Heteroaggregation in Binary Mixtures of Oppositely Charged Colloidal Particles Wei Lin, †,‡ Motoyoshi Kobayashi, †,§ Michal Skarba, † Changdao Mu, †,‡ Paolo Galletto, † and Michal Borkovec* ,† Department of Inorganic, Analytical, and Applied Chemistry, UniVersity of GeneVa, 1211 GeneVa 4, Switzerland ReceiVed August 22, 2005. In Final Form: NoVember 4, 2005 Heteroaggregation (or heterocoagulation) rate constants have been measured in mixtures of well-characterized colloidal particles of opposite charge with multiangle static and dynamic light scattering. This technique permits routine measurements of absolute heteroaggregation rate constants, also in the presence of homoaggregation. Particularly with multiangle dynamic light scattering, one is able to estimate absolute heteroaggregation rate constants accurately in the fast aggregation regime for the first time. Heteroaggregation rate constants have also been measured over a wide range of parameters, for example, ionic strength and different surface charge densities. Amidine latex particles, sulfate latex particles, and silica particles have been used for these experiments, and they were well characterized with respect to their charging and homoaggregation behavior. It was shown that heteroaggregation rate constants of oppositely charged particles increase slowly with decreasing ionic strength, and provided the surface charge is sufficiently large, the rate constant is largely independent of the surface charge. These trends can be well described with DLVO theory without adjustable parameters. 1. Introduction Although aggregation between equal particles, referred to as homoaggregation, has been the focus of numerous studies, 1-15 aggregation between unequal particles, referred to as hetero- aggregation, has been studied to a lesser extent. 16-23 However, many important processes in nature and in industry are governed by heteroaggregation. The classical example of heteroaggregation is papermaking. 24 Thereby, the paper is being formed from the slurry through the heteroaggregation of cellulose fibers and filler particles. Other examples involve aggregate formation in flotation processes and water purification 25,26 or the synthesis of coated particles or nanostructured materials. 16,27 Heteroaggregation is also much more rich from a conceptual point of view because of the inherent asymmetry originating from the two surfaces of the different particles. However, the case of homoaggregation is simpler because the system is symmetric and both particle surfaces are equivalent. 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