The Electrical Double Layer of a Disk-Shaped Clay Mineral Particle: Effects of Electrolyte Properties and Surface Charge Density

Abstract

The purpose of present study was to investigate, using a disk model, the effects of charge density and 1–1 electrolyte properties on the diffuse portion of the electrical double layer near a 2:1 clay mineral particle. Numerical solutions for the inner potential and anion exclusion volume were obtained by solving a Poisson–Boltzmann equation with varying parameter values for electrolyte ion radius, electrolyte concentration, basal plane charge density, and edge surface charge density. Although the negative electrostatic potential of the basal planes may spill over to dominate a positively charged edge surface at low electrolyte concentrations because of the typically large particle radius-to-thickness ratio, the simulation results suggested that the spillover effect diminishes with an increase in 1–1 electrolyte concentration. The electric potential surrounding the edge surface is more positive for large (0.4 nm) hard-sphere electrolyte ions than for small (0.2 nm) or point-charge ions. Reducing the basal plane charge density does not significantly affect the anion exclusion volume over a wide range of electrolyte 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 predictions of anion adsorption or exclusion behavior for 2:1 clay mineral particles were consistent with available experimental observations.