Dipole moment, liquid electrolytes

Work on solid-state DSCs has clearly shown a >100 mV Ti02 conduction band shift between a heteroleptic ruthenium dye and an organic dye, which was interpreted in terms of a dipole-induced Ti02 CB shift of different sign [50], Such shifts are generally more difficult to observe in DSCs based on a liquid electrolyte [31], in which the high ion strength and the effect of thermal motion may hinder the role of interface dipoles. Nevertheless, Kusama et al. reported a combined experimental and theoretical study which showed a clear correlation between the dipole moment of electrolyte additives and their DSC yoc-[48]. 

It is obvious that a non-polar molecule cannot penetrate into the crystals of an electrolyte, nor dissolve it. This explains why, for example, NaCl is insoluble in benzene, CC14, CS2, etc., which are non-polar. On the other hand, nitrobenzene is not a solvent for salts either, yet its dipole moment of 4 X 10-18 is larger than that of water. It must, however, be remembered that the molecule C6H6N02 is also much larger than that of water, and consequently the larger dipole cannot approach as closely to the charges of the ions. A liquid, to produce easy dissociation, must have small molecules with large dipole moments. 

Fig. 5.7 shows schematically the potential across a semiconductor-electrolyte interface. To understand it we have to take two additional effects into account. First, the liquid molecules usually show a preferred orientation at the surface. Their dipole moment causes a jump of the potential. Second, on a solid surface the electrons can occupy surface states. These extra electrons contribute to the potential. 

In this chapter, the properties of polar solvents are discussed, especially as they relate to the formation of electrolyte solutions. Polar solvents are arbitrarily defined here as those liquids with a relative permittivity greater than 15. Solvents with zero dipole moment and a relative permittivity close to unity are non-polar. These include benzene, carbon tetrachloride, and cyclohexane. Solvents with relative permittivities between 3 and 5 are weakly polar, and those with values between 5 and 15 are moderately polar. The latter systems are not considered in the discussion in this chapter. 

In general, because of the differences between the permittivities and conductivities of the dispersed phase and the medium, the induced dipole moment of the particle in an electrolyte solution consists of two contributions. One is due to the polarization (orientation) of the molecular dipoles of both phases, and the other is due to the process of accumulation of charges of different signs on opposite poles of the particle. Thus, at very high frequencies (in practice, several MHz) ionic motions in the electrolyte solution and in the double layer toward and around the particle are too rapid for charge accumulation to proceed. Hence, only orientation of dipoles in both the particle and the liquid medium can participate in the dipole. 

Many of the important chemical applications of ILs will occur at solid surfaces, including electrochemical processes at IL-electrode interfaces, lubrication of ILs, fabrication of IL solid electrolytes and IL solid catalysts, etc. When a solid interface is present, molecules near the interface are subject to diflferent interactions than in the bulk phase, and the free energy of a surface can often be reduced by local changes in molecular orientation, aggregation, density, or composition. Familiar examples include surface adsorption, wetting and the electrochemical double-layer structure, where dipole moments usually lie at the interface. The surfaces of ionic liquids at the solid surface show dramatic changes in local structure, which can be demonstrated using simulations and probed by a number of experimental techniques. Due to a wide variety of experimental, theoretical, and... 

For electrolyte solutions and polar liquids, the amoxmt of slip depends on the electrical properties of the liquid. The sedimentation experiments report that slip is only observed for polar liquids. Drainage force experiments report slip to increase with increase in the dipolar moment of the liquid when liquids are polar. This phenomenon is attributed to the super lattice structure in liquid due to the dipole-dipole interactions. 

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