Two polarizable interfaces

In Fig. 4.10, the DDPV curves corresponding to a membrane system with two polarizable interfaces (solid lines) and also to a system with a single polarizable interface (dashed lines), obtained for two values of the pulse amplitude AE, are shown. The current A/DDPV has been plotted in all the cases versus the... 

In Fig. 5.19, the cyclic voltammogram versus the membrane potential M (given in Eq. (2.79)) obtained for a system with two polarizable interfaces (solid line) is presented. The i//cv curve has been also plotted versus the outer interface oul (dashed line) and the inner interface potential ilm (dotted line) with out and inn given by... 

In this section, the subtractive multipulse techniques DMPV and SWV are applied to reversible ion transfer across different liquid-liquid systems with one or two polarizable interfaces. These electrochemical techniques allow the accurate and easy determination of standard potentials directly from the peak potentials of the current-potential curves since non-faradaic and background currents are minimized [12, 35-40]. 

In the case of DMPV (see Scheme 7.2), the treatment followed in Sect. 4.2.4.1 for Differential Double Pulse Voltammetry (DDPV) for one and two polarizable interfaces can be used because the equilibrium is quickly reestablished during the longer period Therefore, the peak potential of the DMPV curves when the current is plotted versus the index potential is... 

The general expression for the SWV net current applicable to systems with one and two polarizable interfaces is [36-38] ... 

Fig. 7.23 Theoretical /sw — (E — E1/2) curves corresponding to the direct and reverse scans of the square wave (solid lines and empty circles, respectively) for a system with one and two polarizable interfaces obtained for sw = 50mV by using Eq. (7.44). AEs = 5mV. E1/2 is the half-wave potential given by Eqs. (2.53) and (2.81) for system with one or two polarizable interfaces, respectively. Others parameters are the same as given in Scheme 7.5. Reproduced from [38] with permission...

This chapter will include equilibria at non-polarizable interfaces for a metal or semiconductor phase-electrolyte system (a galvanic cell in the broadest sense) and for two electrolytes (the solid electrolyte-electrolyte solution interface, or that between two immiscible electrolyte solutions). 

A polarizable Interface is represented by a (polarizable) electrode where a potential difference across the double layer is applied externally, i.e., by applying between the electrode and a reference electrode using a potentiostat. At a reversible interface the change in electrostatic potential across the double layer results from a chemical interaction of solutes (potential determining species) with the solid. The characteristics of the two types of double layers are very similar and they differ primarily in the manner in which the potential difference across the interface is established. 

It can be observed that the above two types of electric double layer, which have basically similar properties, differ principally in the manner of establishing the potential difference across the electric double layer. One type is fixed by the solubility and other interactions of the solid in contact with solution of electrolyte. In the other type, polarizable interface, the experimenter applies any desired potential difference between one liquid surface and a reference electrode. The resulting Volta potential is fixed by the specific adsorbability of the electrolyte. 

Fig. 2.5 (a) Normalized current-potential curves corresponding to a system with two polarizable liquid/ liquid interfaces (solid line see Eq. (2.84)) and to a system with one polarizable interface (dashed line see Eq. (2.52)). (b) /n/ m (solid line), Is/Ei (dashed line), and/N/(—Ei) (dotted line) calculated from Eqs. (2.84), (2.73), and (2.77), respectively. 

DDPV technique has been also applied to the study of the ion transfer processes in systems with one and two liquid/liquid polarizable interfaces [40-42], The expression for the current corresponding to the transfer of ion X+ is ... 

As for single polarized interface systems, an explicit analytical equation for the CV response for systems with two L/L polarizable interfaces is derived from that corresponding to CSCV when the pulse amplitude AE approaches zero (see also Appendix H). For the case corresponding to the transfer of a cationX+, one obtains... 

In line with what is observed for other techniques, the response obtained in CV for a system with two liquid/liquid polarizable interfaces is lower and broader than that obtained for ion transfers at a single water/organic interface. This has been attributed to different polarization rates at the outer and inner interfaces [66]. 

ET reactions at the polarizable - interface between two immiscible electrolyte solutions (ITIES) have been studied by cyclic voltammetry [ii], AC impedance [iii], and scanning electrochemical microscopy (SECM) [iv]. A simple method was introduced that allows evaluation of the ET rates at the interface between the thin film of an organic solvent and the aqueous electrolyte solution [v]. 

The above discussion applies to a non-polarizable interface with a common electrolyte MX distributed in two immiscible phases a and P ... 

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. 

Fig. 1 An arbitrary scheme of the structure of a perfectly polarizable interface between two immiscible liquids (a, b, c, d) and an ideally polarizable interface (b, c).

---