Sodium polyacrylate adsorption onto anionic and cationic silica in the presence of salts

Sodium polyacrylate is well known for its application as a scale inhibitor in common household products, and the effects of both monovalent and divalent metal cations on its structure have been covered by a range of previous publications. In the present article, we extend this work by using solvent relaxation NMR to look at the adsorption of the polyelectrolyte onto both positively and negatively charged silica and how this is altered by calcium chloride. In the anionic case, we found that polyacrylate adsorption was predictably very weak, and interestingly, perhaps counterintuitively, it was further reduced by calcium ions. This is probably linked to NaPA-Ca2+ binding, which changes the conformation and charge of the polyelectrolyte. In contrast, NaPA adsorbs very strongly on cationic silica, to the point that precipitation often occurs, particularly on addition of salt.