Stable amorphous calcium carbonate (ACC) composite particle with a size-controlled monodispersed sphere was obtained by a new simple carbonate controlled-addition method by using poly(acrylic acid) (PAA) (Mw = 5000), in which an aqueous ammonium carbonate solution was added into an aqueous solution of PAA and CaCl2 with a different time period. The obtained ACC composite products consist of about 50 wt % of ACC, 30 wt % of PAA, and H2O. Average particle sizes of the ACC spheres increased from (1.8 +/- 0.4) x 102 to (5.5 +/- 1.2) x 102 nm with an increase of the complexation time of the PAA-CaCl2 solution from 3 min to 24 h, respectively. The ACC formed from the complexation time for 3 min was stable for 10 days with gentle stirring as well as 3 months under a quiescent condition in the aqueous solution. Moreover, the ACC was also stable at 400 degrees C. Stability of the amorphous phase decreased with an increase of the complexation time of the PAA-CaCl2 solution. No ACC was obtained when the lower molar mass PAAs (Mw = 1200 and 2100) were used. In the higher molar mass case (Mw = 25 000), a mixture of the amorphous phase and vaterite and calcite crystalline product was produced. The present results demonstrate that the interaction and the reaction kinetics of the PAA-Ca2+-H2O complex play an important role in the mineralization of CaCO3.

The crystallization of CaCO3 was examined by changing the addition time of poly(acrylic acid) (PAA) to an aqueous solution of calcium carbonate by selectively interacting with the crystal at different stages during the crystal-forming process. The precipitation of CaCO3 was carried out by a double jet method to prevent heterogeneous nucleation on glass walls, and the sodium salt of PAA was added by a delayed addition method. In the initial presence of PAA in an aqueous solution of calcium carbonate, PAA acted as an inhibitor for the nucleation and growth of crystallization. However, it was found that stable vaterite particles were successfully obtained by delaying the addition of PAA from 1 to 60 min. The vaterite particles were stable in the aqueous solution for more than 30 days, and the CaCO3 particles were formed by a spherulitic growth mechanism. It is suggested that PAA strongly binds with the Ca2+ ion on the surface of CaCO3 particles to stabilize the unstable vaterite form effectively. Upon changing the addition time of PAA, we found that CaCO3 particles were formed through different formation mechanisms in selectively controlled crystallization at different stages during the crystallization process.

The effect of poly(acrylic acid) (PAA) on the mineralization of CaCO 3 was studied by varying the molecular weights (PAA1.2k, M w ¼ 1200; PAA25k, M w ¼ 25000; PAA250k, M w ¼ 250000) as well as by changing the addition time of the sodium salts of PAA (PAA-Na) to an aqueous solution of calcium carbonate. The precipitation of CaCO 3 was carried out by a double jet method. Stable vaterite crystals were successfully obtained by delaying the addition of all the sodium salts of PAA from 1 to 60 min, and the resulting particles were formed by a spherulitic growth mechanism. The vaterite particles modified with PAA25k-Na and PAA250k-Na showed higher stability than the ones stabilized by PAA1.2k-Na in an aqueous solution. However, in the initial presence of PAA-Na, amorphous calcium carbonate (ACC), vaterite or calcite were induced under various conditions. It is interesting to find that the ACC product induced by PAA1.2k-Na was more stable than that induced by PAA25k-Na in an aqueous solution as well as in a dry state. It was also found that the CaCO 3 particles formed through the nano-aggregation mechanism might be induced by the strong inhibiting effects of PAA-Na, while the spherulitic growth mechanism might be due to the insufficient inhibiting efficiency for the crystallization. These results suggest that the selective interaction of PAA-Na with CaCO 3 at different stages as well as the inhibiting strength varied with the chain lengths of PAANa could play an important role for the controlling of the crystal nucleation and growth during the crystallization process.

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