Clay minerals are anisotropic phyllosilicates and have a layer
structure comprised of edges (E) and faces (F), and therefore clay
platelets can associate in edge–edge (E–E), edge–face (E–F), and
face–face (F–F) configurations. It is known that clay minerals have
a basal permanent negative charge caused by isomorphous substitution, whilst the charge on the edges is either positive or negative
depending on the pH [5–8]. Recent work has reported that the two
basal plane surfaces of kaolinite can have positive or negative
charges as well [9]. The anisotropic structure and charge properties