Zdeněk Samec

Charge transfer between two immiscible electrolyte solutions: Part II. The investigation of Cs+ ion transfer across the nitrobenzene/water interface by cyclic voltammetry with IR drop compensation

Abstract The transfer of Cs + ion across the interface of two immiscible electrolyte solutions, 0.05 M LiCl in water and 0.05 M tetrabutylammonium tetraphenylborate in nitrobenzene, was investigated by cyclic voltammetry with four-electrode system. The positive feedback was used for the elimination of the ohmic potential drop between the tips of the Luggin capillaries. The standard potential differences for Cs + and tetrabutylammonium cations and for tetraphenyl-borate anion, which were deduced from the present experimental results, are in very good agreement with those calculated from the extraction data. The value of the diffusion coefficient of Cs + ion in water D Cs +(w)=2.4×10 −5 cm 2 s −1 was estimated. The kinetics of Cs + ion transfer across the nitrobenzene/water interface is characterized by the apparent rate constant k app o =5.5×10 −2 cm s −1 and by the formal charge transfer coefficient α=0.46.

Standard Gibbs energies of transfer of univalent ions from water to 1,2-dichloroethane

Abstract Standard Gibbs energies of ion transfer from water to 1,2-dichloroethane have been evaluated from voltammetric measurements at the interface between two immiscible electrolyte solutions for a series of univalent ions (in kJ mol −1 ): H + (53), Li + (57), Na + (57), Cl − (51), Br − (43), and I − (33). Thermodynamic scales derived from solubility, partition or electrochemical measurements have been compared and found mutually consistent, except for highly hydrated cations, in which case the electrochemical scale is superior. The stability constants of crown ether complexes of alkali metal cations in 1,2-dichloroethane have been revised.

Charge transfer between two immiscible electrolyte solutions: Part I. Basic equation for the rate of the charge transfer across the interface

Abstract The theoretical fundamentals of the kinetics of ion and electron transfer reactions at the interface between two immiscible electrolyte solutions have been discussed on the basis of the present theory of chemical reactions in polar media.

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.

Charge transfer between two immiscible electrolyte solutions: Part IV. Electron transfer between hexacyanoferrate(III) in water and ferrocene in nitrobenzene investigated by cyclic voltammetry with four-electrode system

Abstract The electron transfer reaction Fe(CN) 6 3− (w)+(C 5 H 5 ) 2 Fe(n)Fe(CN) 6 4− (w)+(C 5 H 5 ) 2 Fe + (n) between hexacyanoferrate redox couple in water (w) and ferrocene in nitrobenzene (n) has been investigated by cyclic voltammetry with a four-electrode system. The phenomenological relations found have been discussed in view of the theory of the stationary current-potential curve worked out in a preceding paper providing a qualitative understanding of the observed phenomena.

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.

Ion amperometry at the interface between two immiscible electrolyte solutions in view of realizing the amperometric ion-selective electrode

This article reviews the development in ion amperometry at the interface between two immiscible electrolyte solutions (ITIES) in view of realizing the amperometric ion-selective electrode (ISE). The concept of polarizability of ITIES in a multi-ion system is outlined. Principle aspects of ion amperometry at ITIES are discussed including the use of amperometry as a tool for the clarification of the ion sensing mechanism, and for determining the concentrations of ions in the solution. The reference is made to recent amperometric measurements at the supported liquid membrane (SLM) and polymer composite liquid membranes (PCLM), which, together with the micro-hole supported ITIES, appear to be particularly suitable for realization of the amperometric ISE.

Charge transfer between two immiscible electrolyte solutions: Part x. Kinetics of tetraalkylammonium ion transfer across the water/nitrobenzene interface

The kinetics of tetrabutylammonium, tetrapropylammonium and tetraethylammonium ion transfer across the water/nitrobenzene interface were investigated by using the galvanostatic pulse technique and convolution potential sweep voltammetry. The apparent ionic rate constants were corrected for the ion distribution in the electrical double layer and the corrected rate constants together with those reported in the literature for the transfer of tetramethylammonium, choline and acetylcholine ions, were correlated with the corrected Gibbs energy. The data for the tetraalkylammonium ions fall on a single line with a slope corresponding to the true charge transfer coefficient, αt ≈ 0.5 showing some tendency to drop from this line as the corrected Gibbs energy, ΔGrp, becomes large and negative. The conclusions was made that on crossing the interface the ion must overcome a symmetric potential energy barrier.

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.

Oxygen Reduction Catalyzed by a Fluorinated Tetraphenylporphyrin Free Base at Liquid/Liquid Interfaces

The diprotonated form of a fluorinated free base porphyrin, namely 5-(p-aminophenyl)-10,15,20-tris(pentafluorophenyl)porphyrin (H(2)FAP), can catalyze the reduction of oxygen by a weak electron donor, namely ferrocene (Fc). At a water/1,2-dichloroethane interface, the interfacial formation of H(4)FAP(2+) is observed by UV-vis spectroscopy and ion-transfer voltammetry, due to the double protonation of H(2)FAP at the imino nitrogen atoms in the tetrapyrrole ring. H(4)FAP(2+) is shown to bind oxygen, and the complex in the organic phase can easily be reduced by Fc to produce hydrogen peroxide as studied by two-phase reactions with the Galvani potential difference between the two phases being controlled by the partition of a common ion. Spectrophotometric measurements performed in 1,2-dichloroethane solutions clearly evidence that reduction of oxygen by Fc catalyzed by H(4)FAP(2+) only occurs in the presence of the tetrakis(pentafluorophenyl)borate (TB(-)) counteranion in the organic phase. Finally, ab initio computations support the catalytic activation of H(4)FAP(2+) on oxygen.