Liquid junction potentials immiscible liquids

The membrane phase m is a solution of hydrophobic anion Ax (ion-exchanger ion) and cation Bx+ in an organic solvent that is immiscible with water. Solution 1 (the test aqueous solution) contains the salt of cation Bx+ with the hydrophilic anion A2. The Gibbs transfer energy of anions Ax and A2 is such that transport of these anions into the second phase is negligible. Solution 2 (the internal solution of the ion-selective electrode) contains the salt of cation B with anion A2 (or some other similar hydrophilic anion). The reference electrodes are identical and the liquid junction potentials A0L(1) and A0L(2) will be neglected. 

Interface between two liquid solvents — Two liquid solvents can be miscible (e.g., water and ethanol) partially miscible (e.g., water and propylene carbonate), or immiscible (e.g., water and nitrobenzene). Mutual miscibility of the two solvents is connected with the energy of interaction between the solvent molecules, which also determines the width of the phase boundary where the composition varies (Figure) [i]. Molecular dynamic simulation [ii], neutron reflection [iii], vibrational sum frequency spectroscopy [iv], and synchrotron X-ray reflectivity [v] studies have demonstrated that the width of the boundary between two immiscible solvents comprises a contribution from thermally excited capillary waves and intrinsic interfacial structure. Computer calculations and experimental data support the view that the interface between two solvents of very low miscibility is molecularly sharp but with rough protrusions of one solvent into the other (capillary waves), while increasing solvent miscibility leads to the formation of a mixed solvent layer (Figure). In the presence of an electrolyte in both solvent phases, an electrical potential difference can be established at the interface. In the case of two electrolytes with different but constant composition and dissolved in the same solvent, a liquid junction potential is temporarily formed. Equilibrium partition of ions at the - interface between two immiscible electrolyte solutions gives rise to the ion transfer potential, or to the distribution potential, which can be described by the equivalent two-phase Nernst relationship. See also - ion transfer at liquid-liquid interfaces. 

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. 

The establishment of such interfacial potentials is readily envisaged for cases where the net transport of an electrolyte is prevented because one of its constituents cannot partition. What is perhaps less obvious is that such potentials arise continually within solution phases, even where there is no physical separation into distinct phases. These so-called liquid junction potentials or diffusion potentials play an important role in electrochemical experiments, but because there is no well-defined phase boundary, they are intrinsically more difficult to measure. This chapter discusses how these potentials arise, how they may be calculated, what quantities are associated with them, and how they may be minimised. Finally, interfaces between electrolytes (i.e. those interfaces between immiscible electrolyte solutions (ITIES)) and the application of some of the concepts developed earlier in the chapter to non-standard electrolyte systems, such as polymer electrolytes and room-temperature ionic liquids, will be discussed. 

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