Swelling behavior of poly(acrylamide-co-sodium acrylate) hydrogels in aqueous salt solutions: theory versus experiments

The predictions of the Flory-Rehner theory of swelling equilibrium includin g the ideal Donnan equilibria were compared with the experimental swelling data obtained from poly(acrylamide-co-sodium acrylate) hydrogels swollen in water and in aqueous salt (NaCl) solutions. For this comparison, the fract ion of counterions which are effective in the gel swelling was taken into a ccount. The ionic hydrogels were prepared from acrylamide and sodium acryla te (NaAc) monomers with 0 to 5 mol% NaAc and using N,N'-methylenebis(acryla mide) as the crosslinker: As expected. at a fixed crosslinker ratio, the vo lume swelling ratio of hydrogels in water increases sharply when the mole f i action of NaAc increases or the NaCl concentration in the external soluti on decreases. Taking into account the wasted counterions within the hydroge ls, the theory correctly predicts the swelling behavior of hydrogels in wat er and in aqueous salt solutions, Not predicted by the theory is the increa sed swelling ratio of hydrogels with less than 4 mol% NaAc content with inc reasing salt concentration from 10(-1) to 10(0) M. (C) 1999 Elsevier Scienc e Ltd. All rights reserved.