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Experimental evidence of a fully reversible aggregation process due to ionic specificity is presented for the first time. The results indicate the existence of repulsive structural forces between particles coming from the specific accumulation of poorly hydrated ions around hydrophobic surfaces. This result emphasizes the key role that ionwater structure near interfaces plays in Hofmeister effects, as recently suggested by molecular dynamics simulations.

Electrostatic heteroaggregation processes arising in 1:1 mixtures of oppositely charged microspheres at low and very low electrolyte concentrations were investigated by means of single-cluster light scattering. Cluster discrimination, i.e., the fact that clusters differing by only one constituent particle behave quite differently, was found for monomers and dimers. This effect was recently predicted by Brownian dynamic simulations but, to the best of our knowledge, not yet confirmed by experiments. The experimental data were confronted with the simulations and a good qualitative and quantitative agreement was obtained. The origin of the cluster discrimination phenomenon could be related to the range of the attractive electrostatic interactions.

Binary diffusion-limited cluster-cluster aggregation processes are studied as a function of the relative concentration of the two species. Both, short and long time behaviors are investigated by means of three-dimensional off-lattice Brownian Dynamics simulations. At short aggregation times, the validity of the Hogg-Healy-Fuerstenau approximation is shown. At long times, a single large cluster containing all initial particles is found to be formed when the relative concentration of the minority particles lies above a critical value. Below that value, stable aggregates remain in the system. These stable aggregates are composed by a few minority particles that are highly covered by majority ones. Our off-lattice simulations reveal a value of approximately 0.15 for the critical relative concentration. A qualitative explanation scheme for the formation and growth of the stable aggregates is developed. The simulations also explain the phenomenon of monomer discrimination that was observed recently in single cluster light scattering experiments.