Transfer at the ITIES

Although most amperometric SECM experiments involved ET reactions at the tip and/or substrate, interfacial IT processes can also be probed. Historically, the first IT reactions studied by SECM were ion-exchange processes at ionically and electronically conductive polymer films (48). The ions of interest were electrochemically active (e.g., Ec(CN)f or Br ) to enable amperometric detection at the tip. It was shown more recently that the tip process can be an IT reaction rather than an ET process if a micropipet electrode is used as an amperometric probe (49). In this section we consider two different types of IT reactions employed in SECM studies, i.e., facilitated IT and simple IT. 

In a typical facilitated IT reaction an ion (most often, a cation, M+) is transferred from aqueous solution into the organic phase. A complex species formed by this ion and a ligand (L, initially present in organic phase) is easier to transfer than M+ itself. Reactions of this type are widely used in chemical sensors [ion-selective electrodes, liquid ion-exchangers (50)]. For SECM experiments, an aqueous solution of M+ is placed inside a micropipet, which serves as a tip electrode. The facilitated IT reaction at the micropipet tip is 

With the tip biased at a sufficiently positive potential and the concentration of M+ inside a pipet much higher than concentration of L in the outer solvent, the tip current is limited by diffusion of L to the pipet orifice. When the tip approaches the bottom (aqueous) layer, M+ is released from the complex and transferred to the aqueous solution, and L is regenerated (Fig. 18A)  

The ITIES formed at the pipet tip is polarizable, and the voltage applied between the micropipet and the reference electrode in organic phase provides the driving force for facilitated IT reaction. The interface between organic (top) and water (bottom) layers is nonpolarizable, and the potential drop, A) p, is governed by the ratio of concentrations of the common ion (e.g., 

18 Schematic representation of the SECM operating in the facilitated (A) and simple (B) IT feedback mode. (A) Potassium ions are transferred from the pipet into DCE by interfacial complexation with DB18C6 [Eq. (26)] and from DCE to 

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Progress in experimental and theoretical studies of the mechanistic and dynamic aspects of charge transfer at the ITIES is developing swiftly. The present reviews is therefore deemed to be a status report concerning the charge transfer kinetics at the ITIES, rather than a systematic presentation of the subject. 

Besides improving classical electrochemical methods, newly employed techniques such as second harmonic-generation and time-resolved fluorometry, with either control of the potential drop across the interface or fluctuation analysis, are promising in this respect. Also indispensable are further advances in molecular dynamics and statistical-mechanical treatments of structure and charge transfer at the ITIES. 

Kotov, N. A. and M. G. Kuzmin, Computer analysis of photoinduced charge transfer at the ITIES in protoporphyrin-quinone systems, J Electroanal Chem, Vol. 341, (1992) p. 47. 

Solomon, T., Bard, A. J. Scanning electrochemical microscopy. 30. The application of glass micropipet tips and electron transfer at the ITIES for SECM imaging. Anal. Chem. 1995, 67, 2787. 

To extend the applicability of the SECM feedback mode for studying ET processes at ITIES, we have formulated a numerical model that fully treats diffusional mass transfer in the two phases [49]. The model relates to the specific case of an irreversible ET process at the ITIES, i.e., the situation where the potentials of the redox couples in the two phases are widely separated. A further model for the case of quasireversible ET kinetics at the ITIES is currently under development. For the case where the oxidized form of a redox species, Oxi, is electrolytically generated at the tip in phase 1 from the reduced species, Red], the reactions at the tip and the ITIES are ... 

FIG. 26 Typical DPSC data for the oxidation of DMFc to DMFc+ (forward step of 0.371 s duration, upper solid line) and the collection of DMFc" by reduction to DMFc (reverse step, lower solid line) at a tip in a DCE phase, positioned at a distance of 4.4 pm from the DCE-aqueous interface. The upper dashed line shows the theoretical characteristics for hindered diffusion of DMFc to the tip. The lower dashed line is the theoretical response when there is no DMFc+ transfer across the ITIES. (Reprinted from Ref. 86. Copyright Elsevier Science.)... 

The electrolyte dropping electrode has found particular application in the study of ion transfer at the polarized ITIES, with an emphasis on analysis. A range of species have been detected amperometrically by measuring the transport-limited current ... 

A hanging electrolyte drop has also been applied to determine ionic species in solution using differential-pulse-stripping voltammetry procedures [69]. Particular emphasis was given to assessing the selectivity and sensitivity of the method. The technique of current-scan polarography has also been applied in the study of electron-transfer [70] and coupled electron-transfer-ion-transfer [71,72] reactions at the ITIES in this configuration. 

The use of micropipette electrodes for quantitative voltammetric measurements of ion transfer (IT) and electron transfer (ET) reactions at the ITIES requires knowledge of geometry of the liquid interface. For the micrometer-sized micropipettes, both the orifice radius and the thickness of the pipette wall can be measured microscopically. A typical error of the microscopic determination of a radius was estimated to be 0.5/am for a micropipette and 1 /am for a microhole [24]. 

Since the mass-transfer coefficient at a micropipette is inversely proportional to its radius, the smaller the pipette the faster heterogeneous rate constants can be measured. Micrometer-sized pipettes are too large to probe rapid CT reactions at the ITIES. Such measurements require smaller (nm-sized) pipettes. Nanopipettes are also potentially useful as SECM tips (see Section IV.D) because they can greatly improve spatial resolution of that technique. The fabrication of nanopipettes was made possible by the use of a micro-processor-controlled laser pipette puller capable of puling quartz capillaries [26]. Using this technique, Wei et al. produced nanopipettes as small as 20 nm tip radius and employed them in amperometric experiments [9]. 

In this case, both the top and the bottom liquid phases contain the same ion at equilibrium. A micropipette tip is used to deplete concentration of this ion in the top solvent near the ITIES. This depletion results in the ion transfer across the ITIES (Fig. 13), which can produce positive feedback if the bottom phase contains a sufficiently high concentration of M ... 

In Ref. 30, the transfer of tetraethylammonium (TEA ) across nonpolarizable DCE-water interface was used as a model experimental system. No attempt to measure kinetics of the rapid TEA+ transfer was made because of the lack of suitable quantitative theory for IT feedback mode. Such theory must take into account both finite quasirever-sible IT kinetics at the ITIES and a small RG value for the pipette tip. The mass transfer rate for IT experiments by SECM is similar to that for heterogeneous ET measurements, and the standard rate constants of the order of 1 cm/s should be accessible. This technique should be most useful for probing IT rates in biological systems and polymer films. 

The description of the ion transfer process is closely related to the structure of the electrical double layer at the ITIES [50]. The most widely used approach is the combination of the BV equation and the modified Verwey-Niessen (MVN) model. In the MVN model, the electrical double layer at the ITIES is composed of two diffuse layers and one ion-free or inner layer (Fig. 8). The positions delimiting the inner layer are denoted by X2 and X2, and represent the positions of closest approach of the transferring ion to the ITIES from the organic and aqueous side, respectively. The total Galvani potential drop across the interfacial region, AgCp = cj) — [Pg.545]

When a monolayer of phospholipids is adsorbed at the ITIES, there must be a modification of the electrical structure of the interface [60]. Since we aim at describing the effect of this monolayer on the rate of ion transfer in a simple way, we assume a sharp interface also in the presence of phospholipids. The hydrophobic tails are located in the organic phase (negative x region), and the hydrophilic headgroups are located in the aqueous phase (positive X region). 

A theoretical approach based on the electrical double layer correction has been proposed to explain the observed enhancement of the rate of ion transfer across zwitter-ionic phospholipid monolayers at ITIES [17]. If the orientation of the headgroups is such that the phosphonic group remains closer to the ITIES than the ammonium groups, the local concentration of cations is increased at the ITIES and hence the current observed due to cation transfer is larger than in the absence of phospholipids at the interface. This enhancement is evaluated from the solution of the PB equation, and calculations have been carried out for the conditions of the experiments presented in the literature. The theoretical results turn out to be in good agreement with those experimental studies, thus showing the importance of the electrostatic correction on the rate of ion transfer across an ITIES with adsorbed phospholipids. 

Various types of research are carried out on ITIESs nowadays. These studies are modeled on electrochemical techniques, theories, and systems. Studies of ion transfer across ITIESs are especially interesting and important because these are the only studies on ITIESs. Many complex ion transfers assisted by some chemical reactions have been studied, to say nothing of single ion transfers. In the world of nature, many types of ion transfer play important roles such as selective ion transfer through biological membranes. Therefore, there are quite a few studies that get ideas from those systems, while many interests from analytical applications motivate those too. Since the ion transfer at an ITIES is closely related with the fields of solvent extraction and ion-selective electrodes, these studies mainly deal with facilitated ion transfer by various kinds of ionophores. Since crown ethers as ionophores show interesting selectivity, a lot of derivatives are synthesized and their selectivities are evaluated in solvent extraction, ion-selective systems, etc. Of course electrochemical studies on ITIESs are also suitable for the systems of ion transfer facilitated by crown ethers and have thrown new light on the mechanisms of selectivity exhibited by crown ethers. 

In both cases, the half-wave potential shifts by RT/ ziF)vaN per pH unit, and a typical example of such a behavior is given in Fig. 9 for the transfer of two acidic fi-diketones at the water-nitrobenzene interface. These results were unexpected, since a current wave is measured at a pH where the compound of interest is by a very large majority neutral, but they in fact represent the typical behavior of ionizable compounds at the ITIES and prove that the interfacial potential and the transfer of protons plays a key role for the distribution in biphasic systems. 

Potential differences at the interface between two immiscible electrolyte solutions (ITIES) are typical Galvani potential differences and cannot be measured directly. However, their existence follows from the properties of the electrical double layer at the ITIES (Section 4.5.3) and from the kinetics of charge transfer across the ITIES (Section 5.3.2). By means of potential differences at the ITIES or at the aqueous electrolyte-solid electrolyte phase boundary (Eq. 3.1.23), the phenomena occurring at the membranes of ion-selective electrodes (Section 6.3) can be explained. 

The electrodes used in conventional polarography and voltammetry are electronic conductors such as metals, carbons or semiconductors. In an electrode reaction, an electron transfer occurs at the electrode/solution interface. Recently, however, it has become possible to measure both ion transfer and electron transfer at the interface between two immiscible electrolyte solutions (ITIES) by means of polarography and voltammetry [16]. Typical examples of the immiscible liquid-liquid interface are water/nitrobenzene (NB) and water/l,2-dichloroethane (DCE). 

Many of the systems used for electrochemical studies of ion transfer processes taking place at the ITIES are systems of a single polarized interface. In these kinds of systems, the polarization phenomenon is only effective at the sample solution/... 

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