Daniel Homolka

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.

Charge transfer between two immiscible electrolyte solutions part VIII. Transfer of alkali and alkaline earth-metal cations across the water/nitrobenzene interface facilitated by synthetic neutral ion carriers☆

Facilitated transfer of proton, alkali and alkaline earth-metal cations across the water/nitrobenzene interface was observed in the presence of either N, N′-di[(11′-ethoxycarbonyl)undecyl]-N, N′,4,5-tetramethyl-3,6-dioxaoctanediamide (DODA) or 7,19-dibenzyl-2,3-dimethyl-7,19-diaza-1,4,10,13,16-pentaoxacycloheneicosane-6,20-dione (PEDA) in the nitrobenzene phase. These neutral synthetic compounds act as the ion carriers which, through the complex formation, mediate the translocation of the ion across the water/nitrobenzene interface. Using the convolution potential sweep voltammetry, the stoichiometry (r:s) and the thermodynamic, transport as well as the kinetic parameters were evaluated for the single-step charge-transfer model: rM2+ (w) + sL(n)=MrLsr2+(n), where M2+(w) is the metal cation with the charge number z in water and L(n) is the neutral ligand in nitrobenzene. The formation of 1 : 1 or 1 : 2 (cation to ligand) complexes is involved in the case of the monovalent or divalent cations respectively. The highest stability constant K in nitrobenzene was found for calcium ion: K= 5.8 × 1021 M−2 or 2.0 × 1018 M−2 for the acyclic (DODA) or the cyclic (PEDA) ligands respectively. The kinetic parameters were evaluated only for the transfer of the divalent metal cations, since the transfer of the monovalent cations is too fast. While the apparent charge-transfer coefficient is invariably very close to 0.5, the apparent rate constants at the formal potentials of the charge-transfer reactions differ considerably from each other and are sensitive to the nature of the ligand. For the acyclic ligand (DODA) the following sequence of the apparent rate constants was observed: Ca2+⪢Ba2+∼Sr2+⪢Mg2+, whereas for the cyclic ligand (PEDA) it was: Ba2+⪢Sr2+∼Mg2+⪢Ca2+. Among the factors underlying the kinetic behaviour there seem to be the double-layer effects, the internal as well as the solvent contributions to the activation energy.

Facilitated ion transfer across the water/nitrobenzene interface Theory for single-scan voltammetry applied to a reversible system☆

The transfer of the metal cation across the interface between two immiscible electrolyte solutions facilitated by complex formation with a ligand at the interface was investigated both theoretically and experimentally. The theory of single-scan voltammetry was derived which enables the complex stoichiometry (1:1, 1:2 or 1:3. cation to ligand) to be determined as well as the thermodynamic and transport parameters of the facilitated charge transfer controlled by the diffusion of the ligand. Application of the theoretical results was illustrated for the transfer of Li+ and Cd2+ ions across the water/nitrobenzene interface facilitated by complexation with the neutral macrocyclic polyether diamine.

The partition of amines between water and an organic solvent phase

Abstract The partition of a series of protonated amines (aniline, benzylamine, 2-phenylethylamine and related compounds) between water and nitrobenzene was investigated using electrochemical approaches (cyclic voltammetry and differential pulse stripping voltammetry) which made it possible to infer the transport and thermodynamic parameters of the partition process. Micromolar concentrations of the protonated amines in the aqueous phase can be determined by DPSV at the electrolyte hanging drop electrode. The function of an amine as the proton acceptor in the facilitated proton transfer across the water/organic solvent phase was discussed.

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.

The double layer at the interface between two immiscible electrolyte solutions: Part II. Structure of the water/nitrobenzene interface in the presence of 1:1 and 2:2 electrolytes

From fast galvanostatic pulse measurements at 25°C the capacitance of the water/nitrobenzene interface was evaluated as a function of the interfacial potential difference Δowϕ for systems consisting of NaBr, LiCl or MgSO4 in water and tetrabutylammonium tetraphenylborate, tetraphenylarsonium tetraphenylborate or tetraphenylarsonium dicarbollylcobaltate in nitrobenzene. The modified Verwey—Niessen model, in which an inner layer of solvent molecules separates two space-charge regions (the diffuse double layer), describes the structure of the water/nitrobenzene interface well at electrolyte concentrations above ca. 0.02 mol dm−3, provided that the ions are allowed to penetrate into the inner layer over some distance. For all the systems studied the zero-charge potential difference was found at Δwoϕpzc ≈ 0 on the basis of the standard potential difference Δwoϕ0TMA + = 0.035 V for tetramethylammonium cation which was used as a reference ion. At zero surface charge a comparison was made with the theoretical capacitance calculated using the mean spherical approximation for a model consisting of two ion and dipole mixtures facing each other. The effect of ion penetration on the interfacial capacitance was estimated from the solution of the linearized Poisson-Boltzmann equation for a triple dielectric model with a continuous distribution of the point ions. The concentration-independent inner layer potential difference and capacitance can only be inferred from the capacitance data if the ion size effect is taken into account. A non-iterative procedure based on the hypernetted-chain equation was used for the evaluation of the potential drop across the diffuse double layer. The extend of the penetration into the inner layer appears to be a function of ion solvation, e.g. the more hydrated ion the less extensive ion penetration is likely.

Charge transfer between two immiscible electrolyte solutions: Part VI. Polarographic and voltammetric study of picrate ion transfer across the water/nitrobenzene interface

Abstract The transfer of the picrate ion across the interface between two immiscible electrolyte solutions, 0.05 M LiCl in water and 0.05 M tetrabutylammonium tetraphenylborate in nitrobenzene was investigated by electrolysis with the electrolyte dropping electrode and by cyclic voltammetry. Under the conditions of the experiments the charge-transfer process is controlled solely by diffusion. The maximum which appears on the polarogram of the picrate ion close to the limiting current can be suppressed by the addition of a surface-active substance (gelatine). The diffusion coefficients of the picrate ion in the aqueous and nitrobenzene phase were determined from the limiting polarographic current and from the peak current on the cyclic voltammogram. The value of the formal potential of the charge-transfer reaction, which was calculated from the half-wave potential or from the peak potential, is in good agreement with that inferred from the extraction data.

Charge transfer between two immiscible electrolyte solutions: Part IX. Kinetics of the transfer of choline and acetylcholine cations across the water/nitrobenzene interface

Abstract Kinetic and thermodynamic parameters of the transfer of choline and acetylcholine cations across the water/nitrobenzen interface were evaluated using convolution potential sweep voltammetry. In order to trace the factors which control the ion-transfer kinetics the semi-phenomenological theory was used, assuming that: (1) the temperature dependence of the rate constant has the form of the Arrhenius equation; (2) the reaction site is located in the outer Helmholtz plane (oHp); (3) there exists a Bronsted-type relationship between the true activation Gibbs energy and the reaction Gibbs energy for the ion transfer from the oHp in water to that in the organic solvent phase. The apparent rate constants were 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 constants arises largely from the effect of the potential on the concentration of the transferred ions at the reaction planes. The correlation of the true (corrected) rate constant with the reaction Gibbs energy for a series of the ions with similar structure indicates that the true charge-transfer coefficient α 1 ≅0.3, which would correspond to the asymmetric potential energy barrier for the ion-transfer step.

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.