Interactions between aluminium ions and acrylic acid-acrylamide copolymers in aqueous solution: 2. Phase separation

Abstract

The stability of aqueous solutions of acrylic acid-acrylamide copolymers (with acrylate content τ varying from 0.3 to 27%) towards Al species has been investigated at different pH values. In our previous work, an electrostatic interaction model was proposed which predicted first that the polymer stability is at a minimum at pH 5 (due to the presence of Al polycations) and secondly that there is no interaction between polymer and Al at pH 7 (due to the presence of non-ionic Al(OH) 3 species). The second prediction is incorrect and phase diagrams were established at pH 5 as well as at pH 7, and allowed us to determine: (a) the critical concentration C∗ a of the Al (for which polymer precipitation occurs) as a function of polymer concentration and (b) the C∗ a dependence on τ. The fraction of precipitated polymer was evaluated using two techniques (potentiometric titration of the supernatant and carbon titration in the precipitated phase). This was compared, at low pH (≤5) with the calculated results of the electrostatic interaction model. An interesting result was the finding of gel formation at pH 5, for one of our polymer samples in the presence of Al.