Distribution of three ions in the thin film experiment

In the theoretical model it is assumed that a graphite disk electrode is covered by a thin film of the solution of decamethylferrocene and some supporting electrolyte in nitrobenzene and immersed in an aqueous solution of the same electrolyte. Oxidation of decamethylferrocene is accompanied by the transfer of anions of the electrolyte from water into nitrobenzene. The flux of decamethylferrocenium cations at the electrode surface and the flux of anions at the liquid/liquid interface are separated by the thickness of the film, but the electroneutrality is ensured by the migration of ionic species through the film. Theoretical concentration profiles of ionic species in the film are reported for cyclic voltammetry.     

Quantification of the chiral recognition in electrochemically driven ion transfer across the interface water/chiral liquid

The electrochemically driven transfer of the chiral anions of d- and l-tryptophan across the interface water/chiral liquid (d- or l-menthol) is stereoselective, and it can be used to determine quantitatively the difference in Gibbs energies for the solvation of chiral ions in chiral liquids. The ion transfer can be achieved in a three-phase arrangement where a droplet of the chiral liquid containing decamethylferrocene as the electroactive redox probe is attached to a graphite electrode immersed in the aqueous solution containing the chiral ions.     

Comparative study of the thermodynamics and kinetics of the ion transfer across the liquid/liquid interface by means of three-phase electrodes

A comparative study of the behavior of different sorts of three-phase electrodes applied for assessing the thermodynamics and kinetics of the ion transfer across the liquid/liquid (L/L) interface is presented. Two types of three-phase electrodes are compared, that is, a paraffin-impregnated graphite electrode at the surface of which a macroscopic droplet of an organic solvent is attached and an edge pyrolytic graphite electrode partly covered with a very thin film of the organic solvent. The organic solvent contains either decamethylferrocene or lutetium bis(tetra-tert-butylphthalocyaninato) as a redox probe. The role of the redox probe, the type of the electrode material, the mass transfer regime, and the effect of the uncompensated resistance are discussed. The overall electrochemical process at both three-phase electrodes proceeds as a coupled electron-ion transfer reaction. The ion transfer across the L/L interface, driven by the electrode reaction of the redox compound at the electrode/organic solvent interface, is independent of the type of redox probe. The ion transfer proceeds without involving any chemical coupling between the transferring ion and the redox probe. Both types of three-phase electrodes provide consistent results when applied for measuring the energy of the ion transfer. Under conditions of square-wave voltammetry, the coupled electron-ion transfer at the three-phase electrode is a quasireversible process, exhibiting the property known as "quasireversible maximum". The overall electron-ion transfer process at the three-phase electrode is controlled by the rate of the ion transfer. It is demonstrated for the first time that the three-phase electrode in combination with the quasireversible maximum is a new tool for assessing the kinetics of the ion transfer across the L/L interface.     

The determination of standard Gibbs energies of transfer of cations across the nitrobenzene|water interface using a three-phase electrode

A three-phase electrode consisting of a droplet of a nitrobenzene solution of iron(III) tetraphenyl porphyrine chloride (Fe(III)-TPP-Cl) attached to a graphite electrode and immersed in an aqueous electrolyte solution was applied to determine the standard Gibbs energies of transfer of cations between water and nitrobenzene. The reduction of Fe(III)-TPP-Cl prompts the transfer of the cations from the aqueous to the organic phase. The system is chemically and electrochemically reversible.     

A comparative study of the anion transfer kinetics across a water/nitrobenzene interface by means of electrochemical impedance spectroscopy and square-wave voltammetry at thin organic film-modified electrodes

The kinetics of the transfer of a series of hydrophilic monovalent anions across the water/nitrobenzene (W/NB) interface has been studied by means of thin organic film-modified electrodes in combination with electrochemical impedance spectroscopy and square-wave voltammetry. The studied ions are Cl-, Br-, I-, ClO4-, NO3-, SCN-, and CH3COO-. The electrode assembly comprises a graphite electrode (GE) covered with a thin NB film containing a neutral strongly hydrophobic redox probe (decamethylferrocene or lutetium bis(tetra-tert-butylphthalocyaninato)) and an organic supporting electrolyte. The modified electrode is immersed in an aqueous solution containing a supporting electrolyte and transferring ions, and used in a conventional three-electrode configuration. Upon oxidation of the redox probe, the overall electrochemical process proceeds as an electron-ion charge-transfer reaction coupling the electron transfer at the GE/NB interface and compensates ion transfer across the W/NB interface. The rate of the ion transfer across the W/NB interface is the limiting step in the kinetics of the overall coupled electron-ion transfer reaction. Moreover, the transferring ion that is initially present in the aqueous phase only at a concentration lower than the redox probe, controls the mass transfer regime in the overall reaction. A rate equation describing the kinetics of the ion transfer that is valid for the conditions at thin organic film-modified electrodes is derived. Kinetic data measured with two electrochemical techniques are in very good agreement.     

Transfer of lanthanum ion across the adsorption of emulsifier OP at the water-nitrobenzene interface

The transfer of lanthanum ion facilitated by diantipyrylmathane (DAM) across the water/nitrobenzene (w/nb) interface and the adsorption of emulsifier OP at the w/nb interface has been studied by the cyclic voltammetry.The mechanism of the charge transfer reactions is discussed.It has been concluded that the transfer of rare earth metal ion (La3+) facilitated by neutral ionophore (DAM) at the w/nb interface is E mechanism and the nonionic surfactant (emulsifier OP) can participate in the charge transfer process as an ionophore,charge transfer catalyst and inhibitor.     

Molecular dynamics study of dicarbollide anions in nitrobenzene solution and at its aqueous interface. Synergistic effect in the Eu(III) assisted extraction

We report a molecular dynamics study of chlorinated cobalt bis(dicarbollide) anions [(B(9)C(2)H(8)Cl(3))(2)Co](-)"CCD(-)" in nitrobenzene and at the nitrobenzene-water interface, with the main aim to understand the solution state of these hydrophobic species and why they act as strong synergists in assisted liquid-liquid extraction of metallic cations. Neat nitrobenzene is found to well solubilize CCD(-), Cs(+) salts in the form of diluted pairs or oligomers, without displaying aggregation. In biphasic nitrobenzene-water systems, CCD(-) anions mainly partition to the organic phase, thus attracting Cs(+) or even more hydrophilic counterions like Eu(3+) into that phase. The remaining CCD(-) anions adsorb at the interface, but are less surface active than at the chloroform interface. Finally, we compare the interfacial behavior of the Eu(BTP)(3)(3+) complex in the absence and in the presence of CCD(-) anions and extractant molecules. It is found that in the absence of CCDs, the complex is trapped at the interface, while when the CCDs are concentrated enough, the complex is extracted to the nitrobenzene phase. These results are compared to those obtained with chloroform or octanol as organic phase and discussed in the context of synergistic effect of CCDs in liquid-liquid extraction, pointing to the importance of dual solvation properties of nitrobenzene or octanol to solubilize the CCD(-) salts as well as the extracted complex.     

A simultaneous study of kinetics and thermodynamics of anion transfer across the liquid/liquid interface by means of Fourier transformed large-amplitude square-wave voltammetry at three-phase electrode

This paper describes a novel application of Fourier transformed large-amplitude square-wave voltammetry (FT-SWV) in combination with three-phase edge plane pyrolytic graphite (EPPG) electrode to investigate both the kinetics and thermodynamics of anion transfer across the liquid/liquid interface using a conventional three-electrode arrangement. The transfer of anion from aqueous phase to organic phase was electrochemically driven by reversible redox transformation of confined redox probe in the organic phase. The kinetics and thermodynamics of anion transfer were inspected by a so-called "quasi-reversible maximum" (QRM) emerged in the profile of even harmonic components of power spectrum obtained by Fourier transformation (FT) of time-domain total current response and formal potential E(f) of first harmonic voltammogram obtained by application of inverse FT on the power spectrum. Besides, a systematic study of patterns of behavior of a variety of anions at the same concentration and a specific anion at different concentrations on kinetics and thermodynamics and the effect of amplitude ΔE on QRM were also conducted, aiming to optimize the measurement conditions. The investigation mentioned above testified that the ion transfer across the liquid/liquid interface controls the kinetics of overall electrochemical process, regardless of either FT-SWV or traditional SWV investigation. Moreover, either the kinetic probe f(max) or the thermodynamic probe E(f) can be served as a way for analytical applications. Interestingly, a linear relationship between peak currents of the first harmonic components and concentrations of perchlorate anion in the aqueous solutions can be observed, which is somewhat in accordance with a finding obtained by Fourier transformed alternating current voltammetry (FT-ACV) [Bond, A. M.; Duffy, N. W.; Elton, D. M.; Fleming, B. D. Anal. Chem. 2009, 81, 8801-8808]. This may open a new door for analytical detection of a wide spectrum of electrochemically inactive analytes of biological and environmental significance. Compared with traditional SWV, FT-SWV is much simpler and faster in ion transfer kinetics estimation and also provides a new access to thermodynamics evaluation.     

Kinetics of electrode reaction coupled to ion transfer across the liquid/liquid interface

In the theoretical model it is assumed that a graphite disk electrode is covered by a thin film of solution of decamethylferrocene (dmfc) and some electrolyte CX in nitrobenzene and immersed in an aqueous solution of the electrolyte MX. Oxidation of dmfc is accompanied by the transfer of anion X ⁻ from water into nitrobenzene since it is also assumed that cations dmfc ⁺ and C ⁺ are insoluble in water and cation M ⁺ is insoluble in nitrobenzene. Kinetic parameters of the electrode reaction can be determined if the total potential difference across the nitrobenzene/water interface is maintained constant by adding the electrolytes CX and MX in concentrations which are much higher than the initial concentration of dmfc in nitrobenzene.     

Determination of the standard Gibbs energies of transfer of cations across the nitrobenzene|water interface utilizing the reduction of iodine in an immobilized nitrobenzene droplet

When a nitrobenzene (NB) droplet containing iodine is attached to a graphite electrode and immersed into a chloride containing aqueous (AQ) solution, the electrochemical reduction of iodine is accompanied by a transfer of chloride ions from NB to water. These chloride ions enter the NB phase in a preceding partition between the AQ and the NB phases, supported by formation of I₂Cl⁻ ions in NB and accompanied by the transfer of stoichiometric amounts of cations. The overall electrode reaction is of CE_rev type, where C refers to the preceding chemical step forming I₂Cl⁻, and E_rev refers to the reversible reduction of iodine at the graphite|NB interface and the simultaneous transfer of chloride from NB to water. If the chloride concentration in NB is insufficient to compensate by leaving the NB the amount of electrochemically produced iodide, a second voltammetric signal occurs at more negative potentials due to the transfer of iodide from NB to water. The kinetics and thermodynamics of the preceding chemical step C, determine the voltammetric behaviour of the system in such way that the ratio of peak currents of the first and second signals depends linearly on the Gibbs energy of transfer of the co-partitioned cations. The method was validated for cations of known Gibbs energies of transfer, and it was applied to cations of amino acids.     

ReO-4、IO-4、MnO-4在水/硝基苯界面上转移过程的伏安法研究

本文用循环伏安法研究了Re0_4~-L、102、MnO_4~-等过氧酸根阴离子在水/硝基苯界面上的转移过程, 详细研究了各种条件对转移离子行为的影响. 三种离子均表现为可逆离子转移过程。转移过程CV峰电流与水相中阴离子的浓度成线性关系, 据此可进行ReO_4~-、TO_4~-、MnO_4~-的测定, 并且有用液/液界面电化学方法监测IO_4~-的氧化还原反应的意义。     

LuIII bisphthalocyanines as mediators for redox reactions at thin-organic-film modified electrodes

Lutetium bisphthalocyanines, dissolved in a thin-film of nitrobenzene covering a graphite electrode, have already been used for ion transfer studies across the liquid∣liquid interface. These sandwich complexes are also good candidates for the electron exchange with species in water, across the interface. Their one-electron oxidation and reduction are fully reversible, the oxidized and reduced forms being chemically stable. Compared with ferrocene derivatives and porphyrins that have been extensively used previously, these compounds offer very interesting properties for the mediation of electron exchanges with redox species in aqueous solutions.     

Electrochemical behaviour of EOP at the liquid/liquid interface and its determination

A new approach to the determination of emulsifier OP (EOP, a nonionic surfactant) is proposed, based on the voltammetry of the EOP facilitating Ca(2+) transfer across the water/nitrobenzene (w/nb) interface. The peaks obtained with cyclic voltammetry at the w/nb interface correspond to the transfer of the coordination complex ion [Ca(EOP)](2+) from the aqueous to the nonaqueous phase. The calibration graph is linear in the range of 1.5x10(-5)-5x10(-4) mol l(-1). The ionic surfactants have a very minor effect on detection.     

Gaseous rust: thermochemistry of neutral and ionic iron oxides and hydroxides in the gas phase

The experimental and theoretical thermochemistry of the gaseous neutral and ionic iron oxides and hydroxides FeO, FeOH, FeO(2), OFeOH, and Fe(OH)(2) and of the related cationic water complexes Fe(H(2)O)(+), (H(2)O)FeOH(+), and Fe(H(2)O)(2)(+) is analyzed comprehensively. A combination of data for the neutral species with those of the gaseous ions in conjunction with some additional measurements provides a refined and internally consistent compilation of thermochemical data for the neutral and ionic species. In terms of heats of formation at 0 K, the best estimates for the gaseous, mononuclear FeO(m)H(n)(-/0/+/2+) species with m = 1, 2 and n = 0-4 are Delta(f)H(FeO(-)) = (108 +/- 6) kJ/mol, Delta(f)H(FeO) = (252 +/- 6) kJ/mol, Delta(f)H(FeO(+)) = (1088 +/- 6) kJ/mol, Delta(f)H(FeOH) = (129 +/- 15) kJ/mol, Delta(f)H(FeOH(+)) = (870 +/- 15) kJ/mol, Delta(f)H(FeO(2)(-)) = (-161 +/- 13) kJ/mol, Delta(f)H(FeO(2)) = (67 +/- 12) kJ/mol, Delta(f)H(FeO(2)(+)) = (1062 +/- 25) kJ/mol, Delta(f)H(OFeOH) = (-84 +/- 17) kJ/mol, Delta(f)H(OFeOH(+)) = (852 +/- 23) kJ/mol, Delta(f)H(Fe(OH)(2)(-)) = -431 kJ/mol, Delta(f)H(Fe(OH)(2)) = (-322 +/- 2) kJ/mol, and Delta(f)H(Fe(OH)(2)(+)) = (561 +/- 10) kJ/mol for the iron oxides and hydroxides as well as Delta(f)H(Fe(H(2)O)(+)) = (809 +/- 5) kJ/mol, Delta(f)H((H(2)O)FeOH(+)) = 405 kJ/mol, and Delta(f)H(Fe(H(2)O)(2)(+)) = (406 +/- 6) kJ/mol for the cationic water complexes. In addition, charge-stripping data for several of several-iron-containing cations are re-evaluated due to changes in the calibration scheme which lead to Delta(f)H(FeO(2+)) = (2795 +/- 28) kJ/mol, Delta(f)H(FeOH(2+)) = (2447 +/- 30) kJ/mol, Delta(f)H(Fe(H(2)O)(2+)) = (2129 +/- 29) kJ/mol, Delta(f)H((H(2)O)FeOH(2+)) = 1864 kJ/mol, and Delta(f)H(Fe(H(2)O)(2)(2+)) = (1570 +/- 29) kJ/mol, respectively. The present compilation thus provides an almost complete picture of the redox chemistry of mononuclear iron oxides and hydroxides in the gas phase, which serves as a foundation for further experimental studies and may be used as a benchmark database for theoretical studies.     

Neurotransmitter Biomolecule Transfers Across Liquid/Liquid Interface Through A Thick Organic Membrane-Modified Electrode

Electrochemical studies at liquid/liquid interfaces (L/L, or soft interfaces) have disclosed a biomimetic model to mimic charge transfers at cytomembrane surface. Herein, we reported two neurotransmitter biomolecules of dopamine and adrenalone across the L/L interface by a thick organic membrane-modified electrode. This system comprised polarized electrode/oil and oil/water interfaces in series in which the electron transfer (ET) of redox 7,7,8,8-tetracyanoquinodimethane (TCNQ) at electrode/oil interface drove ion transfer (IT) of biomolecules at oil/water interface. This ET-IT coupled reaction overcame the limitation of polarized potential window at conventional single polarized L/L interface. The crucial design of a thick organic membrane could ensure the generated TCNQ anions maintained at electrode/oil interface during the voltammetry, which could not result in interruptions to biomolecule transfers. Through this system, their Gibbs transfer free energies were accurately determined (44.4 and 39.4 kJ mol^?1 for dopamine and adrealone, respectively). Moreover, facilitated biomolecule transfers were evaluated by crown ionophores where both facilitated numbers and constants were determined simultaneously. Owing to the simple electrochemical setup, this system would hold great potentials in future hydrophilic biomacromolecule transfers, such as DNA, peptides and proteins.     

Modelling reversible cyclic voltammetry at the one-dimensional junction established by three phases: an organic liquid containing a neutral electroactive solute,an aqueous electrolyte solution,and an electronic conductor

Scholz et al. (Electrochem. Commun. 2 (2000) 112) recently performed cyclic voltammetry on droplets of a solution of decamethylferrocene (dmfc) in nitrobenzene (nb) lying between graphite and an aqueous solution of potassium chloride. Those authors hypothesized that the reaction dmfc(nb)+Cl⁻(aq)→dmfc⁺(nb)+Cl⁻(nb)+e⁻ took place at the hoop where the three phases meet. We have modelled this hypothesis on the basis that the cyclovoltammetric current is controlled by Nernst’s law and by the cylindrical diffusion of dmfc to, and of dmfc⁺Cl⁻ from, the three-phase junction. Model and experiment do not match.     

Mechanistic study of the oxidation of -ascorbic acid by chloranil at the nitrobenzene water interface

The redox reaction between -ascorbic acid in water and chloranil in nitrobenzene has been studied by means of polarography with an ascending water electrode as well as cyclic voltammetry with a stationary interface. Through accurate measurement of the limiting currents, it has been suggested that the redox reaction should be a two-electron reaction rather than a one-electron reaction described previously. A spectrophotometric technique has also been used to observe that the redox reaction proceeds spontaneously under certain conditions even without electrochemical control. Based on these findings, it has been concluded that the present heterogeneous charge transfer reaction is the ion transfer of chloranil semiquinone radical, which is driven by the homogeneous electron transfer between ascorbic acid and chloranil in the aqueous phase.     

Enzymatic formation of ions and their detection at a three-phase electrode

An electrochemical method for the detection of enzymatically created anions is described that uses a thin-film electrode with decamethylferrocene as an electroactive redox probe. The enzymatic oxidation of glucose with enzyme glucose oxidase produces gluconic acid as a final product. The oxidation of decamethylferrocene dissolved in the thin-nitrobenzene film, that is spread on the working graphite electrode and submerged in the aqueous solution containing glucose and glucose oxidase, is followed by the up-take of gluconate anions from the aqueous phase to nitrobenzene. The peak currents of the square-wave voltammetric responses of that system are a linear function of the glucose concentration in the milimolar range from 0.1 mmol/L to 0.7 mmol/L (R²=0.994).     

Improvement in the assessment of direct and facilitated ion transfers by electrochemically induced redox transformations of common molecular probes

A new strategy based on a thick organic film modified electrode allowed us, for the first time, to explore the voltammetric processes for a series of hydrophilic ions by electrochemically induced redox transformations of common molecular probes. During the limited time available for voltammetry, this thick organic film ensured that the generated product of the molecular probe, which is within a limited diffusion layer, was kept far away from the aqueous-organic solvent interface; therefore, regardless of the degree of hydrophobicity, the generated product never participates in ion exchange across the interface and the charge neutrality of the organic film (containing an extremely hydrophobic electrolyte) can only be maintained by the injection of ions from the aqueous phase. Taking advantage of this fact, common redox probes, such as ferrocene (Fc) and 7,7,8,8-tetracyanoquinodimethane (TCNQ), which are almost useless for both three-phase electrode (TPE) and thin-layer cyclic voltammetry (TLCV) methods, can induce the transfer of numerous highly hydrophilic anions and cations. Consequently, the majority of their Gibbs transfer energies have been accurately determined for the first time to the best of our knowledge. With this in mind, using TCNQ as a redox probe to induce facilitated cation transfer, a stategy that is more advantageous than traditional methods has been developed. The main advantages are that: (i) voltammetric experiments performed on this system were free from the polarized potential window (ppw) in the aqueous phase and, as a result, this allowed the assessment of weakly assisted ion transfers, which appear at the terminal of the ppw at single polarized interfaces; (ii) without introducing the tetraphenylarsonium-tetraphenylborate (TPAs-TPB) thermodynamic assumption, one can conveniently evaluate both the association constant and the stoichiometric parameter between the ion and its ionophore by comparison of their direct and facilitated ion transfer voltammograms. These encouraging results illustrated the exciting innovation for assessing direct and facilitated ion transfers based on this new thick organic film modified electrode.     

Specific ion effects on the electrochemical properties of cytochrome c

The range of salts used as supporting electrolytes in electrochemical studies of redox proteins and enzymes varies widely, with the choice of an electrolyte relying on the assumption that the electrolyte used does not affect the electrochemical properties of the proteins and enzymes under investigation. Examination of the electrochemical properties of the redox protein cytochrome c (cyt c) at a 4,4'-bipyridyl modified gold electrode demonstrates that both the redox potential (E(o')) and the faradaic current are influenced by the nature of the electrolyte used, in a manner explained primarily by Hofmeister effects. The faradaic peak currents display an atypical trend on switching from kosmotropic to chaotropic anions, with a maximum current observed in the presence of Cl(-). For a series of cations, the peak current increased in the sequence: Li(+) (0.34 μA) < guanidinium(+) (0.36 μA) < Na(+) (0.37 μA) < K(+) (0.38 μA) < Cs(+) (0.40 μA) and for anions it decreased in the sequence: Cl(-) (0.37 μA) > Br(-) (0.35 μA) > ClO(4)(-) (0.35 μA) > SCN(-) (0.31 μA) > F(-) (0.30 μA). E(o') decreased by a total of 24 mV across the series F(-) > Cl(-) > Br(-) > ClO(4)(-) > SCN(-) whereas no specific ion effect on E(o') was observed for cations. Factorisation of E(o') into its enthalpic and entropic components showed that while no specific trends were observed, large changes in ΔH(o') and ΔS(o') occurred with individual ions. The effect of anions on the faradaic peak current can be qualitatively explained by considering Collins' empirical rule of 'matching water affinities'. The effect of cations cannot be explained by this rule. However, both anion and cation effects can be understood by taking into account the cooperative action of electrostatic and ion dispersion forces. The results demonstrate that the choice of a supporting electrolyte in electrochemical investigations of redox proteins is important and emphasize that care needs to be taken in the determination and comparison of E(o'), ΔH(o') and ΔS(o') in different solutions.