At pH รต p.z.c. (the point of zero charge, 5), the positively The purpose of present study was to investigate, using a disk charged edge surface of one clay mineral particle may intermodel, the effects of charge density and 1-1 electrolyte properties act attractively with the negatively charged basal planes of on the diffuse portion of the electrical double layer near a 2:1 clay other particles. Although the edge surface is only about 1% mineral particle. Numerical solutions for the inner potential and of the total surface area for a smectite particle with a typical anion exclusion volume were obtained by solving a Poisson-Boltzaspect ratio (particle radius/thickness) of 100, such an mann equation with varying parameter values for electrolyte ion ''edge-to-face interaction'' (6) could lower the critical coagradius, electrolyte concentration, basal plane charge density, and ulation concentration (ccc, 1) for particles to undergo flocedge surface charge density. Although the negative electrostatic culation. At pH รต p.z.c., anions may be simultaneously potential of the basal planes may spill over to dominate a positively charged edge surface at low electrolyte concentrations because of adsorbed by the positively charged edge surface and exthe typically large particle radius-to-thickness ratio, the simulation cluded by the negatively charged basal planes. Anion adsorpresults suggested that the spillover effect diminishes with an intion by the edge surface would compensate anions excluded crease in 1-1 electrolyte concentration. The electric potential surby the basal planes and could result in a net positive anion rounding the edge surface is more positive for large (0.4 nm) hardadsorption. Therefore, in addition to surface charge neutralsphere electrolyte ions than for small (0.2 nm) or point-charge ization (7, 8) and quasicrystal formation (9), the spatial ions. Reducing the basal plane charge density does not significantly heterogeneity of anion exclusion and adsorption should be affect the anion exclusion volume over a wide range of electrolyte taken into account in the mechanisms of anion reactions concentration. A highly positive edge surface, however, may spill with clay minerals. over to dominate the basal plane and thereby adsorb significant Because of electrostatic field overlap (2), the basal planes amounts of anions at low electrolyte concentrations. The disk and edge surface do not interact independently with ionic model predictions of anion adsorption or exclusion behavior for 2:1 clay mineral particles were consistent with available experimental species in an aqueous solution. The properties of the diffuse observations.