Inorganic particles as colloidal models. Effects of size and shape on the electrokinetics of hematite (alpha-Fe2O3)

Abstract

This paper deals with the electrokinetics of alpha-Fe2O3 (hematite) particles with the aim of checking their possible use as colloidal models. The key idea is that, since hematite can be prepared, in a controlled and reproducible way, as colloidal particles homogeneous in size and shape, these could be considered as model colloids to which theories developed for systems of given geometry might be applied. In this contribution, we first describe the electrophoretic mobility, mu(e), of pseudospherical hematite samples with very different diameters as a function of NaCl and CaCl2 concentrations. Their electrokinetic behavior is compared with the predictions of a rigorous theory for the mobility of spheres, assuming that both types of particles have equal zeta potentials (zeta); apparently, smaller particles show a considerable effect of anomalous (Stern-layer) ionic conduction, less appreciable for particles larger in diameter. However, in the electrolyte concentration range (above 10(-3) M) in which this surface contribution is masked by the bulk solution conductivity, a satisfactory agreement is found between measured and predicted mobility values. The study is repeated with spheroidal particles: we have used hematite spheroids of two different average sizes and axial ratios. The effect of pH and Na(NO3) concentration on the electrophoretic mobility of both spheroidal samples is compared to the predictions of models of the mobility of particles with spheroidal geometry; these models are valid for either thin double layers and arbitrary zeta potentials or low zeta and arbitrary ionic layer thickness. The coincidence in the predictions of zeta for both particles in most experimental conditions (and, furthermore, the fact that the zeta potential turns out to be the same for spherical and nonspherical particles) points not only to the usefulness of the theories in dealing with spheroids, but also, equally important, to the possibilities of hematite with nonspherical, but controlled, geometries as colloidal models.