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 the reversibility of MCC aggregation under high-shear conditions. A correlation was established between the mean chord length output of FBRM probing a high-shear zone with the mean particle size (laser diffraction) of an aliquot extracted from the low-shear bulk mixing zone. Flocs formed by addition of a cationic polyelectrolyte were ruptured by shear forces of mixing and did not reaggregate at low mixing intensities. Flocs formed by addition of both polyelectrolyte and colloidal silica sols were found to reaggregate at low shear quite reversibly following high-shear degradation. The Kolmolgoroff microscale, eta, was determined using a three-compartment mixing model for the FBRM experiments, and the minimum aggregate adhesion forces were calculated to be similar to 3 nN under the experimental mixing conditions. Shear-dependent FBRM studies are also used to estimate the radial dependence of particle adhesion forces within an aggregate. AFM-based surface force measurements between model anionic surfaces (mica and glass beads) showed more reversible adhesion forces in the presence of colloidal silica than with cationic polyelectrolyte only. A descriptive model of the interfaces giving rise to the observed MCC aggregation and adhesion behavior is proposed. (C) 2000 Academic Press.