Evidence for a crossover region in the aggregation of PEO-coated polystyrene particle suspensions

The crossover region in the salt-induced aggregation processes occurring in polystyrene latex suspensions has been investigated by means of dynamic li ght scattering measurements. The possible existence of a region between rea ction-limited and diffusion-limited regimes has been suggested in the last few years on the basis of various experiments and computer simulations, but no definitive agreement has been reached, since reaction-limited aggregati on can be viewed as the initial regime crossing over towards diffusion-limi ted aggregation after a certain time and, in any case, in the long time lim it. In the present work we have varied the overall interaction potential be tween particle monomers and thus the corresponding aggregation regime by va rying the amount of adsorbed polymer [poly(ethylene oxide) (PEO), in this c ase] on the particle surface. The aggregation is induced by adding an appro priate simple salt (NaCl, 0.5 M) to the PEG-coated latex particles, at diff erent polymer surface coverages, giving rise to different aggregation regim es, from fast aggregation in the absence of PEG, to slow aggregation at mod erate PEO concentration, up to complete stabilization of the colloidal susp ension at higher polymer concentration. In the present case, deviation from a reaction-limited regime extends for a period of time long enough to allo w accurate analysis of the data. Comparison of the experimental data with t he prediction of the Smoluchowski equation, modified by Tartaglia et al. (P hys. Rev. E, 1994, 50, 1649) to take into account the intermediate regime, is quite satisfactory and values of the parameters of the interaction poten tial between monomers have been evaluated. The present analysis gives suppo rt to the suggestion that slow, reaction-limited cluster aggregation is onl y a transient regime of the early stage of aggregation.