Polarizability, liquid electrolytes

Heterogeneous ET reactions at polarizable liquid-liquid interfaces have been mainly approached from current potential relationships. In this respect, a rather important issue is to minimize the contribution of ion-transfer reactions to the current responses associated with the ET step. This requirement has been recognized by several authors [43,62,67-72]. Firstly, reactants and products should remain in their respective phases within the potential range where the ET process takes place. In addition to redox stability, the supporting electrolytes should also provide an appropriate potential window for the redox reaction. According to Eqs. (2) and (3), the redox potentials of the species involved in the ET should match in a way that the formal electron-transfer potential occurs within the potential window established by the transfer of the ionic species present at the liquid-liquid junction. The results shown in Figs. 1 and 2 provide an example of voltammetric ET responses when the above conditions are fulfilled. A difference of approximately 150 mV is observed between Ao et A" (.+. ... 

Most of the events in electrochemistry take place at an interface, and that is why interfacial electrochemistry constitutes the major part of electrochemical science. Relevant interfaces here are the metal-liquid electrolyte (LE), metal-solid electrolyte (SE), semiconductor-electrolyte, and the interface between two immiscible electrolyte solutions (ITIES). These interfaces are chargeable, that is, when the external potential is applied, charge separation of positive and negative charges on the two sides of the contact occurs. Such an interface can accumulate energy and be characterized by electric capacitance, within the range of ideal polarizability beyond which Faraday processes turn on. 

The value of the induced potential depends on not only the externally applied electric field but also the electric properties of both the solid surface and the liquid electrolyte and geometry of the solid surface. To date, more attention has been given to the induced potential on ideally polarizable surfaces. To obtain the induced potential of an ideally polarizable sphere, two assumptions are applied after the polarization, the electric field lines near the conducting surface are distorted and go around the conducting sphere (expressed as ,. = 0 as shown in Fig. 3) and the potential at the ideally polarized surface is identical (expressed as = 0 at r < a). Utilizing the assumptions above, Dykhin and Bazant solved the Laplace s equation [2, 3] and got the analytical solution of the induced potential on a polarized cylinder and sphere. The analytical solutions to the Laplace s equation are only limited to cylinder and sphere geometries, because the boundary conditions for the Laplace s equation are much easier in these cases. The obtained induced potential on an ideally polarizable sphere is... 

An atomistic simulation MD simulation of a common carbonate-based organic electrolyte, ethylene carbonate dimethyl carbonate (EC DMC = 3 7) with approximately 1 mol/kg LiPFs, referred to as the organic liquid electrolyte or OLE, and an ionic liquid-based electrolyte (ILE), 1-ethyl 3-methyl-imidazolium bis (fluorosulfonyl)imide (EMIM iFSE) with 1 mol/kg LiFSI, in contact with LiFeP04 has been carried out [107]. Simulations were carried out using quantum chemistry-based polarizable force at 363 K on a 3-D periodic orthorhombic... 

Zaitsev, N., Gorehk, O., Kotov, N. et al. (1988) A photoelectrochemical effect at the polarizable interface between liquid electrolyte-solutions in protoporphyrin quinone systems. Soviet Electrochemistry, 24 (10), 1243-1247. 

It has been demonstrated by Koryta (25, 26) that for such cases, there exists a range of potential differences (potential window) in which is controlled by the charge in the double layer rather than by the ion activities (see Figure 17.3.5A). This situation is completely analogous to that of an ideal-polarizable metal/electrolyte interface. We call this case the ideal-polarizable liquid/liquid interface. One of such example is the following... 

Recent progress and main problems of the study of electrochemical equilibrium properties are reviewed for interfaces between two immiscible liquid electrolyte solutions. The discussed properties are mainly described in terms of the Galvani, Volta, zero charge, and surface (dipolar) potentials at the liquid-liquid interfaces and free liquid surfaces. Different galvanic and voltaic cells with liquid-liquid, mainly water-nitrobenzene interfaces, are described. These interfaces may be polarizable or reversible with respect to one or several ions simultaneously. 

Induced-charge electro-osmosis refers to nonlinear electro-osmotic flow of a liquid electrolyte, when an electric held acts on its own induced diffuse charge near a polarizable surface. 

Metal/molten salt interfaces have been studied mainly by electrocapillary833-838 and differential capacitance839-841 methods. Sometimes the estance method has been used.842 Electrocapillary and impedance measurements in molten salts are complicated by nonideal polarizability of metals, as well as wetting of the glass capillary by liquid metals. The capacitance data for liquid and solid electrodes in contact with molten salt show a well-defined minimum in C,E curves and usually have a symmetrical parabolic form.8 10,839-841 Sometimes inflections or steps associated with adsorption processes arise, whose nature, however, is unclear.8,10 A minimum in the C,E curve lies at potentials close to the electrocapillary maximum, but some difference is observed, which is associated with errors in comparing reference electrode (usually Pb/2.5% PbCl2 + LiCl + KC1)840 potential values used in different studies.8,10 It should be noted that any comparison of experimental data in aqueous electrolytes and in molten salts is somewhat questionable. 

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. 

The above conclusions have resulted from an analysis of computer simulation data carried out on pure liquids and supercritical fluids, and on liquids in equilibrium with their vapor. One immediate question one should ask concerns thus a more general validity of the reached conclusions. Particularly important problem is to what extent they may remain valid for mixtures. Due to polarizability and other possible effects brought about by electrostatic interactions between unlike species, the pair interaction, and hence the local and, particularly, orientational arrangement may be changed considerably. With respect to a wide variety of mixtures this problem will require rather an extensive investigation. The most difficult mixtures will evidently be solutions of charged objects as e.g. electrolytes. 

The liquid liquid interface is ideally polarizable when the electrolyte in phase ot is insoluble in p, and that in p, insoluble in a. Such a system may be designated as... 

The investigation of the effect made by the applied potential difference on the interfacial tension can be most conveniently carried out on the ideally polarizable surface of liquid metal (most commonly mercury) in aqueous electrolyte solution. It is important that in these experiments one be able to simultaneously measure the potential difference between phases (with respect to some standard electrolyte) and the interfacial tension. The latter is usually... 

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. 

In the case where the ionic species in the aqueous electrolyte are fairly hydrophilic and the organic phase features hydrophobic ions, the liquid]liquid junction behaves similarly to an ideally polarizable metal electrode. Under this condition, the Galvani potential difference can be effectively controlled by a four-electrode potentiostat [4,5]. A schematic representation of a typical electrochemical cell is shown in Fig. 1 [6]. Cyclic voltammo-grams illustrating the potential window for the water] 1,2-dichloroethane (DCE) interface for various electrolytes are also shown in Fig. 1. In the presence of bis(triphenylpho-sphoranylidene)ammonium hexafluorophosphate (BTPPA PFe) the supporting electrolyte in DCE, the potential window is limited to less than 200 mV due to the hydrophilicity of the anion. Wider polarizable potential ranges are obtained on replacing... 

In practice, the ratio of active materials of both the electrodes was often chosen such that the capacitor should operate in the mode, in which capacitance was determined by the polarizable carbon electrode. Owing to the relative smallness of the volume of electrolyte in HSC, a change in its volume and concentration occurs in the course of charging and discharge. It was found that a 5-20% decrease in the liquid phase volume occurs in the course of charging, and a 15-50% decrease in the concentration of electrolyte is observed in the course of discharge. Herewith, an increase... 

When the particle polarizability is much less than the electrolyte, the net dipole points in the opposite direction (Fig. 2b). This case is an insulating sphere suspended in a liquid with a high dielectric constant or a high conductivity. 

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