Surface treatment and bulk density of ions in nematic liquid crystals

The frequency dependence of the electrical impedance of a planar nematic sample in the shape of a slab is investigated. The measurements are performed by means of an external voltage of small amplitude (a few tens of millivolts) in such a manner that the liquid crystal behaves as a linear system from the electrical point of view. In this framework, the nematic orientation induced by the external field is absent because the applied voltage is smaller than the critical one for the transition of Freedericksz for the considered geometry. Our measurements indicate that the real part of the impedance presents a large plateau, finishing at the frequency of Debye. Whereas the imaginary part diverges in the low frequency region, indicating that the electrodes of the cell behave as blocking electrodes. From the value of the plateau of the resistance of the cell and from the frequency of Debye, we can conclude that the surface treatment to orient the nematic liquid crystal has a strong influence on the bulk density of ions. The experimental data relevant to the real and imaginary part of the impedance of the cell have been analyzed with a model based on the equations of continuity for the positive and negative ions and on the equation of Poisson for the actual electrical potential across the sample. By assuming that there is only a type of positive and negative ions and that the electrodes are perfectly blocking, except adsorb ions, we show that the agreement between theoretical predictions and experimental data is good.