Transport Phenomena in Non-Aqueous Solutions

In the development of the theory of ionic conductance it has been shown that the viscosity of the solvent is an important parameter determining ionic mobility. Initially, conductivity data were only available in water so that attention was focused on the effects of ionic size, structure, and charge in determining mobility and its concentration dependence. More recently, data have become available in a wide variety of non-aqueous solvents [11, 12], that is, in media with a wide range of permittivities and viscosities. On the basis of these data one may examine in more detail the role of solvent viscosity in determining the transport properties of single ions. Values of the limiting ionic molar conductance for selected monovalent cations and anions are summarized in tables 6.4 and 6.5, respectively. 

By combining the Stokes-Einstein equation (equation (6.7.27)) with equation (6.7.23), the following expression for the limiting ionic molar conductance is obtained  

The product X,oTi is known as the Walden product and is the focus of the discussion in the present section. To a first approximation, A,oT1 should be independent of solvent nature if the ion moves in the solution without any accompanying solvent molecules. However, some variation in A,oT1 is seen on the basis of an analysis of the data in tables 6.4 and 6.5 using the viscosities reported in table 6.1[13]. 

The results for the Stokes radii of anions are more eomplex and are eonsidered separately for protic and aprotie solvents. In the ease of aprotie solvents, the value of fsT decreases in the aprotie solvents with inerease in solvent radius (fig. 6.12). Since the interaction between the anion and solvent is weak, the number of solvent molecules which move with the ion decreases with inerease in solvent size. This trend is weaker for the larger CIO4 ion than for the smaller CP ion, demonstrating that the CP anion is more strongly solvated in solvents of the smallest molecular size. In the case of protic solvents, hydrogen bonding is involved in anion solvation. Data are available in only a few solvents so that a detailed analysis is not possible at present. 

The results presented here demonstrate elearly that the solvent radius is the important parameter to be considered in diseussing the solvent dependence of the 

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