DFT study on characterization of hydrogen bonds in the hydrates of MgSO4

Magnesium salt hydrates are potential thermo-chemical energy storage materials considering its high energy storage density and its availability. However, in practical applications, these materials suffers 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 which is significantly influencing the chemical structure of the molecules. These hydrogen bonds appear to cause distortions in the hydrated structures and even hindering the coordination of water with magnesium resulting in lower energy isomers. In the case of hexa-hydrated isomers, the hydrogen bond stabilizes a conformation which 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 intra-molecular proton transfer reaction. This suggests that the strong hydrogen bond interactions potentially dissociates water molecules during hydration.