Electrosurface properties of kaolin and bentonite particles in solutions of electrolytes and surfactants

Abstract

The influence of pH and KCl, CaCl₂, AlCl₃, sodium dodecyl sulfate, and cetyltrimethylammonium bromide (CTAB) concentrations on electrokinetic potential ζ of particles and the electrical conductivity of aqueous kaolin and bentonite suspensions has been studied. At “natural” pH values of the suspensions (7.2–7.4), the ζ potential of bentonite is much higher (–45 mV) than that of kaolin (–20 mV), and this difference takes place in a wide pH range (2–11). In both cases, a rise in pH leads to an increase in the absolute values of ζ due to the enhanced dissociation of surface OH groups. Charge reversal is observed at pH of approximately 2 and pH 2.5 for kaolin and bentonite, respectively. The dependence of the ζ potential of the particles on KCl concentration passes through a maximum, while the addition of CaCl₂ and AlCl₃ causes a substantial reduction in ζ and particle charge reversal, respectively. An increase in CTAB concentration C in a suspension leads to a decrease in the negative ζ values followed by particle charge reversal, with the ζ(C) curve tending to reach a plateau. It has been shown that the initial regions of the dependences of the relative electrical conductivities of the suspensions on the volume fraction of the solid phase are almost linear, with their slopes decreasing with a rise in electrolyte concentration. Hence, the contribution of the surface conductivity to the total suspension conductivity decreases with a rise in the ionic strength of a solution. The values of the electrokinetic charge calculated from the data on the ζ potential, i.e., the charge of the slipping plane, have appeared to be one to two orders of magnitude lower than the surface charges of the minerals. This finding suggests that counterions are predominantly located in the hydrodynamically motionless part of the electrical double layer.