Predicting Aggregation Rates of Colloidal Particles from Direct Force Measurements

Direct force measurements between negatively charged colloidal latex particles of a diameter of 1 μm were carried out in aqueous solutions of various inorganic monovalent and multivalent cations with the multiparticle colloidal probe technique based on the atomic force microscope (AFM). The observed force profiles were rationalized within the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). In the presence of monovalent and divalent cations, this theory was capable to describe the force profiles correctly down to distances of a few nm. At shorter distances, however, a strong non-DLVO attraction was identified. For more highly charged cations, an additional and more long-ranged non-DLVO attractive force is observed, and it was interpreted by surface charge heterogeneities. On the basis of these force profiles, the aggregation rates, which were independently measured by light scattering, can be predicted relatively well. The main conclusion of this study is that, in the present system, direct force measurements do capture the principal interactions driving aggregation in colloidal suspensions.