The Intimate Role of Imidazole in the Stabilization of Silicic Acid by a pH-Responsive, Histidine-Grafted Polyampholyte

A methacrylate-based polyampholyte homopolymer was synthesized starting from N-methacryloyl-L-histidine (MHist). This paper reports the inhibitory effects of poly-N-methacryloyl-L-histidine (poly-MHist) on the in vitro silicic acid condensation. In particular, the ability of poly-MHist to retard silicic acid condensation in aqueous supersaturated solutions at three pH values, 5.5, 7.0, and 8.5, is studied. The direct role of the imidazole ring was confirmed by substantial changes in silicic acid stabilization efficiency based on the following observations: (a) the protonation degree of the imidazole ring affects stabilization. At a relatively low pH of 5.5, the imidazole is protonated and the entire polymer acquires a zwitterionic character (-COO- is also present). Inhibitory activity increases considerably. In contrast, at a high pH of 8.5, the imidazole ring is neutral and the polymer backbone is anionic due to the presence of -COO- moieties. This results in total inactivity with respect to silicic acid stabilization. (b) The use of a similar, but pH-insensitive derivative poly(N-acryloyl-L-phenylalanine) (poly-PHE), which contains a phenylalanine instead of histidine, results in total loss of silicic acid stabilization activity. Finally, poly-MHist also shows effects on colloidal silica particle morphology. Increasing the poly-MHist concentration results in a reduced size of silica particles.