Between Two Immiscible Electrolyte Solutions

Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Prague, Czech Republic 

An interface between two immiscible electrolyte solutions (ITIES) is formed between two liqnid solvents of a low mutual miscibility (typically, 1% by weight), each containing an electrolyte. One of these solvents is usually water and the other one is a polar organic solvent of a moderate or high relative dielectric constant (permittivity). The latter requirement is a condition for at least partial dissociation of dissolved electrolyte(s) into ions, which thus can ensure the electric conductivity of the liquid phase. A list of the solvents commonly used in electrochemical measurements at ITIES is given in Table 32.1. 

Here A and X represent a highly hydrophilic and hydrophobic anion, respectively, equilibrium concentrations of which in the other phase are practically zero. Typical examples are chloride and tetrakis (4-chlorophenyl)borate anions, respectively. 

Fundamentals of Electrochemistry, Second Edition, By V. S. Bagotsky Copyright 2006 John Wiley Sons, Inc. 

TABLE 32.1 Polar Organic Solvents Exhibiting Low Miscibility with Water and Their Relative Dielectric Constants e and Densities p at 298 K 

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ITIES interface between two immiscible electrolyte solutions... 

The interface separating two immiscible electrolyte solutions, e.g., one aqueous and the other based on a polar organic solvent, may be reversible with respect to one or many ions simultaneously, and also to electrons. Works by Nernst constitute a fundamental contribution to the electrochemical analysis of the phase equilibrium between two immiscible electrolyte solutions [1-3]. According to these works, in the above system electrical potentials originate from the difference of distribution coefficients of ions of the electrolyte present in the both phases. 

A. R. Brown. Photoelectrochemical Processes at the Interface Between Two Immiscible Electrolyte Solutions. PhD Thesis, University of Edinburgh, Edinburgh, 1992. 

The structure of the interface between two immiscible electrolyte solutions (ITIES) has been the matter of considerable interest since the beginning of the last century [1], Typically, such a system consists of water (w) and an organic solvent (o) immiscible with it, each containing an electrolyte. Much information about the ITIES has been gained by application of techniques that involve measurements of the macroscopic properties, such as surface tension or differential capacity. The analysis of these properties in terms of various microscopic models has allowed us to draw some conclusions about the distribution and orientation of ions and neutral molecules at the ITIES. The purpose of the present chapter is to summarize the key results in this field. 

In particular, the coupling between the ion transfer and ion adsorption process has serious consequences for the evaluation of the differential capacity or the kinetic parameters from the impedance data [55]. This is the case, e.g., of the interface between two immiscible electrolyte solutions each containing a transferable ion, which adsorbs specifically on both sides of the interface. In general, the separation of the real and the imaginary terms in the complex impedance of such an ITIES is not straightforward, and the interpretation of the impedance in terms of the Randles-type equivalent circuit is not appropriate [54]. More transparent expressions are obtained when the effect of either the potential difference or the ion concentration on the specific ion adsorption is negli-... 

ITIES interface between two immiscible electrolyte solutions K tautomeric equilibrium constant between the zwitterionic and the neutral forms of a compound... 

V. Gobry, F. Re5miond and H. H. Girault, Refinment of Ionic Partition Diagrams and Determination of Partition Coefficients of Multiprotic Compounds by Electrochemistry at the Interface between Two Immiscible Electrolyte Solutions, submitted. 

The determination of the standard Gibbs energies of transfer and their importance for potential differences at the boundary between two immiscible electrolyte solutions are described in Sections 3.2.7 and 3.2.8. 

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). 

The procedure described in the preceding section can form a basis for unambiguous determination of the Galvani potential difference between two immiscible electrolyte solutions (the Nernst potential) considering Eqs (3.1.22) and (1.4.34), e.g. for univalent ions,... 

Potential differences at the interface between two immiscible electrolyte solutions (ITIES) are typical Galvani potential differences and cannot be measured directly. However, their existence follows from the properties of the electrical double layer at the ITIES (Section 4.5.3) and from the kinetics of charge transfer across the ITIES (Section 5.3.2). By means of potential differences at the ITIES or at the aqueous electrolyte-solid electrolyte phase boundary (Eq. 3.1.23), the phenomena occurring at the membranes of ion-selective electrodes (Section 6.3) can be explained. 

Electrical double layers are also characteristic of the semiconductor-electrolyte solution, solid electrolyte or insulator-electrolyte solution interface and for the interface between two immiscible electrolyte solutions (ITIES) (Section 4.5). 

For semiconductor electrodes and also for the interface between two immiscible electrolyte solutions (ITIES), the greatest part of the potential difference between the two phases is represented by the potentials of the diffuse electric layers in the two phases (see Eq. 4.5.18). The rate of the charge transfer across the compact part of the double layer then depends very little on the overall potential difference. The potential dependence of the charge transfer rate is connected with the change in concentration of the transferred species at the boundary resulting from the potentials in the diffuse layers (Eq. 4.3.5), which, of course, depend on the overall potential difference between the two phases. In the case of simple ion transfer across ITIES, the process is very rapid being, in fact, a sort of diffusion accompanied with a resolvation in the recipient phase. 

This type of sensor often does not have a membrane it instead utilizes the properties of a water-oil interface, a boundary between an aqueous and a non-aqueous (organic) phase. Traditionally, sensors based on non-equilibrium ion-selective transport phenomena were distinguished as a separate group and considered as the electrochemistry of the ion transfer between two immiscible electrolyte solutions (IT1ES). Here, we will not distinguish polymeric membrane electrodes and ITIES-based electrodes due to the similarity in the theoretical consideration. 

Z. Samec, E. Samcova, and H.H. Girault, Ion amperometry at the interface between two immiscible electrolyte solutions in view of realizing the amperometric ion-selective electrode. Talcmta 63, 21—32 (2004). 

This definition requires some explanation. (1) By interface we denote those regions of the two adjoining phases whose properties differ significantly from those of the bulk. These interfacial regions can be quite extended, particularly in those cases where a metal or semiconducting electrode is covered by a thin film. Sometimes the term interphase is used to indicate the spatial extention. (2) It would have been more natural to restrict the definition to the interface between an electronic and an ionic conductor only, and, indeed, this is generally what we mean by the term electrochemical interface. However, the study of the interface between two immiscible electrolyte solutions is so similar that it is natural to include it under the scope of electrochemistry. 

As mentioned in section 2.2, an electric double layer is formed at the boundary between two immiscible electrolyte solutions as a result of the different tendencies of electrically charged components in each of the phases to pass into the other phase. 

Similar types of electric double layer may also be formed at the phase boundary between a solid electrolyte and an aqueous electrolyte solution [7]. They are formed because one electrically-charged component of the solid electrolyte is more readily dissolved, for example the fluoride ion in solid LaFs, leading to excess charge in the solid phase, which, as a result of movement of the holes formed, diffuses into the soUd electrolyte. Another possible way a double layer may be formed is by adsorption of electrically-charged components from solution on the phase boundary, or by reactions of such components with some component of the solid electrolyte. For LaFa this could be the reaction of hydroxyl ions with the trivalent lanthanum ion. Characteristically, for the phase boundary between two immiscible electrolyte solutions, where neither solution contains an amphiphilic ion, the electric double layer consists of two diffuse electric double layers, with no compact double layer at the actual phase boundary, in contrast to the metal electrode/ electrolyte solution boundary [4,8, 35] (see fig. 2.1). Then, for the potential... 

Consider a system in which a potential difference AV, in general different from the equilibrium potential between the two phases A 0, is applied from an external source to the phase boundary between two immiscible electrolyte solutions. Then an electric current is passed, which in the simplest case corresponds to the transfer of a single kind of ion across the phase boundary. Assume that the Butler-Volmer equation for the rate of an electrode reaction (see p. 255 of [18]) can also be used for charge transfer across the phase boundary between two electrolytes (cf. [16, 19]). It is mostly assumed (in the framework of the Frumkin correction) that only the potential difference in the compact part of the double layer affects the actual charge transfer, so that it follows for the current density in our system that... 

This chapter is based on the thermodynamic theory of membrane potentials and kinetic effects will be considered only in relation to diffusion potentials in the membrane. The ISE membrane in the presence of an interferent can be thought of as analogous to a corroding electrode [46a] at which chemically different charge transfer reactions proceed [15, 16]. Then the characteristics of the ISE potentials can be obtained using polarization curves for electrolysis at the boundary between two immiscible electrolyte solutions [44[Pg.35]

It follows from study of the kinetics of transfer of ions across the phase boundary between two immiscible electrolyte solutions (see chapter 9) that ion-exchanger ions, where the ion is as nearly as possible symmetrically surrounded by hydrophobic groups on all sides, are especially suitable. Amphiphilic (amphipathic) substances, in whose molecules the hydrophobic part is separated from the hydrophilic part, are less suitable because they have a tendency to become adsorbed on the membrane/water phase boundary, thus retarding ion transfer across this boundary. 

The previous chapters dealt with ISE systems at zero current, i.e. at equilibrium or steady-state. The properties of the interface between two immiscible electrolyte solutions (ITIES), described in sections 2.4 and 2.5, will now be used to describe a dynamic method based on the passage of electrical current across ITIES. Voltammetry at ITIES (for a survey see [3, 8, 9, 10, 11, 12,18]) is an inverse analogue of potentiometry with liquid-membrane ISEs and thus forms a suitable conclusion to this book. 

Voltammetry at the Interface Between Two Immiscible Electrolyte Solutions... 

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). 

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