Path-Dependent Hydration and Dehydration of CaCl₂

Area of Science:

• Materials Science
• Chemical Engineering
• Solid-State Chemistry

Background:

• Calcium chloride (CaCl2) is a widely used salt hydrate for heat storage due to its availability and high water uptake capacity.
• The phase transitions of CaCl2 hydrates, particularly involving the tritohydrate (CaCl2·1/3H2O), exhibit path dependency, complicating their application.
• Understanding these transitions is crucial for optimizing heat storage materials.

Purpose of the Study:

• To investigate the reasons behind the path dependency in CaCl2 hydrate phase transitions.
• To clarify the position of the tritohydrate in the CaCl2-water phase diagram.
• To elucidate the kinetic and structural factors influencing hydration and dehydration pathways.

Main Methods:

• Thermogravimetric analysis (TGA) was employed to study hydration and dehydration pathways under varying water vapor pressures.
• Crystalline structures of the tritohydrate and monohydrate were resolved using X-ray diffraction.
• Kinetic parameters and activation energies for phase transitions were analyzed.

Main Results:

• Hydration pathway observed: 0-1/3-2; Dehydration pathways: 2-1-0 (low vapor pressure) and 2-1/3-0 (high vapor pressure).
• The 1/3-1 transition exhibits poor kinetics, and the tritohydrate is metastable.
• Resolved crystal structures reveal significant structural rearrangements during the 1/3-1 transition, leading to high activation energy.

Conclusions:

• The path dependency of CaCl2 hydrate transitions is attributed to the metastability of the tritohydrate and slow kinetics of the 1/3-1 transition.
• High activation energy barriers for 1/3-1 hydration and 1-1/3 dehydration steps significantly impede these phase changes.
• This research provides critical insights into the behavior of CaCl2 hydrates, essential for designing efficient heat storage systems.