Surface interactions in polyelectrolyte-cellulose systems and their implications for flocculation mechanisms

Abstract

The overall purpose of this work was to gain a better understanding on how the different polyelectrolyte systems affect on the interactions between cellulose surfaces, and thus how the different flocculation systems worked on a molecular level. The focus was on studying the effect of cationic polyelectrolytes, polyelectrolyte complexes and architectures formed by polyelectrolyte and nanoparticles on interactions. The main part of the results was obtained using atomic force microscope (AFM). The forces between cellulose surfaces in cationic polyelectrolyte solutions depended on the molecular weight and charge density of the polyelectrolyte. Repulsion at high polymer concentrations was mainly steric in the solution of high molecular weight, low charged polyelectrolyte, whereas repulsion was mainly electrostatic in the solution of the low molecular weight, high charged polyelectrolyte. Also forces on separation were different. While a sharp minimum was found in the presence of a polyelectrolyte with high charge density and low molecular weight close to charge neutralization point, the detachment was gradual and long ranged for a polyelectrolyte with low charge density and high molecular weight. The main mechanism for flocculation was bridging in both cases. The interactions between adsorbed polyelectrolyte complex layers were mainly steric and repulsion was long ranged, even at low polymer concentration. The largest complex, formed by high molecular weight, low charged polyelectrolytes, showed the longest ranged repulsion and the best dewatering behaviour. In contrast to single polymer systems, the pull-off force was long ranged even in high polymer concentrations. It was concluded that the flexible and large complex was beneficial for flocculation, when the anionic polyelectrolyte can act as a link between the complex layers. The stratifying of silica nanoparticles and cellulose nanofibrils (NFC) with polyelectrolyte were compared. The adsorption and repulsion increased during the layer formation in the silica nanoparticle systems, whereas the second addition of cationic polyelectrolyte decreased repulsion in the case of nanofibrillar cellulose multilayer formation due to the collapse of the topmost nanofibrils. A high repulsion between nanofibril covered surfaces indicated together with high dissipation changes that NFC formed loose and thick layer containing a large amount of water. Silica nanoparticles were concluded to be able to penetrate inside the loose polyelectrolyte structure due to their small size. On the other hand, NFC formed individual layers between the polyelectrolyte layers.