Systematic study of the effects of polyamines on calcium carbonate precipitation

While negatively charged organic additives are widely used as an effective means to control CaCO3 precipitation, positively charged additives are generally considered to be much less active. Nevertheless, the cationic polyelectrolyte poly(allylamine hydrochloride) has recently been shown to exert significant control over CaCO3 precipitation, driving the formation of thin films and fibers, and other examples suggest that many positively charged additives promote vaterite formation. This article aims to bring together these sometimes conflicting views of the activity of positively charged additives. The effect of a series of polyamines on CaCO3 precipitation was studied, where the polyamines were selected such that the amine group type, the pKa value (of the corresponding conjugated acid), the molecular weight, and the side chain length of the polymers could be evaluated. The results unambiguously demonstrate that polyamines carrying primary amine groups are capable of exerting a significant effect and that the activity of this class of polyamines is strongly dependent on the length of the side chain. In contrast, polyamines comprising with quaternary amines have negligible effect, despite carrying a permanent positive charge. The activity of the most active polyamines therefore depends on their ability to complex with carbonate ions present in solution, and electrostatic attraction alone is not sufficient.