Polarizability ITIES

Thermodynamic analysis of the ideally polarizable ITIES in the absence of the ion association yields the electrocapillary equation (for T,p = const) (Kakiuchi and Senda, 1983)... 

As mentioned earlier, a great deal of literature has dealt with the properties of heterogeneous liquid systems such as microemulsions, micelles, vesicles, and lipid bilayers in photosynthetic processes [114,115,119]. At externally polarizable ITIES, the control on the Galvani potential difference offers an extra variable, which allows tuning reaction paths and rates. For instance, the rather high interfacial reactivity of photoexcited porphyrin species has proved to be able to promote processes such as the one shown in Fig. 3(b). The inhibition of back ET upon addition of hexacyanoferrate in the photoreaction of Fig. 17 is an example of a photosynthetic reaction at polarizable ITIES [87,166]. At Galvani potential differences close to 0 V, a direct redox reaction involving an equimolar ratio of the hexacyanoferrate couple and TCNQ features an uphill ET of approximately 0.10 eV (see Fig. 4). However, the excited state of the porphyrin heterodimer can readily inject an electron into TCNQ and subsequently receive an electron from ferrocyanide. For illumination at 543 nm (2.3 eV), the overall photoprocess corresponds to a 4% conversion efficiency. 

While conventional studies of the ITIES have been carried out at externally biased polarizable ITIES, in SECM measurements, a nonpolarizable ITIES is poised by the concentrations of the potential-determining ions providing a controllable driving force for the ET process [38, 39]. 

Another approach to heterogeneous electron transfer relies on photoexcitable dye molecules adsorbed at the polarizable ITIES [xi]. General mechanism for a pho-toinduced electron transfer between a water-soluble dye sensitiser and a redox quencher located in the organic... 

The main objective of this chapter is to illustrate how fundamental aspects behind catalytic two-phase processes can be studied at polarizable interfaces between two immiscible electrolyte solutions (ITIES). The impact of electrochemistry at the ITIES is twofold first, electrochemical control over the Galvani potential difference allows fine-tuning of the organization and reactivity of catalysts and substrates at the liquid liquid junction. Second, electrochemical, spectroscopic, and photoelectrochemical techniques provide fundamental insights into the mechanistic aspects of catalytic and photocatalytic processes in liquid liquid systems. We shall describe some fundamental concepts in connection with charge transfer at polarizable ITIES and their relevance to two-phase catalysis. In subsequent sections, we shall review catalytic processes involving phase transfer catalysts, redox mediators, redox-active dyes, and nanoparticles from the optic provided by electrochemical and spectroscopic techniques. This chapter also features a brief overview of the properties of nanoparticles and microheterogeneous systems and their impact in the fields of catalysis and photocatalysis. 

Photocatalytic processes in two-phase systems involve either a homogeneous photoreaction followed by the transfer of intermediate species across the interface, or a heterogeneous electron transfer between the photoactive species and the substrate. At the polarizable ITIES, both processes would manifest themselves by an increase in the current on illumination at constant potential, i.e., a photocurrent response. Indeed, photocurrent measurements have been recorded for the transfer of photogenerated ions at a liquid liquid boundary, as well as for heterogeneous redox quenching. We shall review some of these studies in this section. 

In Section II.C, we described the reactivity of adsorbed dye species at liquid liquid junctions in heterogeneous photoredox reactions. The properties of these systems can be used to catalyze electron-transfer processes. The behavior of dyes at interfaces has been vigorously studied in micelles and microemulsion systems, and many excellent reviews and books are available on this subject [94-97]. In this section, we shall consider some basic aspects of photoprocesses in microheterogeneous systems that are relevant to polarizable ITIES. This is not intended to cover comprehensively the recent developments in the active area of photochemistry at organized assemblies, but to highlight how spatial confinement, hydrophilic hydrophobic forces, and local potentials can affect the course of a photochemical process. We shall also revise some recent developments in photocatalysis and photosynthesis at polarizable liquid liquid interfaces, highlighting advantages and limitations in relation to two-phase catalysis. 

Through these well-known examples the effect of three basic features of organized assemblies can be visualized (1) inhomogeneous solvation properties, (2) interfacial potentials, and (3) spatial confinement. Apart from the last, the other aspects can be finely controlled at the polarizable ITIES. As we have seen in Section II, the Galvani potential difference not only affects the dynamics of photoinduced electron transfer, but also the concentration ratio of ionic species. 

Steady-state voltammetry was used to investigate ET reactions at the polarizable ITIES formed at the tip of 50-nm to 400-nm radius pipets. The pipet was filled with aqueous solution containing a mixture of two forms of redox species (Oi and Rj) and immersed in organic solution containing water-insoluble redox species (02)- The application of a sufficiently negative potential to the internal reference electrode with respect to the external (organic) reference resulted in the electric current across the nano-ITIES... 

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