Interface Between Two Immiscible Electrolyte Solutions Research Articles

Voltammetry at Micropipet Electrodes

The use of micropipet electrodes for quantitative voltammetric measurements of ion-transfer (IT) and electron-transfer (ET) reactions at the interface between two immiscible electrolyte solutions (ITIES) requires knowledge of geometry of the liquid interface. The shape of the meniscus formed at the pipet tip was studied in situ by video microscopy under controlled pressure. The shape of the interface can be changed from a complete sphere to a concave spherical cap by varying the pressure applied to the pipet, and the diffusion current to the pipet changes accordingly. With no external pressure applied, the water/organic interface turned out to be flat, and the voltammetric response of a pipet must follow the well-known theory for a microdisk electrode. The large deviations from this theory observed previously can be attributed to a small amount of the filling aqueous solution which escapes from the pipet and forms a thin layer on its outer wall. This effect can be eliminated by making the outer pipet wall hydrophobic. Procedures have been developed for independent silanization of the inner and outer walls of the pipet. Pipets with a silanized inner wall can be filled with an organic solvent (e.g., 1,2-dichloroethane) and be used for voltammetric measurements in aqueous solutions. Another mode of voltammetry is based on trapping of a thin layer of organic solvent in the narrow shaft of a pipet between the filling solution and the aqueous outer phase. This arrangement is potentially useful for electrochemical catalysis and sensor applications.

Liquid/Liquid Interface as a Model System for Studying Electrochemical Catalysis in Microemulsions. Reduction of trans-1,2-Dibromocyclohexane with Vitamin B12

A complex electrochemical catalytic reaction was investigated at the interface between water and benzonitrile as a model for interfacial chemistry in microemulsions. Structures similar to those between oil−water microphases in microemulsions were created by using the interface between two immiscible electrolyte solutions (ITIES). While interfacial area in a microemulsion can be uncertain, the ITIES is well-defined and can be used to evaluate relevant heterogeneous interfacial kinetics. The reaction between the Co(I) form of vitamin B12 generated electrochemically in the water phase, and trans-1,2-dibromocyclohexane (DBCH) in benzonitrile was probed directly at the ITIES by scanning electrochemical microscopy (SECM). Apparent heterogeneous rate constants for the interfacial reaction were extracted from SECM current−distance curves. Influences of reactant concentration, potential drop across the ITIES, and adsorbed surfactants were investigated. Results suggest that the kinetics of reduction of DBCH by B12 ...

Study of ion transfer across phospholipid monolayers adsorbed at micropipette ITIES

Abstract The transfer of tetraethylammonium cations across phospholilpid monolayers deposited at a micropipette ITIES (interface between two immiscible electrolyte solutions) was studied using short potential step techniques. The observed current transients consisted of a capacitive and a faradaic current component, which were separated using a non-linear fit to a model function. Fits were very good. From the capacitive component, the surface coverage of lipids was calculated by applying a simple model for the monolayer configuration. It appeared that the coverage varied between 80 and 99%. The rate of ion transfer was increased slightly after the lipid addition, but no significant variations between different lipids were found. The transfer mechanism is discussed briefly.

Scanning Electrochemical Microscopy. 34. Potential Dependence of the Electron-Transfer Rate and Film Formation at the Liquid/Liquid Interface

The potential drop across the interface between two immiscible electrolyte solutions (ITIES), φ, can be quantitatively controlled and varied by changing the ratio of concentrations of the potential-determining ion in the two liquid phases. This approach was used to study the potential dependence of the rate constant for electron transfer (ET) at the ITIES (kf) by scanning electrochemical microscopy (SECM) with no external potential bias applied. The Tafel plot obtained for ET between aqueous Ru(CN)64- and the oxidized form of zinc porphyrin in benzene was linear with a transfer coefficient, α = 0.5, determined from the slope of a plot of ln kf vs φ, in agreement with conventional ET theory. The observed change in the ET rate with the interfacial potential drop cannot be attributed to concentration effects and represents the potential dependence of the apparent rate constant. This result is discussed in relation to the interface thickness and structure. The SECM was also used to study solid phase formation at the interface at high concentrations of supporting electrolyte (tetrahexylammonium perchlorate, THAClO4) in benzene. The precipitation of the THA+ and Ru(CN)64- compound occurred when its solubility product was exceeded. This process leads to the formation of a thin three-dimensional interfacial layer, which can be unambiguously distinguished from monolayer adsorption. The approach curve analysis yields the composition of such a layer. Its thickness can also be probed.

Dynamic effects at the interface between two immiscible electrolyte solutions (ITIES)

A new apparatus is described in which the effects on interfacial charge transfer of reconstruction of an adsorbed lipid layer can be studied. It involves rapid contraction or expansion of the interface, with control of the interfacial potential difference and measurement of current and interfacial tension. First results on the effects of lecithin on ion transport across a 2-nitrophenyl octyl ether/water interface are given. Reconstruction of the adsorbed layer, dependent on the interfacial potential difference and surface coverage, is inferred. Since the effects on transport of the probe ion tetraethylammonium were small, a model in which the surface reconstruction was in the form of islands of an ordered monolayer within a disordered one is proposed.

Potentials of Thermodynamic and Free Zero Charge at the Interface between Two Immiscible Electrolytes

Electrocapillary equations are analyzed for partially polarizable interfaces between two immiscible electrolyte solutions (ITIES), and the determination of thermodynamic and free zero charge potentials at the oil/water interface is considered. The difference between potentials corresponding to the minimum of differential capacitance and maximum of electrocapillarity curves is caused by faradaic processes and nonideal polarizability of ITIES. The difference depends on the contribution of penetrating ions to the surface charge. Because the inner layer parameters are nonlinear functions of the potential difference at ITIES, the minimum capacitance potential shifts with increasing electrolyte concentrations even when there is no specific ion adsorption.

Total internal reflection second harmonic generation from the interface between two immiscible electrolyte solutions

Total internal reflection second harmonic generation (TIR SHG) is used to investigate the interface between two immiscible electrolyte solutions (ITIES) under potential control. The observed potential dependent second harmonic response is attributed to a resonance enhanced process arising from the tetraphenylborate (TPB −) ion. As the aqueous phase is biased positive of the organic phase, a large increase in the SH response is observed and is attributed to the adsorption of the TPB − ion at the interface. The potential dependent SH data allows for the determination of the TPB − anion concentration at the interface. TIR SHG compliments measurements of the electrolyte surface excess and can function as a means of measuring the relative concentration of electrolytic species responsible for the potential drop across the electrochemical double layer.

Investigation of the kinetics of ion and assisted ion transfer by the technique of ac impedance of the micro-ities

The microhole and micropipette tip-supported interface between two immiscible electrolyte solutions (ITIES) were used to study the kinetics of ion and assisted ion transfer reactions, respectively, using the technique of ac impedance. The ion transfer reaction studied was that of tetramethylammonium transfer between water and 1,2-dichloroethane (1,2-DCE). The assisted ion transfer process investigated was that of potassium, facilitated by dibenzo-18-crown-6, between the same solvents. In each case, the standard apparent rate constant, k 0, was demonstrated to be a quantity which is inaccessible to measurement by electrochemical means. Both reactions were illustrated to be controlled by the diffusion of the participating species only.

Scanning Electrochemical Microscopy. 31. Application of SECM to the Study of Charge Transfer Processes at the Liquid/Liquid Interface

The kinetics of the electron transfer and ion transfer at the interface between two immiscible electrolyte solutions (ITIES) were probed directly by scanning electrochemical microscopy (SECM). The liquid/liquid (Le., waterhitrobenzene) interface appeared to be sharp and wave-free on the submicrometer scale. The use of SECM allowed the electron transfer between ferrocene species in nitrobenzene and other redox species in the aqueous phase to be quantitatively separated from the ion transfer processes. The rate constants were extracted from the dependence of the steady-state current at a micrometer-sized tip electrode on the distance between the tip and the phase boundary by comparison to theoretical working curves. In some experiments, the ultramicroelectrode tip penetrating the ITIES trapped a micrometer-thick layer of water inside the nitrobenzene, forming a thin-layer cell.

Scanning Electrochemical Microscopy. 30. Application of Glass Micropipet Tips and Electron Transfer at the Interface between Two Immiscible Electrolyte Solutions for SECM Imaging

Electron transfer at the interface between two immiscible electrolyte solutions (ITIES) supported at the tip of a micropipet is demonstrated using the 7,7,8,8-tetracyanoquinodimethane (in 1,2 dichloroethane)/ferrocyanide (in water) system. This micro-ITIES is then used as a probe in scanning electrochemical microscopy for imaging purposes. A micro-ITIES can successfully be used to image surfaces with a resolution comparable to that obtained when using a metallic tip of the same size.

Optical second-harmonic generation measurements of molecular adsorption and orientation at the liquid/liquid electrochemical interface

The surface-sensitive spectroscopic technique of optical second-harmonic generation (SHG) is applied to the in situ study of molecular adsorption at the interface between two immiscible electrolyte solutions (ITIES). The resonant SHG form molecules which exhibit a large non-linear optical response at a specific wavelength can be used to measure the relative surface coverage of surfactants at the ITIES as a function of the external electro-chemical parameters. In addition, the polarization dependence of the resonant surface SHG can be used to estimate the average molecular orientation of adsorbates at the liquid/liquid electrochemical interface. As an example, the adsorption of the surfactant 4-(4′-dodecyloxyazobenzene)benzoic acid at the water/1,2-dichloroethane interface is characterized as a function of applied potential, surfactant concentration and aqueous pH with in situ resonant molecular SHG measurements. An analysis of the SHG data results in the determination of the local potential and surface pH experienced by the surfactant.

Potential-step chronofluorometric response of fluorescent-ion transfer across a liquid /vb liquid interface

Abstract Potential-step transients for the reversible transfer of fluorescent ions across the interface between two immiscible electrolyte solutions (ITIES) have been calculated theoretically by combining the Lambert-Beer law with the concentration profile in the organic phase. When so many fluorescent ions are present in the organic phase that the absorption of excitation light becomes significant, the half-wave potential of fluorescence-potential curves shifts in the negative direction and both the fluorescence-time and fluorescence-concentration curves show a tendency to saturate. At lower fluorescent ion concentrations, the fluorescence intensity is proportional to the total charge transferred across the interface. Experimental data corresponding to these two cases are presented for Eosin Y transfer across the 1,2-dichloroethane /vbwater interface.

Nafion® adsorption | anion transfer at the interface between two immiscible electrolyte solutions (ITIES)

The adsorption/transfer of the perfluorosulphonate ionomer Nafion® at the interface between two immiscible electrolyte solutions, (ITIES) is studied by cyclic voltammetry. The transfer of two adsorbed forms is observed and it is postulated that these two forms correspond to a morphological difference associated with the adsorption of Nafion® in the mixed solvent layer. Thermodynamic data are calculated for the two transferring forms. The observable transfer of the SO 3CF 3 − (triflate) anion, which is structurally representative of Nafion®'s polymeric backbone, confirms the feasibility of the transfer process presented.

Nature of the processes of charge-carrier generation at ITIES by the photoexcitation of porphyrins

The kinetics of the photocurrent through the interface between two immiscible electrolyte solutions (ITIES) was investigated under high intensity irradiation when the organic phase was a concentrated solution of tetraphenylporphine or protoporphyrin. A number of different processes of charge-carrier generation are analysed. The reaction of the excited porphyrin molecule with ground state porphyrin and with oxidized forms of porphyrins was found to generate charge carriers in the volume near the interface. The mathematical description of the generation, diffusion and transformation of charge carriers, as well as relationships between these processes, is presented in the form of a system of differential equations. Numerical simulation has shown that the depletion of the irradiated zone by the oxidized forms of porphyrins strongly affects the kinetics of the photocurrent.

Polarization phenomena at ionic membrane/electrolyte interfaces: A nafion membrane between two electrolyte solutions

The distribution of ions at the phase boundaries of two ionic conductors may give rise to both faradaic and non-faradaic currents, and to a Nernst-, Donnanor diffusion-type interfacial potential difference. A multi-phase electrochemical cell can be designed, which makes it possible to measure the ionic current as a function of the cell potential difference in a polarization experiment 111. In recent years, attention has focussed on the ion transfer across an interface between two immiscible electrolyte solutions (ITIES) [2], or across an ionic membrane placed between two electrolyte solutions [ll. While in the former case the ion transfer across a single ITIES can be studied [2], in the latter case one has to make do with at least two interfacial and intervening bulk membrane processes [l]. However, when concentrations and mobilities of charge carriers inside the membrane are high enough, the behaviour of the system can be controlled mainly by processes occurring in contacting electrolyte solutions. It is the aim of the present communication to show that in a simple system of this type the ion current can be measured and understood in terms of reasonably well separated contributions of the diffusion-migration ion transport, the double-layer charging and the interfacial or bulk membrane ion transport resistances. Recently, two pioneering studies dealing with such a system have appeared [3,4], but a quantitative resolution has not been provided.

Current—potential characteristic of ion transfer across the interface between two immiscible electrolyte solutions based on the Nernst—Planck equation

A current—potential characteristic for ion transfer across the polarized interface between two immiscible electrolyte solutions (ITIES) is derived using a Goldman-type approximation to solve the Nernst—Planck equation which describes ion transfer across the inner part of the double layer. The proposed model can predict salient features of the current—potential characteristic experimentally observed in monovalent ion transfer across ITIES. The nonlinearity in the current—overpotential characteristic, i.e. the Goldman-type rectification, provides a new way of studying charge transfer kinetics at ITIES.

Electrocapillary phenomena at polarizable and reversible interfaces between two immiscible liquids: the generalized electro-capillary equation in hansen's representation

Abstract The thermodynamic theory of electrocapillary phenomena at the interface between two immiscible electrolyte solutions is considered. A generalized electrocapillary equation for polarizable and reversible interfaces is derived based on Hansen's method. The Gibbs equation for electrolytic systems is obtained both in variables corresponding to ion concentrations and concentrations of neutral ion combinations. The relationship between the ion concentrations in the system and EMF of the circuit consisting of immiscible electrolyte solutions whose terminals are made of identical metals is discussed. The formulae for the thermodynamic charge and the free charge of the interface between two immiscible electrolyte solutions (ITIES) are analysed. The cases of both perfectly and ideally polarizable, and also reversible (non-polarizable) interfaces are considered. Relationship between thermodynamic charges in different sets of variables is obtained.

Steady state current for ion transfer reactions at a micro liquid/liquid interface

The advantages of microelectrodes in electrochemistry, both for the study of fast charge transfer reactions, and for investigations in resistive media have been widely reported [ 11. Charge transfer reactions across the Interface between Two Immiscible Electrolyte Solutions (ITIES) which include ion transfer, assisted ion transfer and electron transfer reactions are generally fast (10e3 < k” < 10-l cm s-‘) and the measurement of reliable kinetic data for these processes is largely hampered by the high resistivity of the organic phase. For this reason, the use of micro ITIES offers advantages in circumventing most of the difficulties generally encountered with larger interfaces where planar diffusion mass transport dominates. The first method proposed to obtain a micro ITIES was to support the interface at the tip of a micropipette similar to those used in electrophysiology for intracellular measurements. In this case, however, it was shown that an asymmetric diffusion field prevailed comprising a linear component inside the pipette and a radial component outside [2]. This first approach permitted the study of charge transfer with solvents of low permittivity such as 1,2-dichloroethane but kinetic studies had to rely on a model derived from the computer simulation of this asymmetric mass transport regime [3]. The only kinetic studies at micropipettes which could be analysed following the methodology generally used at solid microelectrodes were pseudo-first order bimolecular reactions where the concentration of the reactant inside the pipette was in excess compared to that of the reactant outside. With this approach the mass transport was controlled by the radial diffusion of the reactants and products outside the pipette. In this way it was possible to study ion/ionophore interfacial complexation reactions [4] such as

Photochemical ion transfer across the interface between two immiscible electrolyte solutions

Irradiation of the interface between two immiscible electrolyte solutions (ITIES) by visible or UV light can give rise to a photocurrent [l-3] or photopotential [4]. This effect has been ascribed either to the photoexcited electron transfer between a sensitizer in one solvent phase and an electron acceptor in the other solvent phase [1,3], or to the transfer of an ionic product of the homogeneous photoassociated electron transfer reaction [2]. Eventually, the non-radiative dissipation of the absorbed light can result in a microscopic thermal gradient at the ITIES bringing about a change in the interfacial potential difference - a sort of ~e~~lect~c effect [2]. The purpose of this communication is to show that the base electrolyte ions themselves can yield a photocurrent which is closely related to their excited states. This applies in particular to aromatic ions like tetraarylborate anions or tetraarylarsonium cations. The interface between water and 1,2-~chloroeth~e (geometric area of 2.5 cm2) was formed in an all-glass four-electrode cell [5], with the organic solvent phase placed in the bottom part of the cell. The top of the cell was left open, so that the light from a 150 W xenon arc source (Applied Photophysics, Model 4060) reflected by a mirror at 45’ passed only through the layer of the non-absorbing aqueous phase prior to reaching the interface. Mon~hromatic light was obtained from a monochromator (Applied Photophysics) placed between the xenon lamp and the reflection mirror. The incident intensity of the monochromatic light was measured by means of a calibrated photodiode detector (Macam Photometrics, Model SD

Electrocapillary phenomena at the interface between two immiscible liquids

The thermodynamic theory of capillary phenomena is considered. The comparative analysis of Gibbs, Hansen's and Guggenheim's methods is presented. The Gibbs and Hansen's reference systems are described. An alternative geometrical interpretation of Gibbs method is developed. The adsorption equation is obtained in Hansen's representation. A symmetrized form of the adsorption equation has been derived and the relationship between Gibbs and Hansen's surface excesses established. Frumkin's interpretation of surface excesses is discussed. A description is given of the specific volume and entropy of an interface formation in a multicomponent system. The difficulties encountered in the interpretation of Hansen's method are explained. Hansen's method has been generalized for the curved surface of an arbitrary shape. The Gibbs phase rule for immiscible electrolyte solutions is considered. Methods of choosing the independent system variables that are most suitable for the electrocapillarity theory are analyzed. The thermodynamic theory of electrocapillarity phenomena at the interface between two immiscible electrolyte solutions (ITIES) is presented. A generalized electrocapillary equation for polarizable and reversible interfaces is derived using Hansen's method. The Gibbs equation for electrolytic systems is obtained both in ion concentrations and in variables corresponding to concentrations of neutral ion combinations. The relationship between the ion concentrations in the system and the EMF of the circuit consisting of immiscible electrolyte solutions with terminals made of identical metals is discussed. The formulas for the thermodynamic and the free charge of the interface between two immiscible electrolyte solutions are analysed. The cases of perfectly, ideally polarizable and reversible (nonpolarizable) interfaces are considered. Relationships between thermodynamic charges in different sets of variables are obtained.