Influence of Salt and pH on the Swelling Equilibrium of Ionizable N-IPAAm Based Hydrogels: Experimental Results and Modeling

The influence of sodium chloride and of pH on the equilibrium swelling of ionic hydrogels consisting of N-isopropyl acrylamide (N-IPAAm), a weak cationic comonomer N-[3-(dimethylamino)propyl] methacrylamide (DMAPMA) and/or a strong anionic comonomer 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) was investigated in aqueous solutions at 298 K. The mass fraction of the polymerizable substances in the aqueous solution was kept at 0.1. The sum of the mole fractions of the comonomers in the gel was 0.05 while the mole fraction of a single comonomer was varied between 0 and 0.05. The mass fraction of NaCl in the aqueous solution was varied between 3 · 10⁻⁶ and 0.25. The pH-sensitive comonomer DMAPMA causes an influence of pH on the degree of swelling. The pH of the aqueous solutions was adjusted by adding HCl or NaOH. A thermodynamic framework was applied to model (i.e., correlate and predict) the experimental results. The calculations revealed that dissolved carbon dioxide (resulting from air that was in contact with the condensed phases) might have a remarkable influence on the swelling behavior.