Effects of some nonionic polymeric additives on the crystallization of calcium carbonate

The effect of water-soluble, nonionic polymers on the crystallization of calcium carbonate is reported. The polymers used were poly(vinyl alcohol) (PVA), polyacrylamide, poly(N-isopropyl acrylamide), poly(N-vinyl pyrrolidone), and poly(ethylene oxide). In addition, several smaller molecules containing hydroxyl groups (methanol, ethanol, ethylene glycol, glycerol, dextran, and maltodextran) were also examined. Crystallization began by mixing together, with or without a polymer or other additive, calcium chloride and ammonium carbonate. The first crystals to form in all cases were vaterite, possibly with a very small amount of calcite. Subsequent behavior depended on the polymer or other additive present. With most additives, the vaterite transformed within ca. 1 h to calcite, probably by dissolution and recrystallization. However, with PVA, the formation of calcite was inhibited, and when enough PVA was added, the vaterite remained. With less PVA, aragonite formed. After ca. 9 days, aragonite was the exclusive crystalline phase, except for a very small amount of early-forming calcite. The distinguishing characteristics of PVA seem to be its ability to hydrogen bond and its forming nearly theta solutions in water over a range of temperatures including room temperature. From a theta solution, the polymer tends to adsorb nonspecifically on all solid surfaces. The common formation of aragonite in biological organisms may arise from the organism's proteins acting in a way similar to that of PVA.