Bicarbonate-mediated dissolution of chitosan-based polyelectrolyte complex gels

Using chitosan/alginate polyelectrolyte complexes (PECs) as the primary model system, we demonstrate and systematically analyze mild and facile chitosan-based PEC gel dissolution in sodium bicarbonate (NaHCO3) solutions. The reversible conversion of chitosan amine groups to carbamate groups dissociates the ionic bonds between chitosan and its complexing polyanion while simultaneously solubilizing the dissociated chitosan, which causes the gels to dissolve. The PEC composition-dependent minimum NaHCO3 concentrations needed for this dissolution, for the materials examined herein, do not exceed a few hundred mM, and the dissolution rate increases with both the NaHCO3 concentration and the stoichiometric imbalance between the chitosan amine and alginate carboxylate groups within the PECs. The PEC dissolution rates also generally increase with the stirring speed but (depending on the PEC composition) can become insensitive to stirring when (1) the gels float and (2) at higher stirring speeds where transport of NaHCO3 to the PEC gel surface ceases to be a rate-determining step in the dissolution process. While the above effects are primarily demonstrated using chitosan/alginate PECs, extension of this NaHCO3-mediated dissolution approach to other chitosan-based PECs (i.e., those formed with synthetic polyanions) is also demonstrated. Collectively, these findings advance a non-destructive method of dissolving chitosan-based PEC gels (which could be advantageous for their compositional analysis or recycling) and provide guidelines for this method's application.