| CaCO3 was precipitated in ethanol-water binary solvent with CaCl2/Na2CO3 reaction system. SEM, XRD and some other characterization were employed to investigate the effect of binary solvent on the morphology, polymorphs and aging process of CaCO3. Compared with the case in water, metastable polymorphs were more abundant in the CaCO3 that crystallized in binary solvent, especially when the initial CaCO3concentration ([CaCO3]t) and the volume fraction of ethanol (rE) were high. Binary solvent exerted a morphogenetic effect on CaCO3. Alteration in [CaCO3]t, rE and the Ca2+/CO32" molar ratio (rCa/CO3) could significantly change the morphology. During aging process, the metastable vaterite was stabilized in binary solvent and transformed to other polymorphs with a very low rate. rCa/CO3 substantially effects the pathway and rate of polymorphic transformation.Based on the stepwise crystallization and aggregation-mediated crystallization models, and from the angle of microheterogeneity of the reaction system and the solvation behavior of ions in binary solvent, we theoretically analyzed the experimental results. Ca2+ and CO32+ had difference in charge density and distribution. And water and ethanol were different in polarity. These two differences had three important outcomes: the differentiating effect on Ca2+ and CO32+ in binary solvent, the compartmentalized microenvironment, and the novel surface properties ofprecipitate particles. They played important roles in the stepwise crystallization, aggregation-mediated crystallization anddissolution-recrystallization, and were the key reasons of ethanol-water binary solvent's effects on the morphology and polymorphs of CaCO3.Additional investigation was made for the crystallization of CaCO3 in glycol-water, ether-water, isopropanol-water and tetrahydrofuran-water binary solvents. The results supported and supplemented our theoretical analysis, and revealed that, to exert efficient morphogenetic effect on CaCO3, the polar difference between water and organic cosolvent must be proper, that is, neither too large nor too minimal. |
