Surface forces between cellulose surfaces in cationic polyelectrolyte solutions: The effect of polymer molecular weight and charge density

The purpose of this study was to gain a better understanding on how the cationic polyelectrolytes influence the forces between the cellulose surfaces and the importance of these forces in papermaking. The role of molecular weight and charge density of the polyelectrolytes was of particular interest. Surface forces between cellulose spheres in the presence of cationic polyelectrolytes were analysed with an atomic force microscope. The repulsion between the cellulose surfaces decreased when a small amount of high charged low molecular weight cationic polyelectrolyte was added. Charge reversal took place at high polyelectrolyte concentrations. Surface forces at large distances were well explained by the DLVO theory, implying that the origin of repulsive interactions at large distances was purely electrostatic. The Situation clearly changed, when a low charged, high molecular weight polyelectrolyte was added to the solution. Purely electrostatic behavior turned into steric repulsion at high polymer concentrations. The forces on separation were also different. While a sharp minimum was found in the presence of a polyelectrolyte with high charge density and low molecular weight, the detachment was gradual with attractive forces observed at separation of hundreds of nanometers for a polyelectrolyte with low charge density and high molecular weight. The pull-off force was at its largest near the charge neutralization point, where the surfaces were only partially covered and the polyelectrolyte was able to effectively bind the surfaces together. The difference between high charged, low molecular weight and low charged, high molecular weight polyelectrolytes indicates that the flocculation mechanism is bridging, but the effect of molecular weight is different. These observations on a molecular level can clearly be correlated with the flocculation and floc strength observed for fiber floes in papermaking suspensions.