Determination of aggregate structures by combined light-scattering and rheological studies

Abstract

The structures of aggregates of a clay, sodium bentonite, formed during the rapid (diffusion-limited) aggregation of a destabilised colloidal suspension have been studied using a combination of three experimental techniques. The first two of these were static and dynamic (quasi-elastic) light-scattering measurements, while the third involved rheological studies. The system was studied as a function of the pH of the aggregating suspension, between pH 2.3 and 10.2. Analysis of the light-scattering results leads directly to a determination of the fractal dimension, d_f, of the aggregates formed. The value of d_f for the aggregates showed a rapid transition from 3.0 below pH 4.3, to 1.8 above pH 4.3. This implied a close-packed structure for aggregates formed under highly acidic conditions, but a more open one in less acidic and in slightly alkaline suspensions. Rheological measurements showed minima in the Bingham yield stresses for both aggregated and unaggregated bentonite suspensions, at the cross-over point, pH 4.3, between the two structures. In addition, the ratio of the storage modulus to the loss modulus took only two values: a higher one below pH 4.3, and a lower one above pH 4.3. It was concluded that hetero-flocculation is induced in the aggregation of sodium bentonite below a certain acidity, resulting in aggregates which have a card-house, as opposed to band-like, structure. Furthermore, these open aggregates showed more viscous, and less elastic, behaviour than those with a higher fractal dimension.