No abstract

We discuss the influence of the ions dissolved in a liquid on the impedance spectroscopy of a cell. Our analysis is performed in the small-voltage regime, where the actual bulk density of ions is only slightly perturbed by the external electric field. In this framework, we show that the presence of the ions can be taken into account by a surface density of charge. The agreement between the theoretical prediction, on the basis of the assumption that the ionic mobility is frequency independent, and the experimental data for the real and imaginary parts of the impedance is fairly good for frequencies larger than 100 Hz. In the low-frequency range, the agreement of the theory with the experiment is rather poor. In this region, the experimental data can be successfully fitted by introducing the impedance of the metal-electrolyte interface, which is accurately represented by Zi = w(i omega)(-nu), where w and nu are two constants, with 0 < nu < 1. From the analysis of the experimental data, we determine w and nu. The theoretical predictions of our model are in good agreement with the experimental data in the investigated frequency range.

The frequency dependence of the real and imaginary parts of the electrical impedance of an electrolytic cell in the low frequency range is experimentally investigated. We show that a possible candidate able to explain the anomalous increasing of the real part of the electrical impedance of the cell is the adsorption-desorption phenomenon at the bounding surfaces. Measurements are performed on cells of water solution containing KCl, of different thickness and concentration of KCl, limited by the same type of gold electrodes. The experimental data can be fitted with the same adsorption and desorption coefficients, appearing in the kinetic equations, in the Langmuir approximation. This result confirm that the electrical impedance of the interface introduced to interpret the observed increasing of the resistance of the cell, in the low frequency region, could be connected with the adsorption-desorption phenomenon.

The intensity dependence of optical nonlinearity in a nematic liquid crystal doped with an azo-dye is investigated. The reorientational part of the nonlinearity changes from self-defocusing to self-focusing character while the intensity passes through the saturation value of trans-cis photoisomerization. This observation, in accordance with previous models, indicates that the optical torque generated by the trans-isomers is of opposite sign than the torque generated by the cis-isomers. At very low intensities a further reorientational nonlinearity was found, which is attributed to light-induced orientational redistribution of adsorbed molecules at the surface.