Quantum chemical analysis of the structures of MgSO4 hydrates
Iype, E., Ozen, C., Nedea, S.V., Rindt, C.C.M., Zondag, H.A. (2012). Quantum chemical analysis of the structures of MgSO4 hydrates. Proceedings of the 12th International conference on Energy Storage (Innostock 2012), 16-18 May 2012, Lleida, Spain (pp. INNO-ST). Technische Universiteit Eindhoven.
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Magnesium sulfate salts can form hydrated compounds with up to seven degree of hydration
with an energy exchange of the order of 2.8GJ/m3 . In addition, this salt is abundant in nature
and thus this material is a potential candidate for storing energy in seasonal heat storage
systems. One of the main issues in using this material for seasonal heat storage system is its
slow kinetics and low extent of water take-up under normal atmospheric conditions . In
addition, the salt undergoes considerable changes in its crystalline structure during hydration
and dehydration, and often they encounter the formation of cracks and pores in the crystal
structure . This significantly affects the efficiency of the salt in storing energy and also
reusability of the material.
A molecular level investigation is necessary to understand the process of hydration and
dehydration in detail. Presence of an extensive network of hydrogen bonds in MgSO4.7H2O
crystal is identified by Allan Zalkin et al . Significant delocalization of hydrogen atoms
within the hydrogen bonds are reported in the study. The 7th water molecule in a hepta-hydrate
crystal is captured in the interstitial space within the crystals due to coulombic forces and they
are very easily removable. Thus modeling a stable molecule of magnesium sulfate hepta hydrate
is difficult. So we undertake the hexa hydrated magnesium sulfate to study the equilibrium
molecular structure. The hydrogen bonds present in the structure, which stabilizes the molecule,
is a focus of attention in this study. In addition, we report Natural Bond Orbital (NBO) 
charges of Mg and S as a function of degree of hydration in this study. The NBO analysis gives
information about electronic occupations in the molecule. In addition, the variation of the
natural charges give information about the nature of inters atomic interactions involved in the
hydration process of magnesium sulfates.
The hydration process is accompanied by a considerable amount of energy exchange with the
surroundings. In addition, significant changes in the crystal structure are predicted to happen
during hydration. The binding of a water molecule on a slab of magnesium sulfate will resemble
the hydration phenomena of a real crystal. Maslyuk et al  have performed such an analysis on
kieserite structures and found the influence of hydrogen bonds during hydration. A similar study
has done towards the last part of this account, which gives important information about
hydration process of magnesium sulfate crystal.