Ionophore ITIES

A composite polymer membrane has also been used as an effective amperometric detector for ion exchange chromatography [42] and showed detection limits similar to those obtained with a conductivity detector. An advantage of the amperometric detector based on micro-ITIES over the conductometric detector is that selectively can be tailored by proper choice of the ionophore. For instance, the selectivity of the membrane toward ammonium in the presence of an excess of sodium could be substantially increased by introducing an ammonium-selective ionophore (such as valinomycin) in the gel membrane [42]. 

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. 

Theoretical insight into the interfacial charge transfer at ITIES and detection mechanism of this type of sensor were considered [61-63], In case of ionophore assisted transport for a cation I the formation of ion-ionophore complexes in the organic (membrane) phase is expected, which can be described with the appropriate complex formation constant, /3ILnI. 

Another analytically useful phenomenon in electrolysis at ITIES is ion transfer faciUtated by ionophores present in the non-aqueous phase [8]. If the ionophore is present at a low concentration in the non-aqueous phase and the aqueous phase contains a large concentration of the cation that is bound in a complex with the ionophore (for example as a component of the base electrolyte), then a voltammetric wave controlled by diffusion of the ionophore toward the ITIES or by diffusion of the complex formed away from the ITIES into the bulk of the organic phase appears at a potential lower than the potential of simple cation transfer. The peak height of this wave is proportional to the ionophore concentration in the solution and can be used for the determination (fig. 9.8). This effect has been observed with valinomycin, nonactin, cycUc polyethers and other substances [2,3,23]. The half-wave potential of these waves is... 

The primary purpose of this review is to summarize comprehensively advances in the study of this kinetic aspect of charge transfer across ITIES since 1981, when Koryta and Vanysek gave a timely review at that early stage of the development of electrochemistry at ITIES. Reviews [5-14] and monographs [15, 16] are available of other aspects of the electrochemistry at ITIES, e.g., ion transfer facilitated by ionophores, applications to analytical purposes or to liquid extraction, and instrumentation. In a recent review on charge transfer across ITIES, Girault [14] addressed key issues regarding the mechanism of ion transfer the dependence of the rate constant of ion transfer on the applied potential, the presence of an activation barrier, the double layer correction, the effect of solvent viscosity, theoretical treatments, etc. Since the author s [14] opinions differ in several respects from ours, we have tried to review this subject as systematically and critically as possible. 

The simple ion transfer across ITIES was observed with alkali metal ions, tetraalkyl-ammonium cations, choline, acetylcholine, picrate, perchlorate, iodide, thiocyanate, nitrate, dodecylsulphate, cationic forms of various tetracycline derivatives, etc. The facilitated ion transfer was mainly studied with alkah and alkaline earth metal ions the transfer of which was mediated by ionophores already mentioned in the first section of this lecture (for reviews see [13,18]). 

In the case of ionophore determination a rather high concentration of the ion to be complexed in the aqueous phase is used while the ionophore is present at a low concentration in the organic phase. Under increasing potential scan the current peak is controlled by diffusion of the ionphore to ITIES and by diffusion of the complex formed from ITIES into the bulk of the organic phase while after scan reversal opposite processes take place. The peak currents are proportional to ionophore concentration. This method has been applied to the determination of monensin in cultures of Streptomyces cinnamonensis [33]. 

In summary, it is clear that ITIES provide a unique support to study phospholipid adsorption and the interaction of the phosphatidyl moiety with aqueous cation. It has been observed many times that a compact monolayer can hinder the transfer of some cations such TEA+, but does seem to be an effective barrier to the transfer of anions. Finally, charge-transfer studies combined to electrocapillary data have clearly shown that phosphatidylcholine acts as a strong ionophore for alkali metal cations and peptides. 

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