electrolyte solutions. The standard Gibbs energies of transfer of these ions from water to nitrobenzene or 1,2-dichloroethane have been revised.

Charge transfer between two immiscible electrolyte solutions: Part VII. Convolution potential sweep voltammetry of Cs+ ion transfer and of electron transfer between ferrocene and hexacyanoferrate(III) ion across the water/nitrobenzene interface

Abstract Convolution analysis was used in the evaluation of the thermodynamic and kinetic parameters of two charge-transfer systems at the water/nitrobenzene interface: Cs+ ion transfer and the electron transfer between ferrocene in nitrobenzene and hexacyanoferrate(III) in water. Attention was focused in particular on the potential dependence of the rate constant of the ion or electron transfer. The apparent rate constant was corrected for the double-layer effect using the capacity data and the Gouy-Chapman theory. It is concluded that the observed potential dependence of the apparent rate constant of Cs+ ion transfer arises from the effect of the total potential difference on the concentration of reactants at the reaction planes. In the electron transfer the analysis is considerably complicated by the possibility of ion-pairing, the bridge mechanism of electron transfer and the existence of the different planes of the closest approach for the reactant and the base electrolyte ions. Nevertheless, an attempt at analysis indicates that an intrinsic potential dependence of the rate constant is involved.

The double layer at the interface between two immiscible electrolyte solutions: Structure of the water/nitrobenzene interface

Abstract From ac impedance measurements the capacity of the water/nitrobenzene interface was evaluated as a function of the potential difference between two phases in contact. In each phase an electrolyte was dissolved: LiCl in water and tetrabutylammonium tetraphenylborate in nitrobenzene. The experimental results were interpreted in terms of the compound double-layer model in which the layer of the oriented solvent molecules (the inner or compact layer) separates two space-charge regions (diffuse double layer). The capacity of the diffuse double layer calculated using the Gouy-Chapman theory was found to fit well for the capacity of the interface. It was concluded that the potential drop across the inner compact layer remains constant and close to zero when the total potential drop across the interface is varied.

Kinetic analysis of the picrate ion transfer across the interface between two immiscible electrolyte solutions from impedance measurements at the equilibrium potential

Abstract The kinetics of the picrate ion transfer across the water/nitrobenzene interface was analysed at the equilibrium (Nernst) potential by using a faradaic impedance method with a four-electrode cell system. The potential-dependent apparent ionic rate constant ( K 0 app = 8.3 × 10 −2 cm s −1 at the standard potential E 0 pi (vs. TBA + = 0.318 ±0.002 V) and the apparent charge transfer coefficient (α app =0.56) were corrected for the ion distribution in the electrical double layer and the true charge transfer coefficient α t = 0.6±0.1 was determined. The conclusion was drawn that the kinetic measurements at the ITIES, which are based on the application of a small signal exciting the system in the thermodynamic equilibrium, offer more reliable data than measurements involving large potential sweeps or variations.

Charge transfer across the interface of two immiscible electrolyte solutions

Abstract The theory, experimental methods and results of the study of processes at the interface of two immiscible electrolyte solutions (ITIES) are described. The theory is concerned with equilibrium potential, double layer structure and ion transfer kinetics. Experimental methods for double layer studies as well as for transfer kinetics are discussed. Experimental results of the study of simple and facilitated ion transfer across ITIES are described. Finally, the perspective of ITIES study is discussed.

The double layer at the interface between two immiscible electrolyte solutions: Part III. Capacitance of the water/1,2-dichloroethane interface

Abstract From ac impedance and galvanostatic pulse measurements the capacitance of the interface between solutions of LiCl in water and tetrabutylammonium tetraphenylborate in 1,2-dichloroethane has been evaluated at various electrolyte concentrations. The experimental data have been interpreted on the basis of the modified Verwey-Niessen model, according to which a layer of oriented solvent molecules (inner layer) separates two space charge regions (diffuse double layer). As for the water/nitrobenzene interface, the interfacial potential difference is spread mainly within the diffuse double layer. In the sequence nitrobenzene

Effect of temperature on the ion transfer across an interface between two immiscible electrolyte solutions : ion transfer dynamics

Abstract The effect of temperature on ion transfer across the water—nitrobenzene interface was studied for a series of six quaternary ammonium and phosphonium cations and two anions using cyclic voltammetry and the faradaic impedance method at the equilibrium potential. Structure-breaking properties of small ions and enhancement of water structure in the presence of larger cations were manifested in both entropic and enthalpic contributions to the standard Gibbs energy of ion transfer. At the phase boundary, the ion is forced to overcome a relatively low potential barrier of about 5 kJ mol −1 , but the ion transfer rate is lowered considerably owing to hydrodynamic friction. The hydrodynamic factor of about 10 −4 reflects a drop in ion mobility in the transition state, which in turn makes the translational part of the activation entropy more negative. The stochastic method has provided a consistent theory for experimentally observed effects of interfacial potential difference, standard Gibbs energy of ion transfer (Bronsted correlation), viscosi

Voltammetric determination of nitrate, perchlorate and iodide at a hanging electrolyte drop electrode

Abstract The use of a hanging electrolyte drop electrode is examined for the determination of nitrate, perchlorate and iodide. A three-electrode system was used with a polarographic analyzer. Crystal violet dicarbollylcobaltate(III) electrolyte in the nitrobenzene phase and magnesium sulphate in the aqueous phase with a Pb/PbSO 4 reference electrode made it possible to increase the viable potential range. For nitrate, the peak current/concentration relation was linear over the range 0–5 × 10 −5 M, and nitrate in potable water was easily determined.

A new model of membrane transport: Electrolysis at the interface of two immiscible electrolyte solutions*

Summary A simple membrane model is the interface between water and an organic liquid immiscible with water, with a strongly hydrophilic electrolyte dissolved in the aqueous phase and a strongly hydrophobic electrolyte in the organic phase. This interface can be electrochemically polarized in the same way as the interface electrode electrolyte solution using various modes of voltammetry or the galvanostatic method. A fourelectrode potentiostatic system is required for such studies. An electrolyte dropping electrode, analogous to Heyrovskýs DME, was also constructed. The voltammograms fully resemble those obtained with metallic electrodes. The faradaic processes studied so far are mainly connected with the transfer of hydrophobic ions across the interface. These processes are quite rapid and the half-wave potential of a particular ion is related to its standard Gibbs transfer energy. Observed electron-transfer effects model redox processes at membranes. Macrocyclic ionophores facilitate transfer of alkali metal ions across this interface. Very fast ion transfer as well as complex formation was observed in the systems under investigation so that, generally, the diffusion of the ionophore toward the interface and of the complex into the organic phase is the rate-controlling step, no surface reaction retarding the overall process. Apart from the investigation of membrane processes, this approach can be used for elucidation of processes in ion-selective electrodes and in phase-transfer catalysis.