Influence of surface conductivity on the apparent zeta potential of homoionic montmorillonite particles
Abstract
Zeta potential is a physicochemical parameter of particular importance in describing ion adsorption
and double layer interactions between charged particles [1]. However, for clay particles, the
conversion of electrophoretic mobility measurements into zeta potentials is difficult. This is due to
their lamellar form, their anisotropic surface charge density distribution, but above all to their very
high 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 can
significantly lower the electrophoretic mobility of the particles. It follows that the magnitude of the
intrinsic zeta potential can be grossly underestimated if surface conductivity is not considered in the
calculation of the zeta potential, in particularly when the aqueous solution is diluted (ionic strength
typically < 0.1 M; [3]).
We use a basic Stern model to describe the electrochemical properties and to calculate the intrinsic
zeta potential of the basal planes of homoionic montmorillonites particles immersed in respectively
NaCl, CaCl2 and MgCl2 aqueous solutions (10-5 to 1 M) (Fig. 1). Only the equilibrium constant of
adsorption of Na+ ions on the basal plane of montmorillonite is adjusted by cation exchange capacity
and 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 complexation
model with Henry's electrophoretic mobility model that considers (1) the retardation force associated
with surface conductivity of the Stern and diffuse layers and (2) the internal conductivity of the clay
aggregate. Our electrophoretic mobility model is also not restricted to low zeta potentials because the
electrical potential distribution at the surface of the particle is calculated by numerically solving the
non-linear Poisson-Boltzmann equation. The very good agreement of calculated and measured
electrophoretic mobilities confirms that the true zeta potential of the basal plane of montmorillonite
particles may correspond to the electrical potential at the onset of the diffuse layer, i.e., at the outer
Helmholtz plane (Fig. 2).