The swelling behavior of poly(acrylamide-co-acrylic acid and trimethyl-N-acryloyl-3-aminopropyl ammonium chloride) gel in electrolyte solutions of NaCl (uni-univalent) and MgCl₂ (bi-univalent) was investigated over the whole range of pH.
The swelling of gel was induced by the ionic pressure due to Na⁺ and/or Mg²⁺ ions in the acid region (pH = 4–7) as well as basic region, where Na⁺ or Mg²⁺ ions added in solution were exchanged with H⁺ ion supplied by the yet undissociated group, –COOH. in the region of pH = 1–4, Cl^– ions generated the ionic pressure. In the case of bivalent cation, Mg²⁺ especially, it was observed that the interactions between the localized fixed charges and the bivalent cation, may result in formation of new weak cross-linking. The swelling and behavior of the ions in the gel were predicted fairly well using a simple model based on Donnan equilibrium theory.