Hydration and deliquescence behavior of calcium chloride hydrates

The phase transitions of calcium chloride between various hydrates and solid–liquid phase transitions are common in many natural and industrial processes. Recent studies have revealed some discrepancies in investigating the hydration and deliquescence of calcium chloride using different methods. In this study, water vapor sorption analysis and Raman measurements on CaCl₂·2H₂O and CaCl₂·6H₂O and their dehydration products were conducted. The results indicate two possible hydration sequences from lower hydrates to deliquescence at 298.15 K: (1) Hydration of the monohydrate to the dihydrate, followed by the formation of β-CaCl₂·4H₂O, ending with its deliquescence at 18.5 % RH; (2) Hydration of the monohydrate to the dihydrate, followed by the formation of α-CaCl₂·4H₂O and of the hexahydrate, ending with its deliquescence at 29 % RH. It was observed that the transition from pure dihydrate to β-CaCl₂·4H₂O occurs spontaneously, instead of hydration to the thermodynamically stable α-CaCl₂·4H₂O. The latter phase is only formed in the presence of crystal seeds of α-CaCl₂·4H₂O that remained after dehydration. Additionally, direct deliquescence of β-CaCl₂·4H₂O and thus absence of hydration to hexahydrate at 298.15 K is reported for the first time, which could be explained by the more similar lattice structure of CaCl₂·2H₂O (orthorhombic) and β-CaCl₂·4H₂O (monoclinic) than α-CaCl₂·4H₂O (triclinic). Apart from that, an explanation for the observed transformation sequence is proposed, considering the impact of the enhanced solubility of β-CaCl₂·4H₂O compared to the α-CaCl₂·4H₂O. The resulting water to salt ratio below six may contribute to the absence of CaCl₂·6H₂O formation. A Raman spectrum of CaCl₂·H₂O not reported previously is also provided.