Characterization of Charge on Clays by Electrophoresis and Diffusiophoresis, 1991 Coating Conference Proceedings

Single clay particles, such as kaolinite, are thought to be plate-like and heterogeneous in their surface charge, with a negative charge on the faces that is relatively insensitive to pH and a pH-dependent charge on the edges. Aqueous suspension flow properties and dry film structure of kaolinite clay coatings are dependent on the magnitudes of these surface charge distributions. To date conventional electrokinetic experiments provide only a rough estimate of the average charge, or zeta potential, of the particles, whereas a more quantitative description of the charge distribution is needed to allow a better understanding of the suspension and coating properties of clays. In this paper we present models for two electrokinetic experiments which, if performed on the same clay suspension, could yield direct values of the zeta potentials of the faces and edges of single plates. The models are based on recently developed theories for the electrokinetic transport of colloidal particles having distributions of zeta potential on their surfaces. Using sample calculations for oblate spheroids that are geometrically similar to clay plates, we demonstrate that measured values of particle transport rates by electrophoresis (electric fields) and diffusiophoresis (electrolyte concentration gradients) would provide data from which the edge and face zeta potentials could be computed. In addition to helping with charge characterization in the laboratory, diffusiophoresis may be an important determinant of structure during coating drying, when electrolyte concentration increases as water evaporates. Calculations show that “neutral” particles (i.e., zero area-averaged zeta potential) can have significant electrophoretic mobilities, and a zero lectrophoretic mobility can be achieved with a strongly charged particle.