Introduction
When a colloidal particle is brought into contact with an aqueous electrolyte environment, the orientation of water molecules at the solid - solution interface induces electrical charging of the surface. Mechanisms for this process may include dissociation or ionisation of surface sites, specific ion adsorption, differential dissolution of surface- based ions and, in the case of clay minerals, exchange of lattice cations with others of lower valence in a process known as isomorphous ion substitution. The presence of the charged surface promotes redistribution of the ions in the surrounding solution, with counter-ions preferentially attracted towards and co-ions repelled away from the surface. The result is an ionic neutralisation of the surface charge over a short distance into the neighbouring aqueous environment. The combination of the charged surface and the unequal distribution of counter-ions and co-ions in the surrounding solution is referred to as the electrical double layer.