Effect of a nonadsorbing polyelectrolyte on colloidal stability: An overview

The effect of nonadsorbing polyelectrolytes (macromolecules) on the interaction between two charged particles was studied. First, a force-balance model was developed in which the particles and macromolecules were simulated as hard charged spheres. The model showed that the presence of charge can greatly increase both the magnitude and range of the depletion interaction between two particles. In addition, a significant, longer-range repulsion can arise due to an ordering of the macromolecules in the gap region. Two types of experiments were then conducted. In the first, the interaction energy between a single colloidal particle and a flat plate in aqueous solutions of nonadsorbing macromolecules was measured using the optical technique of total internal reflection microscopy (TIRM). Second, the stability of a dispersion of charged particles in an aqueous solution of the macromolecules was studied using optical turbidity. Three different species were used to represent the nonadsorbing macromolecular material-hard charged silica spheres, cationic micelles (CTAB), and the anionic polyelectrolyte sodium polystyrene sulfonate (SPSS). With the silica sphere, reasonable agree ment between the predicted and measured profiles was observed. In the stability experiments, both critical flocculation and higher critical restabilization concentrations were found. By comparison, the micelle and SPSS interaction energy profiles showed poor agreement with the model. In addition, both materials were able to induce flocculation of the particles, however no subsequent restabilization occurred. These experiments demonstrate the need for additional modeling work in these relatively complicated polymer-colloid systems.