Engineering rheological response in chitosan-sophorolipid systems through controlled interactions

Objective The purpose of this study was to understand the impact of the biopolymer chitosan on the rheological behaviour of the biosurfactant sophorolipid as well as the effects of ionization and electrolyte addition on the chitosan-sophorolipid system. Methods Rotation mechanical rheometry was used to study the rheological response of the chitosan-SL systems. Frequency sweeps were conducted to analyse the rheological properties of the system at low-frequency ranges, and bulk viscosity of the system was studied at high shear rates for each sample. Results The biosurfactant sophorolipid on its own has very low viscosity. The bulk rheology results show that the addition of chitosan enhances the viscosity and viscoelastic properties of the chitosan-sophorolipid system indicating the presence of synergistic interactions between the two systems. Electrolyte addition had a significant impact on the system's rheological response. Addition of salt built the viscosity of pure chitosan. However due to charge screening effects, it resulted in a decrease in viscosity for the chitosan-sophorolipid system. On further increasing the salt concentration, an increase in viscosity was observed but not beyond the value obtained for the chitosan-SL system without any salt. An increase in pH results in increased ionization of the carboxylic acid groups in acidic SL, which in turn enhances the synergistic interactions between chitosan and SL. Conclusion The strong charge interactions between chitosan and sophorolipid lead to formation of an integrated gel-like network, thus building the viscosity of the system. A variation in parameters like biopolymer concentration, electrolyte and ionic strength has the potential to modify the bulk rheological properties of the chitosan-SL system.