Quantum chemical analysis of the structures of MgSO4 hydrates

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 [1]. 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 [2]. 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 [3]. 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 [4]. 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) [5] 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 [6] 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.