Unexpected counterion exchange influencing fundamental characteristics of quaternary ammonium chitosan salt

N-[(2-hydroxy-3-trimethylammonium)propyl]chitosan salt (HTCS) is used in applications benefiting from its strong cationic charge. The degree of quaternization (DQ) and the type of counterions are the HTCS fundamental characteristics. HTCS with DQ between 15 and 116% were synthesized by reaction between chitosan glucosamine unit and glycidyltrimethylammonium chloride. While this should lead to a chloride polysalt, we unexpectedly identified a massive spontaneous exchange of chloride counterions by other anions originated from aqueous environment during synthesis and purification. Up to 70% lower DQ was obtained by potentiometric titration of chloride counterions – the method widely used in literature. It was shown that the true DQ can be determined by NMR spectroscopy. The phenomenon of counterion exchange was further verified by XPS and demonstrated also for quaternized dextran. The underlying mechanism of the counterion exchange was explained. It should be considered for other polyelectrolytes especially in applications, where the counterion type is important.