The Electrical Double Layer of a Disk-Shaped Clay Mineral Particle: Effect of Particle Size

Abstract

The purpose of the present study is to investigate theoretically, by using a disk model, the effect of finite particle size on the electrical double layer of a 2:1 clay mineral particle suspended in a 1-1 electrolyte solution. The clay mineral particle is represented as a thin disk with negatively charged basal planes and a positively charged edge surface. We applied modified Gouy-Chapman theory to the disk model and solved the Poisson-Boltzmann equation numerically using a self-adaptive finite-element method. To carry out the computer simulation, tree and linked-list structures, nested dissection node ordering, and dynamic memory allocation techniques were implemented in the program DISKMGCT. We found an adsorption barrier at an electrolyte concentration of 0.1 mol m -3 for anions approaching the edge surface. This adsorption barrier, originating from a negative inner potential near the edge surface, is mainly controlled by particle thickness. The anion exclusion volume is also determined by the individual values of particle thickness and radius. For a particle of very small radius (⩽10 nm), the anion exclusion volume as calculated from the disk model was significantly larger than that obtained from the conventional infinite-plane Gouy-Chapman theory in dilute electrolyte solutions. For a particle of thickness ⩾4.9 nm, the anion exclusion volume does not decrease monotonically, but instead changes from negative to positive values with an increase in electrolyte concentration. Differences between the predictions of the disk and the infinite-plane models decrease with increasing electrolyte concentration. Thus, the presence of a positively charged edge surface affects clay-electrolyte interactions most significantly at low electrolyte concentrations.