Four-contact impedance spectroscopy of conductive liquid samples

Compensating for Electrode Polarization in Dielectric Spectroscopy Studies of Colloidal Suspensions: Theoretical Assessment of Existing Methods

Dielectric spectroscopy can be used to determine the dipole moment of colloidal particles from which important interfacial electrokinetic properties, for instance their zeta potential, can be deduced. Unfortunately, dielectric spectroscopy measurements are hampered by electrode polarization (EP). In this article, we review several procedures to compensate for this effect. First EP in electrolyte solutions is described: the complex conductivity is derived as function of frequency, for two cell geometries (planar and cylindrical) with blocking electrodes. The corresponding equivalent circuit for the electrolyte solution is given for each geometry. This equivalent circuit model is extended to suspensions. The complex conductivity of a suspension, in the presence of EP, is then calculated from the impedance. Different methods for compensating for EP are critically assessed, with the help of the theoretical findings. Their limit of validity is given in terms of characteristic frequencies. We can identify with one of these frequencies the frequency range within which data uncorrected for EP may be used to assess the dipole moment of colloidal particles. In order to extract this dipole moment from the measured data, two methods are reviewed: one is based on the use of existing models for the complex conductivity of suspensions, the other is the logarithmic derivative method. An extension to multiple relaxations of the logarithmic derivative method is proposed.

Role of microscopic phase separation in gelation of aqueous gelatin solutions

Using a unique home-made cell for four-contact impedance spectroscopy of conductive liquid samples, we establish the existence of two low frequency conductivity relaxations in aqueous solutions of gelatin, in both liquid and gel states. A comparison with diffusion measurements using pulsed field gradient NMR, and circular dichroism spectroscopy, shows that the faster relaxation process is due to gelatin macromolecule self-diffusion. This single molecule diffusion is mostly insensitive to the macroscopic state of the sample, implying that we have a clear separation of gelatin molecules into a free and network-bound phase. Scaling relationships for the self-diffusion indicate that the gelation process is not a percolative phenomenon, but is caused by aggregation of triple helices into a system-spanning fibre network.

Four-electrode impedance spectrometer for investigation of solid ion conductors

An impedance spectrometer capable of accurately measuring solid ion conducting sample impedance spectra by two- or four-electrode methods in either time or frequency domain has been built. The four-electrode measurement mode is implemented by constructing a differential amplifier with a very high input impedance and common mode rejection ratio over a wide frequency range. All of the measurements can be performed in frequencies ranging from 10 Hz to 2 MHz and sample temperatures up to 800 K. The working principle of the spectrometer as well as its technical parameters and accuracy estimation are presented in this paper. The advantage of four-electrode over two-electrode measurement mode is shown by an example of Ce(0.9)Gd(0.1)O(1.95) solid electrolyte ceramic impedance measurements.