Quantum chemical analysis of the structures of MgSO4 hydrates

Conference Contribution

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 [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.