| Summary: | 博士 === 國立臺灣大學 === 化學工程學研究所 === 89 === Nucleation is the dominant crystallization mechanism for the sparingly soluble system. High nucleation rate and apparent aggregation of small crystals cause the experimental study on the nucleation phenomena of sparingly soluble systems much harder than those of readily soluble systems. The induction period is one of the parameters that are easy and convenient to measure in a crystal nucleation process. Besides, the induction period is closely related to the nucleation mechanism and kinetics. Therefore, the induction period became an important research topic in the nucleation studies. In theoretical, although the classical nucleation theory is widely used to interpret some experimental results of induction period, the related correlation of induction period derived from the classical nucleation theory is difficultly connected with the definition of induction period. Therefore, several theories differing from the classical nucleation theory are applied to investigate the nucleation rate and induction period recently.
In this study, the induction period of CaCO3 precipitated by mixing equalmolar solution of CaCl2 and Na2CO3 is investigated both experimentally and theoretically. In experimental, the induction period is measured by applying a conductivity method in which the induction period can be clearly estimated from the desupersaturation curve. The experimental results show that the induction period increases with a decrease in supersaturation, temperature and pH of solution. The presence of magnesium ion in solution prolongs the induction period. On the other hand, the addition of seed crystals in solution shortens the induction period. In theoretical, the induction period is calculated by using a rapid coagulation theory based on the attractive interactions
between clusters itself or clusters and seed crystals. The results of theoretical analysis indicate that the increase in induction period at lower supersaturation is caused by the smaller concentration of clusters and larger critical nuclei size. The increase in induction period at lower temperature is caused by the smaller diffusivity and lower concentration of clusters, and larger critical nuclei size. The coagulation rate constant of clusters is a function of temperature and independent of cluster size. The decrease in induction period at the presence of seed crystal is caused by the existence of a van der Waals force between the clusters and crystal seeds. The increase in induction period at the presence of Mg2+ is caused by the increase in interfacial tension of CaCO3 in solution and the interfacial tension can be expressed as a function of concentration of magnesium ions. The effect of pH on the induction period is caused by the changes in supersaturation and thus induction period. Further, the results also indicate that the interfacial tension of CaCO3 is about 68.4 erg/cm2 in the aqueous supersaturated CaCl2-Na2CO3 solution. The coagulation rate constant between clusters of CaCO3 is about 2.0 10-18 cm3/s at 25oC and the activation energy of coagulation between clusters is about 200 J/mole.
Finally, the metastable zone widths of CaCO3 at various induction periods can be plotted on the solubility-supersolubility diagram by using the correlation between the induction period and supersaturation. A comparison between the theoretical results and experimental data obtained by using the conductivity and visible methods shows that the present method is a useful tool for predicting the metastable zone width of sparingly soluble system.
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