Investigation on adsorption, configuration and performance of chitosan polyelectrolyte in pulp slurry by ellipsometry technique

Chitosan is one of the biopolymers which possesses unique properties and has been recently considered in various applications such as paper-making. This polymer is soluble in acidic conditions and has the potential to act as a polyelectrolyte under the colloidal condition in the pulp. However, according to the studies, the capability of this polyelectrolyte was better in alkaline condition, in improving the drainage, retention and even strength properties of paper. Therefore, in this study, the performance of chitosan was studied in different conditions and the results were investigated based on the interpretation of the adsorption process and chitosan configuration using silicon wafer model and ellipsometry technique. For this purpose, bleached kraft softwood fibers were refined to a Canadian standard freeness of 300 ml and after addition of chitosan and nano-silica at different pH levels, freeness and fines retention parameters were evaluated. The results of this study confirmed that chitosan polyelectrolyte showed a good performance in increasing drainage and retention at alkaline pH, compared to the other pH levels, lonely and with the addition of nano silica. However in freenes test at acidic pH, the addition of chitosan along with nanosilica reduced the drainage, and no significant effect was observed at neutral and acidic pHs in the case of fine retentions. In order to interpret these observations and to ratiocinate the behavior of chitosan polyelectrolyte, the data obtained from the ellipsometry technique was investigated to determine the configuration and the thickness of the adsorbed chitosan layer which showed a higher thickness of the polymeric adsorbed layer at alkaline pH, and thus, the possibility of the longer tail and loops configurations in the environment and more effective interractions beyond the electrical double layer.