ABSTRACT

In this chapter, the steady state distribution of ionic charges in a liquid, in the presence of surface adsorption, is determined in the framework of a general model. In the first part of the chapter, the effect of an electric field applied by means of blocking electrodes is considered. It is shown that the surface adsorption of ions dissolved in the liquid is responsible for an asymmetry in the electric field distribution. In the model the liquid is supposed dielectric, but containing impurities. These impurities, by means of a chemical reaction, can give rise to ions. The theory takes into account the activation energy for the ionization chemical reaction and the adsorption energy of the ions at the surface. The model is built for isotropic liquids, but the results are used to analyze the behavior of liquid crystalline systems. A set of four non-linear equations governing the charge and field distribution in a typical sample is deduced. These equations come from the condition of conservation of the number of particles, from first integrals of the Poisson-Boltzmann equation and from the appropriate boundary conditions for the electric field at the surfaces limiting the sample. The analysis of this set of equations permits us to establish the existence of the regimes of low and high external voltage, according to the importance of the adsorbed (internal) charges as compared with the charges supplied by an external power supply. In the remaining part of the chapter, the influence of the adsorption energy on the anchoring properties of an NLC sample is investigated for significant values of the material parameters characterizing the phase and by using the electric field distribution calculated in the framework of the model developed in the first part of the chapter. Likewise, the destabilizing effect of the surface electric field is reanalyzed, thus establishing the dependence of the threshold fields on the adsorption energy as well as on the thickness of the sample on more general grounds.