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Poster

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).
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Poster
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https://hal-brgm.archives-ouvertes.fr/hal-01119979
Contributeur : Philippe Leroy <>
Soumis le : mardi 24 février 2015 - 15:05:30
Dernière modification le : mercredi 14 octobre 2020 - 03:43:19

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  • HAL Id : hal-01119979, version 1

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Philippe Leroy, Nicolas Devau, Christophe Tournassat, Olivier Bernard. Influence of surface conductivity on the apparent zeta potential of homoionic montmorillonite particles . Clay Conference : Clays in Natural and Engineered Barriers for Radioactive Waste, Mar 2015, Bruxelles, Belgium. ⟨hal-01119979⟩

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