Low Field Conduction of Nematic Liquid Crystals Studied by Means of Electrodialysis

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Abstract
An elementary theory of electrodialytic purification in insulating liquid crystals is developed by considering both the degree of ionic dissociation and the kinetic rates of dissolved impurities. It is shown that, on application of a uniform electric field with open electrodes, the only species which exhibit strong dissociation, or fast kinetic rates, can be eliminated from the solution; the molecular concentrations of other species remain unaffected by every ionic depletion process. Distinction is made between an irreversible molecular purification, which takes place when electrolytic impurities are definitively removed, and a reversible displacement of the ionic equilibrium by electrodialysis. The ionic concentrations of species having slow kinetic rates are lowered by the electric field, but they grow back to their initial values when the voltage is switched off. These conclusions lead to propose a model for the interpretation of transient and stationary electrical characteristics of several nematic compounds, when no injection phenomena occur. The temperature dependance of the residual conductivity is studied for several nematic liquid crystals. An exponential-like behaviour is observed in both isotropic and nematic phase and a singularity appears near the transition. The conductivity variations are discussed in terms of number and drift mobility of ionic charge carriers and liquid crystal viscosity.

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