Ion transfer facilitated

The first electrochemical observation of a facilitated ion transfer reaction was reported in 1979 by Koryta et al They studied the transfer of potassium from water to nitrobenzene, facilitated by the crown ether ionophore dibenzo-18-crown-6. This original publication has heralded an important part of the field of electrochemistry at liquid-liquid interfaces. The Prague group at the Heyrovsky Institute dedicated a lot of attention to this particular subject, resulting in a large number of publications. The ionophores investigated included nonactin, monensin, calcium ionophore, dibenzo-18-crown-6, tetracycline, valinomycin, and nigericin.  

Following the work of Koryta et Freiser and his group started to investigate these phenomena and questioned the location of the complexation reaction. The 1980s have been marked by a controversy regarding the mechanisms of facilitated ion transfer reactions. The participants to the debate also included Senda et Makrlik et Wendt et and Wang et 

ACT aqueous complexation followed by transfer TOC transfer followed by complexation in the organic phase TIC transfer by interfacial complexation TID transfer by intferfacial dissociation 

The mechanism of facilitated ion transfer reactions is not unique, as it depends on the different concentrations of both the cation and the ligand in the two phases, and also on the association constant values for the complexation equilibria in the water and organic phases. Different limiting situations can be obtained for the different systems discussed here, but any intermediate situation can only be resolved by solving the set of differential equations for mass transport of the different species involved. Furthermore, the liquid-liquid interface, not being a sharp physical boundary, makes it difficult to differentiate cases where the ionophore is distributed between the two phases. 

However, these dif iculties can be circumvented by the choice of easy systems as shown by the numerous publications in this area. Indeed, the main success of this branch of liquid-liquid electrochemistry is undoubtedly because it can provide a rather simple electrochemical route to the measurement of the stoichiometry and association constant of ion-ionophore complexes in organic solvents. Indeed, if we consider the thermodynamic equilibria for a facilitated transfer, for which we assume that 

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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. 

TABLE 2 Thermodynamic Parameters on the Facilitated Ion Transfers Determined Under the TPATPB Assumption... 

The facilitated ion transfers of some alkaline earth metals have been also studied in the DCE systems by the cyclic voltammetry. These systems perhaps have not been studied by any solvent extraction methods yet. Typical voltammograms in the N15C5 diffusion-control systems are shown in Fig. 6. The aqueous supporting electrolyte was MgCl2 instead of MgS04 in these measurements because BaS04 precipitated. 

From a chemical point of view the phenomenon of facilitated ion transfer is intriguing. In this case, the transfer of an ion is aided by complexation in one of the phases, which shifts the equilibrium into the direction desired. Several possible mechanisms are illustrated in Fig. 12.7 for transfer from the aqueous to the organic phase, they are [3] ... 

Figure 12.7 Various mechanisms for facilitated ion-transfer reactions.

Dietz, M. L., Dzielawa, J. A., Laszak, L, Young, B. A., Jensen, M. R, Influence of solvent structural variations on the mechanism of facilitated ion transfer into room-temperature ionic liquids. Green Chem., 5,682-685,2003. 

Examples of these processes are the oxidation of p-aminophenol at platinum electrodes in aqueous acidic solution, the reduction of dopamine at glassy carbon electrodes or that of the cation 2,6-diphenylpyrylium in acetonitrile [9]. Another interesting example arises from the facilitated ion transfer of amines from aqueous to organic media in the presence of crown ethers like the dibenzo-18-crown-6 [52, 53] (see Fig. 3.22). 

Figure 4. Charge-transfer processes at the liquid-liquid interface, (a) Probing ET at the liquid-liquid interface with the SECM. The kinetics of ET between two redox couples confined to different immiscible liquid phases can be measured with the SECM operating in the conventional feedback mode. Electroneutrality is maintained by transfer of the common ion (shown as an anion) across the interface (IT). Adapted with permission from Ref. [38]. Copyright 1995, American Chemical Society, (b) Schematic diagram of facilitated ion transfer reaction studied by SECM.

To decipher this complexity, electrochemistry at the polarized liquid-liquid interface developed over the past two decades has been proven to be a powerful tool, as shown in elucidation of the mechanism of ion-pair extraction [1 ] and the response of ion-selective electrodes of liquid-membrane type to different types of ions [5 7]. Along this line, several attempts have been made to use polarized liquid liquid interfaces for studying two-phase Sn2 reactions [8-10], two-phase azo-coupling [11], and interfacial polymerizations [12]. Recently, kinetic aspects of complexation reactions in facilitated ion transfer with iono-phores and the rate of protonation of amines have been treated quantitatively [13-16]. Their theoretical framework, which was adapted from the theories of kinetic currents in polaro-graphy, can be directly applicable to analyze quantitatively the chemical reactions in the two-phase systems. In what follows is the introduction to recent advances in electrochemical studies of the chemical reactions at polarized liquid liquid interfaces, mainly focusing on... 

B. Facilitated Ion Transfer with Preceding Dissociation of Metal Ion Complexes... 

Voltammetry usually does not provide unequivocal evidence as to the location of the chemical reaction, as has been pointed out in facilitated ion transfer [45]. The difficulty in determining the mechanism is inherent to all electrochemical processes involving chemical reactions in liquid]liquid two-phase systems, including several examples described above. The source of the difficulty is that liquid liquid two-phase systems allow any component in the systems to partition between the two phases. The reaction mechanism in these systems can only have a relativistic meaning the mechanism that dominates the flux of an overall charge-transfer reaction may be considered to be the primary mechanism. The following is a clear-cut example of the Ej-Cj mechanism in liquid liquid two-phase systems in this sense. 

Poly(oxyethylene)octylphenyl ether (Triton X) is a nonionic amphiphilic compound. Triton X adsorbs at the aqueous]organic (W 0) solutions interface when it is added into W or O in a W-0 system. The oxygen atoms in the polyoxyethlene group of Triton X attract the positive charge of certain metal ions (M"+) in W, resulting in a rather hydrophobic M" -Triton X complex. On account of this complex formation, the facilitated transfer of M " " from W to O can be attained in the presence of Triton X at the W 0 interface when a potential difference is applied at this interface [12]. Elere, the appropriate potential difference is that more positive than about —0.4 V versus TPhBE if O is DCE. The facilitated ion transfer is accompanied by the desorption of Triton X (Fig. 11). 

D, kfand are the diffusion coefBcient and the forward (reduction ofTCNQ) and backward (oxidation ofTCNQ ) electron transfer rate constants, respectively. Eqs. (10) and (11) are effectively valid for this system only within the so-called constant phase approximation. In these studies, the concentration of the redox species in the aqueous phase is in large excess with respect to TCNQ in the organic phase, and diffusion profiles are only developed in the latter. The absorbance changes upon several potential steps are exemplified in Fig. 4.5 for the process in Eq. (8). The sudden decrease in the absorbance after 4 s takes place as the potential is stepped back to the initial value (-0.215 V). From the slope and intercept of the Ajir versus curves in Fig. 4.5 b, the heterogeneous electron transfer rate constant can be effectively estimated. This approach, commonly referred to as chronoabsorptometry, has provided valuable kinetic information on interfacial processes such as ion transfer [17, 18] and facilitated ion transfer [21]. 

Figure 6.29 A linear quadrupole ion trap for use as a mass spectrometer using radial ion ejection. Top schematic of the trap showing the ejection slot along the length of one of the x-rods. Bottom an overall view of the complete instrument showing typical potentials and pressures. The first (square) quadrupole is an ion guide to transport ions from the ESI source into the higher vacuum region and the function of the small octapole is similar but to facilitate ion transfer into the trap. Reprinted by permission of Elsevier from A Two - Dimensional Quodrupole Ion Trap. .. , by Schwartz, et al. Journal of the American Society for Mass Spectrometry, 13, p. 659-669, 2002, Eig 1, p. 660 Eig 5, p. 662, by the American Society for Mass Spectrometry.

Shao YH, Mirkin MV (1997) Scanning electrochemical microscopy (SECM) of facilitated ion transfer at... 

One of the characteristics of electrochemistry at liquid/liquid interfaces is the diversity of charge transfer reactions which can be studied by electrochemical methodologies (6). These charge transfer reactions can be classified into three main categories (a) ion transfer (IT) reaction (b) facilitated ion transfer (FIT) reaction (c) electron transfer (ET) reaction. 

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]). 

Sun, P, Z. Q. Zhang, Z. Gao, and Y. H. Shao, Probing fast facilitated ion transfer across an externally polarized liquid-liquid interface by scanning electrochemical microscopy, Angew. Chem., Int. Ed., Vol. 41, 2002 pp. 3445-3448. 

It would be too long and too tedious to list all the facilitated ion-transfer reactions that have been reported over the years. From aUcali-metal ions to transition metal ions, most cations have been studied with different classes of ionophores ranging from the crown family with N, O, or S electron-donating atoms to caUx-arenes, not to mention all the commercial ionophores developed for ion-selective electrode applications or for solvent extraction. In the case of anions, the number of voltammetric studies reported has been much smaller [155-163], although the field of supramolecular chemistry for anion recognition is developing fast as recently reviewed [164]. 

In 1982, Samec et al. studied the kinetics of assisted alkali and alkali-earth metal cation-transfer reactions by neutral carrier and conclnded that the kinetics of transfer of the monovalent ions were too fast to be measured [186]. In 1986, Kakutani et al. published a study of the kinetics of sodium transfer facilitated by di-benzo-18-crown-6 using ac-polarography [187]. They concluded that the transfer mechanism was a TIC process and that the rate constant was also high. Since then, kinetic studies of assisted-ion-transfer reactions have been mainly carried out at micro-lTlES. In 1995, Beattie et al. showed by impedance measurements that facilitated ion-transfer (FIT) reactions are somehow faster than the nonassisted ones [188,189]. In 1997, Shao and Mirkin used nanopipette voltammetry to measure the rate constant of the transfer of K+ assisted by the presence of di-benzo-18-crown-6, and standard rate constant values of the order of 1 cm-S were obtained [190]. A more systematic study was then published that showed the following sequence,, which is not in accordance with... 

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