A theory to determine the surface potentials of clay particles in electrolyte solutions

Abstract

Surface potential, an important parameter at the interface between water and charged particles (such as clays and clay minerals), has a significant effect on a number of chemical and physical properties of particles. The existing theories used to estimate surface potential assume that the double layer medium is a continuous dielectric and the dielectric properties depend on the strong electric field produced by the particle surface charges. In the present study, the analytical relationships between the electric field strength and surface potentials were established based on the nonlinear Poisson-Boltzmann equation to examine the dielectric saturation, and then a new method to determine the surface potential was proposed. Surface potential can be determined using the derived equations from the electric field strength at the particle surface, which was calculated from the surface charge density. For a particle with surface charge density of 0.2C m−2, the surface potentials in 0.001 mol L−1 1:1 and 2:1 electrolytes determined by Gouy-Chapman model were respectively −242 and − 121 mV while those determined by the present method were respectively-286 and -143 mV. The above analyses showed that, due to the influence of dielectric saturation, the surface potential was underestimated using the Gouy-Chapman model for a continuous dielectric medium. The aggregate stability experiments showed that the amount of released small particles for montmorillonite aggregates in different electrolytes was a function of surface electrical potentials calculated based on dielectric saturation. The surface potential was approximate −115 mV for a 3.7% of released <10 μm particles in both KCl and CaCl2 electrolytes when dielectric saturation was taken into account. The surface electrical potentials calculated based on the new method explained the differences between the aggregate stability of clays in different electrolyte solutions.