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.     

Simple electrochemical method for deposition and voltammetric inspection of silver particles at the liquid-liquid interface of a thin-film electrode

A novel experimental methodology for depositing and voltammetric study of Ag nanoparticles at the water-nitrobenzene (W-NB) interface is proposed by means of thin-film electrodes. The electrode assembly consists of a graphite electrode modified with a thin NB film containing decamethylferrocene (DMFC) as a redox probe. In contact with an aqueous electrolyte containing Ag(+) ions, a heterogeneous electron-transfer reaction between DMFC((NB)) and Ag(+)((W)) takes place to form DMFC(+)((NB)) and Ag deposit at the W-NB interface. Based on this interfacial reaction, two different deposition strategies have been applied. In the uncontrolled potential deposition protocol, the electrode is immersed into an AgNO(3) aqueous solution for a certain period under open circuit conditions. Following the deposition step, the Ag-modified thin-film electrode is transferred into an aqueous electrolyte free of Ag(+) ions and voltammetrically inspected. In the second protocol the deposition was carried out under controlled potential conditions, i.e., in an aqueous electrolyte solution containing Ag(+) ions by permanent cycling of the electrode potential. In this procedure, DMFC((NB)) is electrochemically regenerated at the electrode surface, hence enabling continuation and voltammetric control of the Ag deposition. Hence, the overall electrochemical process can be regarded as an electrochemical reduction of Ag(+)((W)) at the W-NB interface, where the redox couple DMFC(+)/DMFC acts as a mediator for shuttling electrons from the electrode to the W-NB interface. Ag-particles deposited at the W-NB interface affect the ion transfer across the interface, which provides the basis for voltammetric inspection of the metal deposit at the liquid-liquid interface with thin-film electrodes. Voltammetric properties of thin-film electrodes are particularly sensitive to the deposition procedure, reflecting differences in the properties of the Ag deposit. Moreover, this methodology is particularly suited to inspect catalytic activities of metal particles deposited at the liquid-liquid interface toward heterogeneous electron-transfer reactions occurring at the at the liquid-liquid interface.     

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.     

Probing non-polarizable liquid/liquid interfaces using scanning ion conductance microscopy

The study of microscopic structure of a liquid/liquid interface is of fundamental importance due to its close relation to the thermodynamics and kinetics of interfacial charge transfer reactions.In this article,the microscopic structure of a non-polarizable water/nitrobenzene(W/NB) interface was evaluated by scanning ion conductance microscope(SICM).Using SICM with a nanometer-sized quartz pipette filled with an electrolyte solution as the probe,the thickness of this type of W/NB interface could be measured at sub-nanometer scale,based on the continuous change of ionic current from one phase to another one.The effects for thicknesses of the non-polarizable W/NB interfaces with different electrolyte concentrations,the Galvani potentials at the interface and the applied potentials on the probe were measured and systematically analyzed.Both experimental setups,that is an organic phase up and an aqueous down,and a reverse version,were employed to acquire the approach curves.These data were compared with those of an ideal polarizable interface under the similar experimental conditions,and several characteristics of non-polarizable interfaces were found.The thickness of a non-polarizable interface increases with the decrease of electrolyte concentration and the increase of applied potential,which is similar to the situation of a polarizable liquid/liquid interface.We also find that the Galvani potential across a non-polarizable interface can also influence the interfacial thickness,this phenomenon is difficult to observe when using polarizable interface.Most importantly,by the comparison of two kinds of liquid/liquid interfaces,we experimentally proved that much more excess ions are gathered in the space charge layer of non-polarizable interfaces than in that of polarizable interfaces.These results are consistent with the predictions of molecular dynamic simulations and X-ray reflectivity measurements.     

细胞色素c在微-液/液界面上电化学行为的研究

本文采用电化学技术,研究了细胞色素c（Cyt c）在玻璃微米管尖端处形成的微-水/1,2-二氯乙烷（W/DCE）界面上的电化学行为.选用四丁基铵四苯硼（TBAT-PB）、四丁基铵四氯代苯硼（TBATPBCl）以及四丁基铵四氟代苯硼（TBATPBF）三种不同的有机相支持电解质来研究Cyt c在W/DCE界面上的反应.在电势窗较窄的含TBATPB体系中只能够观察到吸附过程;在电势窗较宽的含TBATPBCl和TBATPBF的体系中,可以同时观察到吸附与离子转移过程.当Cyt c浓度较低时,两种过程都可以观察到;当Cyt c浓度较高时,主要是吸附.文中对这些过程的机理进行了探讨.     

Electron-conductor separating oil–water (ECSOW) system: a new strategy for characterizing electron-transfer processes at the oil/water interface

A new electrochemical method for studying the electron transfer (ET) at the oil (O)/water (W) interface (or the liquid/liquid) interface has been devised, in which the O- and W-phases are separated by an electron conductor (EC; e.g. Pt). For the EC separating O–W (ECSOW) system, the ET across the EC phase can be observed voltammetrically in a similar manner to the O/W interface, however, no ion-transfer (IT) process can be taken place. Although the ECSOW system is thermodynamically equivalent to the corresponding O/W interface, they may be different from a kinetic viewpoint. In practice, the cyclic voltammograms obtained with the nitrobenzene NB/W interface and the ECSOW system in the presence of ferrocene in NB and hexacyanoferrate in W have shown quite different features, when the concentrations of both redox species are lower. The voltammograms for the NB/W interface have strongly supported the IT mechanism which involves an interfacial transfer of ferricenium ion. Also, the ECSOW system has been shown to be promising for clarification of complicated charge-transfer processes involving biological compounds such as l-ascorbic acid.     

Chiral recognition based on the kinetics of ion transfers across liquid/liquid interface

Three-phase electrodes in combination with square-wave voltammetry are applied to study the transfer kinetics of chiral anions from water to the chiral 2-octanol. The experimental system used consists of a pyrolytic graphite electrode partly modified with a thin film of one of the enantiomers of 2-octanol, which was immersed into an aqueous solution containing anions of chiral 2-chloropropionic acid, 2-bromopropionic acid, or lactic acid. It is demonstrated that the kinetics of the ion transfer is a stereoselective. The rate of the ion transfer is higher when uncomplimentary transferring ion–solvent chiral isomers are used, i.e., (R)-ion and (S)-solvent, or (S)-ion and (R)-solvent. To the best of our knowledge this is the first evidence for the difference in the ion transfer kinetics of chiral isomers across water/chiral organic solvent interface.     

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.     

Electroactive ceramic carbon electrode impregnated with organic liquid

The carbon ceramic electrodes impregnated with hydrophobic organic solvent (toluene, hexadecane, nitrobenzene) containing redox probe (decamethylferrocene) were prepared. The electrode material was obtained by sol–gel process. It consists of graphite powder homogeneously dispersed in hydrophobic silica matrix. After gelation and drying it was filled with organic liquid. The electrochemical properties of the electrode were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. Approximately symmetric cyclic voltammograms were obtained with these electrodes immersed in aqueous electrolyte solution. Their shape and current magnitude and position on the potential scale depends on the organic solvent and the salt present in aqueous phase. It has been concluded that the mechanism of the electrode process involves electron transfer between graphite particle and the redox probe in organic phase, followed by anion transfer from the aqueous phase.     

Analytical Modelling of Electron-coupled Ion Transfers with Immobilized vs Soluble Redox Transducer at Thick Film-modified Electrodes

Analytical theoretical solutions are deduced for the current-potential response, concentration profiles and interfacial potentials of electron-coupled ion transfers in the cyclic voltammetry with thick film-modified electrodes. The theory covers a wide variety of possible situations, namely, the redox transducer can be either immobilized on the working electrode or freely diffusing in the organic film, and the ion transfer can be either simple or complicated by homogeneous chemical processes. A comparative and comprehensive study of each case is performed, establishing the key parameters that define the behaviour of the system, as well as guidelines to analyse its electrochemical signal.     

Carbon ceramic electrode modified with redox liquid

A carbon ceramic electrode modified with a redox liquid, butylferrocene, exhibiting in aqueous salt solution electrochemical behaviour resulting from the redox process of the modifier and ion transfer across the liquid-liquid interface has been prepared.     

Ion insertion into ionic liquid supported toluene generated by electrochemical redox reaction

Ion transfer across the toluene|water, toluene–ionic liquid mixture|water and ionic liquid|water boundary generated by electrochemical redox reaction of tert-butylferrocene (tBuFc) was studied with the glassy carbon (GC) electrode partially covered by the organic liquid deposit and immersed in the aqueous electrolyte solution. The electrooxidation of the redox probe in toluene deposit is followed by ejection of newly formed cation into the aqueous solution. The same reaction in the toluene–ionic liquid deposit promotes anion insertion. This pathway is also found at the electrode modified with ionic liquid.     

The characteristics of highly ordered mesoporous carbons as electrode material for electrochemical sensing as compared with carbon nanotubes

In this paper, the unique properties of highly ordered mesoporous carbons modified glassy carbon electrode (OMCs/GE) are illustrated from comparison with carbon nanotubes modified glassy carbon electrode (CNTs/GE) for the electrochemical sensing applications. Electrochemical behaviors of eight kinds of inorganic and organic electroactive compounds were studied at OMCs/GE, which shows more favorable electron transfer kinetics than that at CNTs/GE. Especially, OMCs/GE exhibits remarkably strong and stable electrocatalytic response toward NADH compared with CNTs/GE. The ability of OMCs to promote electron transfer not only provides a new platform for the development of dehydrogenase-based bioelectrochemical devices, but also indicates a potential of OMCs in a wide range of sensing applications. OMCs prepared are the novel carbon electrode materials, exhibiting more favorable electrochemical reactivity than CNTs for the wide electrochemical sensing applications without pretreatments, while purification or end-opening processing was usually required in case of CNTs.     

Voltammetry at a liquid–liquid interface supported on a metallic electrode

A liquid–liquid interface supported on a metallic electrode has been used to study ion transfer (IT) and electron transfer (ET) reactions by cyclic voltammetry. The system is composed of an aqueous droplet supported on a platinum disc electrode and immersed into an organic electrolyte solution. Depending on the nature of the dissolved species present in the aqueous solution, and in the organic electrolyte solution, different electrochemical coupled reactions can be observed. This method enables a fast and convenient method to measure standard transfer potentials for example of ionised drug molecules.     

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.     

应用扫描电化学显微镜研究极化液/液界面上的电子转移反应

将含有氧化还原电对的水溶液滴涂在铂盘电极表面, 然后将该电极插入到1,2-二氯乙烷溶液中, 形成稳定的油/水界面. 液滴中的K3Fe(CN)6和K4Fe(CN)6氧化还原电对既可以作为水相中的参比电对参与控制液/液界面上的电势差, 同时又可以作为水相的电子授受体参与界面上的电子转移反应. 结合扫描电化学显微镜电化学系统的特点, 利用其双恒电位仪分别控制界面电势差和现场扫描的优点, 通过扫描电化学显微镜的渐进曲线得到了不同界面电势差控制的电子转移反应速率常数. 实验结果表明, 应用此方法获得的液/液界面可以被外加电位极化, 在一定的电势差范围内, 反应速率常数与界面电势差的关系遵守Butler-Volmer公式.     

Amperometric Determination of Creatinine with a Dialysis Membrane‐Covered Nitrobenzene/Water Interface for Urine Analysis

The ion transfer of creatinine (CRE) at a polarized nitrobenzene (NB)/water (W) interface has been studied. When the pH of the W phase is in the range of 1.2 to 4.0, a well‐defined voltammetric wave is observed for a simple transfer of CRE⁺ (protonated form) at the NB/W interface. This transfer reaction has been applied to develop an amperometric method for the determination of CRE in urine. In this system, the NB/W interface is covered with a dialysis membrane to prevent possible interference from urine proteins. The concentration of CRE in a urine control has successfully been determined.     

Voltammetric study of the transfer of fluoride ion at the nitrobenzene / water interface assisted by tetraphenylantimony.

The transfer of F- ion assisted by an organometallic complex cation tetraphenylantimony (TPhSb+) across the polarized nitrobenzene / water (NB / W) interface has been studied by means of ion-transfer voltammetry. A well-defined voltammetric wave was observed within the potential window at the NB / W interface when tetraphenylantimony tetrakis(4-chlorophenyl) borate and F- ion were present in NB and W, respectively. The voltammogram can be interpreted as being due to the reversible transfer of F- ion assisted by the formation of the TPhSbF complex through the coordination of F- to Sb atom in NB. The formal formation constant of TPhSbF in NB has been determined to be 10(1.95 +/- 0.2 M(-1). No voltammetric wave due to the TPhSb(+)-assisted transfer of other anions such as Cl-, Br, I-, NO3-, CH3COO- and H2PO4(-) ions has been observed within the potential window.     

Electrode/Electrolyte Synergy for Concerted Promotion of Electron and Proton Transfers toward Efficient Neutral Water Oxidation

Neutral water oxidation is a crucial half-reaction for various electrochemical applications requiring pH-benign conditions. However, its sluggish kinetics with limited proton and electron transfer rates greatly impacts the overall energy efficiency. In this work, we created an electrode/electrolyte synergy strategy for simultaneously enhancing the proton and electron transfers at the interface toward highly efficient neutral water oxidation. The charge transfer was accelerated between the iridium oxide and in situ formed nickel oxyhydroxide on the electrode end. The proton transfer was expedited by the compact borate environment that originated from hierarchical fluoride/borate anions on the electrolyte end. These concerted promotions facilitated the proton-coupled electron transfer (PCET) events. Due to the electrode/electrolyte synergy, Ir−O and Ir−OO⁻ intermediates could be directly detected by in situ Raman spectroscopy, and the rate-limiting step of Ir−O oxidation was determined. This synergy strategy can extend the scope of optimizing electrocatalytic activities toward more electrode/electrolyte combinations.     

Electrochemically driven transfer of carboxyl-terminated PAMAM dendrimers at the water/dichloroethane interface

The transfer of PAMAM dendrimers bearing carboxylic acid peripheral groups between two immiscible liquids was studied by means of the three phase junction system, using a gold wire vertically crossing the interface and decamethyl ferrocene as the redox probe in the organic phase. While the voltammetric behavior indicates kinetic limitations of the overall ion–electron transfer process, thermodynamic data shows that the phase transfer process is entropically controlled. Four dendrimer generations were analyzed and it was found that the kinetics as well as the thermodynamics of the phase transfer reaction are size dependent.