Effect of the ionic surfactant concentration on the stabilization/destabilization of polystyrene colloidal particles.

One of the most interesting properties of the surfactants is that they are able to alter the stability of colloidal dispersions. Despite its great industrial relevance, only a few works analyze the colloidal stability of these systems at high surfactant concentrations (well above the critical micelle concentration (CMC)). In the present work, the colloidal stability of polystyrene particles is studied under a wide range of ionic surfactant concentrations. The effects of the surface charge of the latex particles (evaluating both sign and value), and surfactant type (cationic or anionic) have been examined. Colloidal stability data have been gathered by monitoring aggregation using a nephelometric technique. As will be shown, it is possible to reach different stability regimes using the same colloidal system just by changing the surfactant concentration. Independently of the sign of both the surfactant and the surface, the destabilization of the system consistently takes place above certain surfactant concentration due to a depletion effect from non-adsorbed micelles. This destabilization can be predicted by adding to the DLVO interaction energy a new contribution addressing the force between two spherical particles in the presence of non-adsorbing spherical macromolecules.