Nitrobenzene ITIES

The electrochemisty of ITIES is developing mainly on the basis of the studies of the water-nitrobenzene and water-1,2-dichloroethane interfaces. The polarizability ranges of these interfaces in the presence of typical electrolytes (Scheme 13) are about 0.30 V. Extension of these ranges has been achieved using other organic ions or/and solvents [2,8]. For example, TBA ions may be substituted by tetraphenylarsonium crystal violet cations and TPhB ions by dicarbollyl cobaltate (III) anions [1,2]. 

In pioneering studies [47], the SECM feedback mode was used to study the ET reaction between ferrocene (Fc), in nitrobenzene (NB), and the aqueous mediator, FcCOO, electrochemically generated at the UME by oxidation of the ferrocenemonocar-boxylate ion, FcCOO. Tetraethylammonium perchlorate (TEAP) was applied in both phases as the partitioning electrolyte. The results of this study indicated that the reaction at the ITIES was limited by the ET process, provided that there was a sufficiently high concentration of TEAP in both phases. 

Recently, the newly developed time-resolved quasielastic laser scattering (QELS) has been applied to follow the changes in the surface tension of the nonpolarized water nitrobenzene interface upon the injection of cetyltrimethylammonium bromide [34] and sodium dodecyl sulfate [35] around or beyond their critical micelle concentrations. As a matter of fact, the method is based on the determination of the frequency of the thermally excited capillary waves at liquid-liquid interfaces. Since the capillary wave frequency is a function of the surface tension, and the change in the surface tension reflects the ion surface concentration, the QELS method allows us to observe the dynamic changes of the ITIES, such as the formation of monolayers of various surfactants [34]. 

Koryta et al. [48] first stressed the relevance of adsorbed phospholipid monolayers at the ITIES for clarification of biological membrane phenomena. Girault and Schiffrin [49] first attempted to characterize quantitatively the monolayers of phosphatidylcholine and phos-phatidylethanolamine at the ideally polarized water-1,2-dichloroethane interface with electrocapillary measurements. The results obtained indicate the importance of the surface pH in the ionization of the amino group of phosphatidylethanolamine. Kakiuchi et al. [50] used the video-image method to study the conditions for obtaining electrocapillary curves of the dilauroylphosphatidylcholine monolayer formed on the ideally polarized water-nitrobenzene interface. This phospholipid was found to lower markedly the surface tension by forming a stable monolayer when the interface was polarized so that the aqueous phase had a negative potential with respect to the nitrobenzene phase [50,51] (cf. Fig. 5). 

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. 

A dropping electrolyte electrode (fig. 9.3) was also developed for the study of electrolysis at ITIES. When the density of the organic phase is greater than that of the aqueous phase (for example, as with nitrobenzene), the aqueous drops move upwards. 

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). 

Fig. 1. Change in the potential at a nonpolarized ITIES with the concentration ratio, at 25 °C for the system RPh4B (nitrobenzene)/RCl(water), where R = tetrapropylammonium (Pr4N + ), tetrabutylammonium (Bu4N ), tetrapentylammonium (Pn4N ), or tetrahexylammonium (Hx4N ) ion and Ph4B = tetraphenylborate. (Reprinted from [47] with permission. Copyright The Chemical Society of Japan).

Adsorption of proteins at an ITIES has also been reported for cytochrome c at the 1,2-dichloroethane-water interface [102], and for ovalbumin [103] and bovine serum albumin [104] at the nitrobenzene-water interface. 

An ITIES is formed between water and nitrobenzene (NB) using electrolytes with the picrate (Pic ) anion at the same activity (0.1 M). The electrolyte in water is Li Pic and that in NB, tetraphenylarsonium (TA" ") picrate. Given that the standard Gibbs energy of transfer of Pic is -4.6 kJmoU [18], estimate the Galvani potential difference at the interface. Use the data in table 8.11 to estimate the activity of Li" " in NB. 

A potential window, where negligible ion re-partitioning occurs, then exists, with limits defined by the lowest absolute values of the standard Galvani potentials. Experimentally, this window is on the order of 0.5-1.0 V, for an ITIES composed of aqueous electrolyte phases and solvents such as 1,2-dichloroethane (DCE) or nitrobenzene (NB). In this type of system, the interfacial potential difference can be imposed externally using potentiostatic control. 

Figure 6.8.1 Schematic diagram for the apparatus for cyclic voltammetry at the ITIES between water and nitrobenzene. Ref a and Ref p are reference electrodes, Wk a and Wk p are metal working electrodes.

Small LL interfaces have been used by Girault and co-workers (33-38) and by Senda et al. (39, 40). We have used a small hole formed in a thin glass wall (41-43). Figure 16 shows the voltammetric response of lauryl sulfate anion transport between water and nitrobenzene. Recent analytical applications of these microinterfaces have resulted in construction of gel-solidified probes. The advantage of such a modification is ease of handling (44-47). The immobilization can be extended further to studies of frozen interfaces, or even to solid electrolytes. Significantly, ITIES theory also applies to interfaces that are encountered in ion-doped, conductive, polymer-coated electrodes. 

We now return to the example introduced in Section 20.1.1, where two electrolyte solutions a and P were placed in contact and a hypothetical barrier was invoked that prevented the transfer of one of the ions between the phases. Although this example may have seemed obscure, such a system may readily be constructed for the interface between two immiscible electrolyte solutions (ITIES). Such a system can be formed by contacting water with an immiscible organic solvent such as nitrobenzene or 1,2-dichloroethane, as discussed in Section 17.3 in Chapter 17 of this handbook. The two solvents possess a slight mutual solubility. Once the two phases are equilibrated, the system is composed of an organic saturated aqueous phase in contact with a water saturated organic phase. For this reason, measmement of any transport property should always be performed on mutually pre-saturated solutions. 

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