Liquid nonaqueous electrolytes between ions

Electrolytes are ubiquitous and indispensable in all electrochemical devices, and their basic function is independent of the much diversified chemistries and applications of these devices. In this sense, the role of electrolytes in electrolytic cells, capacitors, fuel cells, or batteries would remain the same to serve as the medium for the transfer of charges, which are in the form of ions, between a pair of electrodes. The vast majority of the electrolytes are electrolytic solution-types that consist of salts (also called electrolyte solutes ) dissolved in solvents, either water (aqueous) or organic molecules (nonaqueous), and are in a liquid state in the service-temperature range. [Although nonaqueous has been used overwhelmingly in the literature, aprotic would be a more precise term. Either anhydrous ammonia or ethanol qualifies as a nonaqueous solvent but is unstable with lithium because of the active protons. Nevertheless, this review will conform to the convention and use nonaqueous in place of aprotic .]... 

For most potentiometric measurements either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few millivolts. The discussion in Chapter 5 outlines their characteristics, preparation, and temperature coefficients. The silver/silver chloride electrode also finds application in nonaqueous titrations, although some solvents cause the silver chloride film to become soluble. Some have utilized reference electrodes in nonaqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for nonaqueous systems as are any of the prototypes that have been developed to date. When there is a need to rigorously exclude water, double-salt bridges (aqueous/nonaqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the nonaqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause erratic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

We have discussed the self-assembly of nonionic surfactants that occurs in RTILs. Overall, the self-assembly properties in RTILs are largely similar to the aqueous medium. Notable differences between the aqueous and nonaqueous systems are sometimes seen when nonionic surfactants form micelles or lyotropic liquid crystals at certain compositions and temperatures, and this mainly results from the different affinity of the nonionic surfactants with the liquids. In other words, it may be possible to expect the formation of micelles or lyotropic liquid crystals to a certain degree by considering the solvophobic or solvophilic nature of the nonionic surfactants in the RTILs. An interesting feature of RTILs is their self-assembly in bulk liquids and at interfaces. This feature also makes a significant impact on the self-assembly of nonionic surfactants in RTILs. Particularly, we have demonstrated the importance of this feature when nonionic surfactants adsorb at solid/RTIL interfaces. We believe that the self-assembled structures of amphiphilic molecules with RTILs are of great interest not only from academic but also from industrial standpoints. One of the potential applications based on such self-assembled structures should be high-performance ion-conductive electrolytes as a new device system with nanolevel order [50]. 

At low concentrations of electrolyte, the force between surfaces is found to obey the Derjaguln-Landau-Vervey-Overbeck (DLVO) theory (see Chapter 4) until the separation is 1.5nm then the force oscillates, as shown in Fig. 3, as Individual layers of molecules are displaced. The same behavior is observed in a wide range of nonaqueous liquids. In water containing high concentrations of electrolytes, ions adsorbed on the solid surface develop a layer of hydration that creates an additional component of repulsion that decays exponentially over the range of 1.5 to 4 nm (decay length 0.6-l.l nm). This monotonic component results from hydration forces, not ordering of molecules, and is not observed in liquids other than water [12]. 

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