Salt-induced swelling and deswelling of alginate gels

Alginate is a negatively charged polymer that is often cross-linked in the presence of Ca2+ ions to form alginate gels. Their swelling is often induced by the addition of NaCl via what is believed to be the exchange of the calcium ions responsible for cross-linking and the added Na+ ions. Our experiments, however, indicate that the swelling of the gels with NaCl is followed by deswelling at higher concentrations, a fact that cannot be explained through cross-link degradation. Dialyzing these gels against large volumes of ultrapure water confirms the stability of the cross-links. Assuming that the gel is an ionic gel with some permanent cross-links, we develop a model that shows that the key to our observations is the distribution of mobile ions inside and outside the gel, together with electroneutrality and the valence difference between Na+ and Ca2+ ions. Changing the suspension pH, which allows for changing the charging state of alginate, confirms that the observed swelling behavior is consistent with that of ionic gels. Hence, our results, which we also show to be independent of gel morphology, unavoidably indicate that the conventional way to think about alginate swelling must be revised.