ITIES electrolyte solutions

VAN LUiK AND juRiNAK Heuvij Metdls iti Electrolyte Solution... 

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. 

ITIES interface between two immiscible electrolyte solutions... 

Liquid surfaces and liquid-liquid interfaces are very common and have tremendous significance in the real world. Especially important are the interfaces between two immiscible liquid electrolyte solutions (acronym ITIES), which occur in tissues and cells of all living organisms. The usual presence of aqueous electrolyte solution as one phase of ITIES is the main reason for the electrochemical nature of such interfaces. 

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-... 

Early in the 20th century the first electrochemical study at an ITIES (interface of two immiscible electrolyte solutions) on ion transfer across a water-phenol interface was reported by pioneers Nernst and Riesenfeld [1], However, it was about 70 years before the start of the many interesting electrochemical studies of ITIESs, which successfully continue today. 

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

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. 

Fig. 4.1 Structure of the electric double layer and electric potential distribution at (A) a metal-electrolyte solution interface, (B) a semiconductor-electrolyte solution interface and (C) an interface of two immiscible electrolyte solutions (ITIES) in the absence of specific adsorption. The region between the electrode and the outer Helmholtz plane (OHP, at the distance jc2 from the electrode) contains a layer of oriented solvent molecules while in the Verwey and Niessen model of ITIES (C) this layer is absent...

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. 

Recall that the situation at the interface between a metal and an electrolyte solution is much more favorable By using a large concentration of supporting electrolyte, we can ensure that the potential at the reaction site differs little from the potential in the bulk of the solution. This does not help at ITIES because for high ionic concentrations the... 

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. 

The properties of ITIES are in many respects analogous to those of the metal electrode/electrolyte solution interface [8]. This analogy is especially pronounced when electrolyte Ji Xi, dissolved in one phase (an organic solvent virtually immiscible with water), is strongly hydrophobic, whereas electrolyte J2X2, dissolved in the other phase (water), is strongly hydrophilic. This situation is characterized by the following inequalities for the distribution coefficients of the substances ... 

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). 

The study of the ion transfer through artificial liquid membrane systems is important for the elucidation of the ion transfer through biological membranes. In this respect, the Interface between two inmiscible electrolyte solutions (ITIES) constitutes a biomimetic medium suitable for studying several fundamental processes, ranging from biocatalysis to cellular respiration of photosynthesis, and many others [18-22], The first studies of liquid/liquid interfaces (L/L) under the application of an external potential were carried out by Gavach et al. [23], laying the basis for the current electrochemical treatments of ITIES. 

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]. 

Verwey-Niessen model — Earliest theoretical model of the - interface between two immiscible electrolyte solutions (ITIES) assuming the existence of a diffuse double layer with one phase containing an excess of the positive space charge and the other phase an equal excess of the negative space charge [i] (Figure). The difference of - inner electric potentials, Afcj> = (f>w - [Pg.692]

The general thermodynamic approach yields the - Gibbs-Lippmann equation (- electrocapillary) for the nonpolarizable [v] and ideally polarizable [ix] ITIES. For the interface between the electrolyte solutions of RX in w and SY in o, see also - interface between two immiscible electrolyte solutions, this equation has the form [x]... 

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