CHARACTERIZATION OF THE ELECTRICAL DOUBLE-LAYER OF MONTMORILLONITE

The primary objective of this paper was to determine the relative magnitudes of Ψ0, Ψδ, and ζ, the electrostatic potentials in the plane of the clay-water interface, the outer Helmholtz plane, and the plane of shear, respectively. Another objective was to determine the relative magnitudes of σ0 and σδ, the charge densities in the first two planes, respectively. Four different methods were used to achieve these objectives. All of them gave the same results. It was found that ζ was much smaller than ζ0 but was equal to ζ; also, as was much smaller than a0. Moreover, it was found that ζ was independent of electrolyte concentration, pH, and a0 but dependent on the nature of the exchangeable cations. On the other hand, a6 varied linearly with the square root of the electrolyte concentration. Nevertheless, it remained much smaller than σ0. These results were interpreted to mean that most of the exchangeable cations are condensed on the montmorillonite surface in a Stern layer and that adjustments in the ionic occupancy and/or thickness of this layer maintain Ψδ at a constant, critical value that cannot be exceeded. © 1990, American Chemical Society. All rights reserved.