Surface charge, effective charge and dispersion/aggregation properties of nanoparticles

A careful investigation of the relationship between surface properties and colloidal behaviour of nanometric particles in concentrated media has shed some light on the important parameters that must be controlled in order to improve the dispersion of mineral particles. Experimental methods such as rheology and osmometry reveal that the aggregation/dispersion process is not only a matter of electrostatics as stated by classical theories. In practice, the relationship between the surface charge and the state of dispersion is probably much less straightforward than generally assumed by the classical argument stating that the higher the surface charge, the higher the electrostatic repulsion between particles and the more dispersed the particles. Our results on model hematite and industrial TiO 2 systems suggest that the state of dispersion is not always correlated to the magnitude of the surface charge, but seems to depend on the surface density of condensed monovalent counter‐ions. Accordingly, it is believed that the monovalent species present at the interface of mineral particles are deeply involved in the dispersion process of highly concentrated slurries. Much attention should be addressed to the presence of condensed species onto the surface and to their influence on the structure at the interface, in order to prepare better formulations involving (hydr)oxide particles in aqueous media. © 2003 Society of Chemical Industry