J. P. M. van der Ploeg

Electrode effects in dielectric spectroscopy of colloidal suspensions

We present a simple model to account for electrode polarization in colloidal suspensions. Apart from correctly predicting the ω−32 dependence for the dielectric permittivity at low frequencies ω, the model provides an explicit dependence of the effect on electrode spacing. The predictions are tested for the sodium bis(2-ethylhexyl) sulfosuccinate (AOT) water-in-oil microemulsion with iso-octane as continuous phase. In particular, the dependence of electrode polarization effects on electrode spacing has been measured and is found to be in accordance with the model prediction. Methods to reduce or account for electrode polarization are briefly discussed.

Theory of electrode polarization: application to parallel plate cell dielectric spectroscopy experiments

An extension of the model for electrode polarization of Cirkel et al. [Physica A 235 (1997) 269] is given. The problem is solved using both classical boundary conditions and the new boundary conditions using excess densities presented in a previous paper [J. Phys. Chem. B 105 (2001) 11743]. In the present paper, the electrodes are supposed to be ideal, meaning that charge transfer or adsorption are not considered. The advantage of the new boundary conditions lies in the possibility to extend to more complicated situations including for instance specific ion adsorption. We prove that the new boundary conditions and classical ones give the same results. A comparison of the model predictions, involving no adjustable parameters, experimental dielectric spectroscopy data is performed and fairly good agreement is found.

A low frequency relative permittivity meter for electrically conducting liquids

A technique is described for measuring relative permittivity of conducting liquids (of phase angles 10-6 to 0.3 rad) in the frequency domain below 50 kHz. The low frequency limit of the method, extending down from 20 Hz, is lower, the smaller the conductivity of the solutions, and is determined by the values of the phase angles to be measured and the magnitude of electrode and other parasitic effects that cannot be controlled effectively at very low frequencies. the technique is based on the four-terminal cell principle, but differs from other existing methods in two ways: the amplified potential between the two probes and the voltage converted current through the cell are clipped at practically equal heights before being subtracted and fed into a phase sensitive detector; measurement of the unknown phase angle of the solution investigated is made relative to the known phase angle of a reference solution of the same conductivity.

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.

Chemically induced dynamic nuclear polarization. Decarboxylation of photochemically generated benzoyloxy radicals

Abstract CIDNP has been studied during thermal decomposition, photolysis, and sensitized photolysis of benzoyl chloroacetyl peroxide. The ratio of the CIDNP intensities for the recombination products benzyl chloride and chloromethyl benzoate is dependent on the mode of decomposition, reflecting the extent of rapid decarboxylation of the primary formed benzoyloxy radicals.

CIDNP during photoreactions of formaldehyde in solution

Abstract CIDNP has been studied during photolysis of formaldehyde in solution. The primary photochemical process is the hydrogen abstraction by predominant triplet excited formaldehyde from ground state formaldehyde. In the presence of bromotrichloromethane a singlet reaction occurs.