Broadband Dielectric Spectra of Spheroidal Hematite Particles

In this paper, we contribute new data on the dielectric dispersion of suspensions of two kinds of spheroidal hematite particles (R-Fe2O3). One of the samples consisted of nearly spherical particles, and the other included almost spindle-like particles. We intended to demonstrate the unique potential applications of dielectric dispersion measurements on the characterization of the electrical double layer of nonspherical particles. Thus, we show that while the electrophoretic velocity is practically the same for both systems, the dielectric spectra differ very significantly from each other. In addition to analyzing the role of axial ratios on the electrokinetic behavior, we have also focused on the effect of pH and of the frequency range in which experiments are performed. Thus, two relaxations can be identified in the suspensions: the so-called R-relaxation (typically in the kilohertz region), related to the polarization of the electrical double layer, and the Maxwell-WagnerO’Konski relaxation (of the order of megahertz), a consequence of conductivity and permittivity mismatch between the particle and the supporting solution. We show that the dielectric increment in the R-relaxation is larger when particles have larger aspect ratio and that there is a secondary relaxation process at both low and high frequencies, a manifestation of the anisotropy of the particles. To obtain more information about this secondary relaxation, a logarithmic derivative method is used to approximate the imaginary part of the dielectric constant. The results are interpreted in the light of existing models. A qualitative agreement is found between the main features of the models and the results, although dielectric data appear to require higher ú-potentials to be explained than electrophoresis does. We suggest that, while at low frequencies this could be a result of electrode polarization effects, data at high frequencies, less prone to be affected by such effects, confirm the existence of stagnant-layer conductivity in hematite particles.