Polyelectrolyte-induced aggregation of microcrystalline cellulose: Reversibility and shear effects

The polyelectrolyte-induced aggregation of microcrystalline cellulose (MCC) was studied by focused beam reflectance measurement (FBRM) to determine th e reversibility of MCC aggregation under high-shear conditions. A correlati on was established between the mean chord length output of FBRM probing a h igh-shear zone with the mean particle size (laser diffraction) of an aliquo t extracted from the low-shear bulk mixing zone. Flocs formed by addition o f a cationic polyelectrolyte were ruptured by shear forces of mixing and di d not reaggregate at low mixing intensities. Flocs formed by addition of bo th polyelectrolyte and colloidal silica sols were found to reaggregate at l ow shear quite reversibly following high-shear degradation. The Kolmolgorof f microscale, eta, was determined using a three-compartment mixing model fo r the FBRM experiments, and the minimum aggregate adhesion forces were calc ulated to be similar to 3 nN under the experimental mixing conditions. Shea r-dependent FBRM studies are also used to estimate the radial dependence of particle adhesion forces within an aggregate. AFM-based surface force meas urements between model anionic surfaces (mica and glass beads) showed more reversible adhesion forces in the presence of colloidal silica than with ca tionic polyelectrolyte only. A descriptive model of the interfaces giving r ise to the observed MCC aggregation and adhesion behavior is proposed. (C) 2000 Academic Press.