What Constitutes a Polar Liquid

Polar solvents are those liquids whose relative permittivity is sufficiently high that electrolytes can be dissolved in them. The best-known example of such a liquid is water. The oxygen end of this simple molecule is electron-rich and can stabilize cations. The hydrogen atoms are electron-poor and thus are involved in the solvation of anions. The structure of pure water is very much influenced by the 

There are other polar solvents which are not protic. These involve liquids with large dipole moments. Some examples are acetonitrile, propylene carbonate, and dimethylsulfoxide. In each case, the solvent molecule possesses an electronegative group which is rich in electrons. The opposite end of the molecule is electron deficient but does not have acidic hydrogen atoms which can participate in hydrogen bonding. This class of solvents is called aprotic. 

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. 

Initially, some relevant thermodynamic and molecular properties of polar solvents are considered. Then, their dielectric properties are considered in detail. Ion solvation in these solvents is also discussed with emphasis on some non-thermo-dynamic methods of dividing experimentally measured data for electrolytes into contributions for the cation and anion. Finally, the important characteristics of the solvent in its direct interaction with the solute, namely, its acidity and basicity, are also described. 

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