The Solvent Effect on Mobility at Infinite Dilution

At infinite dilution, neither relaxation nor electrophoretic effects are operative on the drift of ions both these effects depend for their existence on a finite-sized ionic cloud. Under these special conditions, the infinite-dilution mobility can be considered to be given by the Stokes mobility 

Considering the same ionic species in several solvents, one has 

an increase in the viscosity of the medium leads to a decrease in the infinite-dilution mobility and vice versa. 

In fact, the radius of the kinetic entity may change in going from one solvent to another because of changes in the stracture of the solvation sheath. Sometimes these solvation effects on the radius may be as much as 100%. Hence, it is only to a rough degree that one can use the approximate equation (4.340). 

In some cases, the changes in the radii of the solvated ions are mainly due to the changes in the sizes of the solvent molecules in the solvation sheath. Thus, in the case of water, methanol, and ethanol, the size of the three solvent molecules increases in the order 

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