Department of Food Science, University of Copenhagen, Rolighedsvej 26, DK-1958 Frederiksberg C, Denmark.
Food Res Int. 2021 Feb;140:109867. doi: 10.1016/j.foodres.2020.109867. Epub 2020 Nov 3.
Calcium citrate tetrahydrate (CCT) and hexahydrate (CCH) precipitates from aqueous solutions of CaCl and sodium citrate above and below the transition temperature of 52 °C, respectively. The CCT, the dihydrate (CCD) and anhydrate (CCA) as obtained by a stepwise dehydration of solid CCH have enthalpy of dehydration of ΔH = 43.6, ΔH = 43.8, and ΔH = 88.1 kJ∙mol as measured by DSC. WAXS measurements demonstrate a stepwise decrease in unit cell size upon dehydration, and a stronger binding of the two first water compared to additional. The increasing negative enthalpy of dissolution, as calculated from the temperature dependence of solubility (10-90 °C), +21 kJ∙mol (CCH), -20 kJ∙mol (CCT), -22 kJ∙mol (CCD), and -40 kJ∙mol (CCA) shows along the series of hydrates with increasing solubility, enthalpy-entropy compensation with an isoequilibrium temperature of 49 °C. Conversion of CCD and CCA in aqueous solutions yields the more soluble CCT, not the stable CCH in agreement with Ostwald's stage law, increasing calcium bioaccessibility under physiological conditions in intestines.
柠檬酸钙四水合物 (CCT) 和六水合物 (CCH) 分别从 CaCl 和柠檬酸钠的水溶液中在 52°C 以上和以下的转变温度沉淀出来。通过对固体 CCH 进行逐步脱水得到的 CCT、二水合物 (CCD) 和无水物 (CCA) 的脱水焓分别为 ΔH=43.6、ΔH=43.8 和 ΔH=88.1 kJ·mol-1，这是通过 DSC 测量得出的。WAXS 测量表明，随着脱水的进行，晶胞尺寸呈逐步减小的趋势，并且与额外的水相比，前两种水的结合更牢固。根据溶解度随温度的变化（10-90°C）计算出的溶解焓的负增量为+21 kJ·mol-1（CCH）、-20 kJ·mol-1（CCT）、-22 kJ·mol-1（CCD）和-40 kJ·mol-1（CCA），表明随着水合物系列中溶解度的增加，焓-熵补偿具有等平衡温度为 49°C。在水溶液中 CCD 和 CCA 的转化生成更易溶解的 CCT，而不是稳定的 CCH，这与奥斯特瓦尔德阶段定律一致，即在肠道的生理条件下增加了钙的生物利用度。
