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

Abstract

The crossover region in the salt-induced aggregation processes occurring in polystyrene latex suspensions has been investigated by means of dynamic light scattering measurements. The possible existence of a region between reaction-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 aggregation can be viewed as the initial regime crossing over towards diffusion-limited aggregation after a certain time and, in any case, in the long time limit. In the present work we have varied the overall interaction potential between particle monomers and thus the corresponding aggregation regime by varying the amount of adsorbed polymer [poly(ethylene oxide) (PEO), in this case] on the particle surface. The aggregation is induced by adding an appropriate simple salt (NaCl, 0.5 M) to the PEO-coated latex particles, at different polymer surface coverages, giving rise to different aggregation regimes, from fast aggregation in the absence of PEO, to slow aggregation at moderate PEO concentration, up to complete stabilization of the colloidal suspension at higher polymer concentration. In the present case, deviation from a reaction-limited regime extends for a period of time long enough to allow accurate analysis of the data. Comparison of the experimental data with the prediction of the Smoluchowski equation, modified by Tartaglia etal. (Phys. Rev. E, 1994, 50, 1649) to take into account the intermediate regime, is quite satisfactory and values of the parameters of the interaction potential between monomers have been evaluated. The present analysis gives support to the suggestion that slow, reaction-limited cluster aggregation is only a transient regime of the early stage of aggregation.