Fragmentation of Colloidal Aggregates Induced by Polymer Adsorption

Abstract

Using a particle counter technique, we investigated the fragmentation process of aggregates of polystyrene suffonate latices induced by the adsorption of polyvinylpyridine at the particle-liquid interface by determining the aggregate size distribution c( x, t), concentration of aggregates of size x, and the weight S( t) and number N ( t) average sizes at time t. The fragmentation of two typical aggregates were studied. The first aggregates obtained under conditions of perikinetic diffusion-limited aggregation (1 M NaCl and pH 3.0 medium) were transferred to aqueous polyelectrolyte solutions at different concentrations in the presence of 0.15 M NaCl and at pH 3.0. The second aggregates obtained under conditions of reaction-limited aggregation (0.15 M NaCl and pH 3.0 medium) were transferred to aqueous polyelectrolyte solutions in the presence of 0.0225 M NaCl and at pH 3.0. We determined that fragmentation is a very slow process. The size distribution of the fragments was self-similar and the variation in the number average size N( t) and the weight average size S( t) as a function of time followed dynamic scaling laws whose exponents depended on the mode of formation and the initial size of the aggregates, as well as on the concentration and the molecular weight of the polymer. Using Cheng and Redner's theory ( Phys. Rev. Lett. 60, 2450 (1988); J. Phys. A: Math Gen. 23, 1233 (1990)), we calculated the rate of aggregate breakup, which was found to be independent of the initial size of the aggregates and of the polyelectrolyte concentration. The mode of aggregate formation and the molecular weight of the polymer in the fragmenting medium were found to fix the rate of breakup and the fragment size distribution. The discrepancies between the rate of aggregate break-up and the rates of fragmentation determined from the variation of the average sizes were attributed to the existence of a concomitant aggregation during fragmentation.