Polyelectrolyte Complexes: Interactions between Lignosulfonate and Chitosan

The interactions between high molecular weight chitosans (fraction of acetylated units (F(A)) = 0.10 or 0.50) and lignosulfonates of varying molecular weights (5000-400000 g/mol) and degrees of sulfonation (0.39-0.64) were studied. Lignosulfonates and chitosans form primarily insoluble polyelectrolyte complexes when mixed at pH 4.5, where the polymers are oppositely charged. In contrast, no complex formation occurred at pH 8, as shown by using a chitosan with F(A) = 0.50, which is soluble at this pH. Thus, a positively charged chitosan is a prerequisite for interactions leading to insoluble complexes with lignosulfonates. It is therefore unlikely that complex formation involves the formation of covalent sulfonylamide linkages as proposed in the literature. The composition of the complexes varied to some degree with the mixing ratio and molecular weight of lignosulfonate, but in most cases compact complexes with a sulfonate/amino ratio close to 1.0 were formed, suggesting that all sulfonate groups are accessible for interactions with chitosan. The influence of the ionic strength and temperature on the complex formation and the behavior of the precipitated complexes were in agreement with that expected for classical polyelectrolyte complexes where the associative phase separation is primarily governed by the increase in entropy due to the release of counterions.