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 ofa slab is investigated. The measurements are performed by means of an external voltage of smallamplitude a few tens of millivolts in such a manner that the liquid crystal behaves as a linearsystem from the electrical point of view. In this framework, the nematic orientation induced by theexternal field is absent because the applied voltage is smaller than the critical one for the transitionof Freedericksz for the considered geometry. Our measurements indicate that the real part of theimpedance presents a large plateau, finishing at the frequency of Debye. Whereas the imaginary partdiverges in the low frequency region, indicating that the electrodes of the cell behave as blockingelectrodes. From the value of the plateau of the resistance of the cell and from the frequency ofDebye, we can conclude that the surface treatment to orient the nematic liquid crystal has a stronginfluence on the bulk density of ions. The experimental data relevant to the real and imaginary partof the impedance of the cell have been analyzed with a model based on the equations of continuityfor the positive and negative ions and on the equation of Poisson for the actual electrical potentialacross the sample. By assuming that there is only a type of positive and negative ions and that theelectrodes are perfectly blocking, except adsorb ions, we show that the agreement betweentheoretical predictions and experimental data is good.