Influence of surface conductivity on the apparent zeta potential of homoionic montmorillonite particles

International audienceZeta potential is a physicochemical parameter of particular importance in describing ion adsorptionand double layer interactions between charged particles [1]. However, for clay particles, theconversion of electrophoretic mobility measurements into zeta potentials is difficult. This is due totheir lamellar form, their anisotropic surface charge density distribution, but above all to their veryhigh surface electrical conductivity, which is inversely proportional to the sizes of the particles [2].When surface conductivity is similar to or higher than the electrical conductivity of bulk water, it cansignificantly lower the electrophoretic mobility of the particles. It follows that the magnitude of theintrinsic zeta potential can be grossly underestimated if surface conductivity is not considered in thecalculation of the zeta potential, in particularly when the aqueous solution is diluted (ionic strengthtypically < 0.1 M; [3]).We use a basic Stern model to describe the electrochemical properties and to calculate the intrinsiczeta potential of the basal planes of homoionic montmorillonites particles immersed in respectivelyNaCl, CaCl2 and MgCl2 aqueous solutions (10-5 to 1 M) (Fig. 1). Only the equilibrium constant ofadsorption of Na+ ions on the basal plane of montmorillonite is adjusted by cation exchange capacityand electrophoretic mobility measurements [4] at fixed pH (pH = 6.5) and high salinity (1 M).Electrophoretic mobilities are then calculated by coupling our electrostatic surface complexationmodel with Henry's electrophoretic mobility model that considers (1) the retardation force associatedwith surface conductivity of the Stern and diffuse layers and (2) the internal conductivity of the clayaggregate. Our electrophoretic mobility model is also not restricted to low zeta potentials because theelectrical potential distribution at the surface of the particle is calculated by numerically solving thenon-linear Poisson-Boltzmann equation. The very good agreement of calculated and measuredelectrophoretic mobilities confirms that the true zeta potential of the basal plane of montmorilloniteparticles may correspond to the electrical potential at the onset of the diffuse layer, i.e., at the outerHelmholtz plane (Fig. 2)