DFT Study on Characterization of Hydrogen Bonds in the Hydrates of MgSO₄

Magnesium salt hydrates are potential thermo-chemical energy storage materials considering their high energy storage density and their availability. However, in practical applications, these materials suffer from low efficiency due to their sluggish kinetics and significant structural changes during hydration and dehydration. A DFT PW91-TZ2P level optimization is performed on the various hydrates of magnesium sulfate molecules to study their structural properties. The study identifies a wide network of hydrogen bonds that is significantly influencing the chemical structure of the molecules. These hydrogen bonds appear to cause distortions in the hydrated structures and even hinder the coordination of water with magnesium, resulting in lower-energy isomers. In the case of hexahydrated isomers, the hydrogen bond stabilizes a conformation that has only four coordinated water molecules and is energetically more stable than the conformation with six coordinated water molecules. The sluggish hydration kinetics in magnesium sulfate is attributed to the strong hydrogen bond network present in the crystals. In addition, the hexahydrated structure exhibits an intramolecular proton-transfer reaction. This suggests that the strong hydrogen bond interactions potentially dissociate water molecules during hydration.