| Summary: | 碩士 === 國立臺灣大學 === 化學研究所 === 104 === Amorphous calcium carbonate (ACC) is commonly regarded as a precursor of all well-crystallized calcium carbonate. In recent years, many investigations have focused on probing the structural ordering of ACC. However, it remains unclear whether this amorphous precursor has an explicit short-range structure. According to Colfen and Gebauer (2008), ACC precipitated from solutions at pH 8.75 and pH 9.80 have distinctive values of solubility product. Consequently, the concept that ACC can exhibit different structural types has been developed and partially corroborated by the 13C magic-angle spinning NMR in 2010.
In marine organisms such as sea urchin, ACC usually contains substantial amount of magnesium content, ranging from 5 to 45%. The exact amount of Mg depends on the growth state and phyla of the species. The presence of magnesium in ACC (Mg-ACC) would inhibit the crystallization of calcium carbonate and hence would greatly increase the stability of Mg-ACC. In a solution of carbonate salt at pH 8 to 9, most of the carbonate ions are protonated. Thus, the binding behavior of bicarbonate ions and other cations such as Ca2+ or Mg2+ could have substantial effect on the formation of Mg-ACC. Nonetheless, relatively few studies have been carried out to investigate this effect.
Previously, our laboratory successfully developed a method based on micromixing to prepare Mg-ACC with good reproducibility in Mg/Ca ratio. In this thesis, we predominantly focused on investigating the chemical composition and the homogeneity of Mg-ACC prepared from the mother liquid with different pH conditions. To further unravel the formation mechanism of Mg-ACC, we constructed a model based on the thermodynamic equilibrium. The Solid-state NMR data revealed that there are two types of carbonate ions in Mg stabilized amorphous calcium carbonate (ACC), whose short-range orders were identical to those of ACC and amorphous magnesium carbonate (AMC). 13C homonuclear correlation spectroscopy supported that Mg-ACC comprised a homogeneous mixture of the nano-clusters of ACC and AMC. Besides, the good agreement between the simulated results and the experimental data signified that the model constructed in this work is highly analogous with the real situations. Therefore, we could legitimately assume that the formation of Mg-ACC occurred through the process of informing the prunucleation cluster.
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