Ionic transport in nematic liquid crystals: A study of viscous and dielectric frictions

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Abstract
The ionic mobilities of different species have been determined in two nematic solvents, in order to specify the relative importance of the dielectric friction and of the viscous friction in the mechanisms limiting the mobility in a liquid. The determination of the mobilities of a series of ions of different size shows the Stokes’ law can account for the observed mobility changes with the ionic radius. The study of the temperature dependence of the drift mobility in both nematic and isotropic phases shows that no discontinuity is observed at the nematic to isotropic transition, and the activation energy of mobility is found to be equal to the activation energy of viscosity. All experimental results give evidence that the dielectric friction mechanism, as described by the Fuoss–Boyd–Zwanzig model, is not the predominant factor affecting the ionic mobilities. It is concluded that the viscous friction does play the leading part in the mechanisms limiting the mobility, and that the Stokes–Einstein model can be considered as a first satisfactory approach for the description of ionic mobility in the nematic solvents.