Effect of Salt Addition on the Phase Behavior and Rheological Properties of Natural Polyelectrolyte Complexes

Polyelectrolytes and polyelectrolyte complexes (PECs) play an important role in various food, pharmaceutical, and biomedical applications. Oppositely charged polyelectrolytes in water can form polyelectrolyte complexes (PECs) due to the electrostatic interactions. The structure and properties of PECs can be tuned by varying the salt concentration, as the addition of salt can facilitate the associative phase separation in the oppositely charged polyelectrolyte complexes. In this work, PECs are prepared from two biopolymers, positively charged chitosan and negatively charged alginate. Without any salt addition, the water content of the complex phase was >95% and the PEC phase resembled a soft solid. The addition of salt led to a decrease in the complex phase's water content with increasing shear-modulus. However, at a very high salt concentration, both the water content and the shear- modulus of the complex phase decreased. The linear viscoelastic region extended with the increase in salt concentration. In the non-linear regime, the shear modulus decreased with the increase in strain. The non-linear behavior of the PECs with respect to salt addition was further investigated using large amplitude oscillatory shear experiments (LAOS). The compositions of the PECs were determined and the results indicate the preferential partitioning of salt into the complex phase. This study provides insights into the tunability of the properties of PECs towards developing functional materials from natural polyelectrolytes.