THE ELECTRICAL DOUBLE-LAYER OF A DISK-SHAPED CLAY MINERAL PARTICLE - EFFECTS OF ELECTROLYTE PROPERTIES AND SURFACE-CHARGE DENSITY

The purpose of present study was to investigate, using a disk model, t he effects of charge density and 1-1 electrolyte properties on the dif fuse portion of the electrical double layer near a 2:1 clay mineral pa rticle, Numerical solutions for the inner potential and anion exclusio n volume were obtained by solving a Poisson-Boltzmann equation with va rying parameter values for electrolyte ion radius, electrolyte concent ration, basal plane charge density, and edge surface charge density. A lthough the negative electrostatic potential of the basal planes may s pill over to dominate a positively charged edge surface at low electro lyte concentrations because of the typically large particle radius-to- thickness ratio, the simulation results suggested that the spillover e ffect diminishes with an increase in 1-1 electrolyte concentration, Th e electric potential surrounding the edge surface is more positive for large (0.4 nm) hardsphere electrolyte ions than for small (0.2 nm) or point-charge ions, Reducing the basal plane charge density does not s ignificantly affect the anion exclusion volume over a wide range of el ectrolyte concentration, A highly positive edge surface, however, may spill over to dominate the basal plane and thereby adsorb significant amounts of anions at low electrolyte concentrations. The disk model pr edictions of anion adsorption or exclusion behavior for 2:1 clay miner al particles were consistent with available experimental observations. (C) 1996 Academic Press, Inc.