Effects of Salt on Phase Behavior and Rheological Properties of Alginate-Chitosan Polyelectrolyte Complexes

Oppositelycharged polyelectrolytes often form polyelectrolytecomplexes (PECs) due to the association through electrostatic interactions.Obtaining PECs using natural, biocompatible polyelectrolytes is ofinterest in the food, pharmaceutical, and biomedical industries. Inthis work, PECs were prepared from two biopolymers, positively chargedchitosan and negatively charged alginate. We investigate the changesin the structure and properties of PECs by adding sodium chloride(salt doping) to the system. The shear modulus of PECs can be tunedfrom similar to 10 to 10(4) Pa by changing the salt concentration.The addition of salt led to a decrease in the water content of thecomplex phase with increasing shear modulus. However, at a very highsalt concentration, the shear modulus of the complex phase decreasedbut did not lead to the liquid coacervate formation, typical of syntheticpolyelectrolytes. This difference in phase behavior has likely beenattributed to the hydrophobicity of chitosan and long semiflexiblealginate and chitosan chains that restrict the conformational changes.Large amplitude oscillatory shear experiments captured nonlinear responsesof PECs. The compositions of the PECs, determined as a function ofsalt concentration, signify the preferential partitioning of saltinto the complex phase. Small-angle X-ray scattering of the salt-dopedPECs indicates that the Kuhn length and radius of the alginate-chitosanassociated structure qualitatively agree with the captured phase behaviorand rheological data. This study provides insights into the structure-propertyas a function of salt concentration of natural polymer-based PECsnecessary for developing functional materials from natural polyelectrolytes.

Automated summary

In this study, the structure and properties of polyelectrolyte complexes (PECs) were investigated by adding sodium chloride (salt doping). The addition of salt resulted in a decrease in the water content and an increase in the shear modulus of the complex phase. However, at high salt concentrations, the shear modulus of the complex phase decreased without forming a liquid coacervate, which is different from synthetic polyelectrolytes. This research provides insights into the structure-property relationship of natural polymer-based PECs.