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

Abstract

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.

A DFT based equilibrium study on the hydrolysis and the dehydration reactions of MgCl2 hydrates

“…Thus, hydrogen bonds appear to have a small influence on the structures of MgCl 2 hydrates, but not as much as compared to MgSO 4 hydrates. 7 Also, the hydrates of MgCl 2 appear to be noncomplex without the presence of any unusual low energy isomers. This difference in the influence of hydrogen bonds to form meta-stable states may possibly be regarded as the reason for the faster hydration kinetics in MgCl 2 as observed by Ferchaud et al 41 in comparison with MgSO 4 .…”

“…6). This method is, successfully, applied for studying molecular systems involving magnesium, 7 chlorides, 33,34 and hydrogen bonds. 35 A spinrestricted Kohn-Sham method is used with a doubly polarized triple-ζ basis set, while keeping the integration accuracy to the maximum throughout the calculation.…”

“…Molecular structures of MgSO 4 hydrates have been subject of investigation in the literature, [6][7][8][9][10][11][12][13] especially the presa) Electronic mail: s.v.nedea@tue.nl ence of strong hydrogen bond networks catalyzing the formation of meta-stable states. 6,7,14 However, fewer accounts exist on the molecular structure of MgCl 2 hydrates.…”

“…6,7,14 However, fewer accounts exist on the molecular structure of MgCl 2 hydrates. Crystalline structures of the hygroscopic magnesium chloride hexa-hydrate (also known as Bischofite) are reported from Neutron Diffraction studies by Agron and Busing.…”

Molecular dynamics study on thermal dehydration process of epsomite (MgSO4·7H2O)

“…A molecular-scale investigation into the dehydration process seems necessary in order to unveil these details and to identify the factors that affect the diffusion through the surface layers. Iype et al [11] quantified the energies of distinct magnesium sulphate hydrates [11] and identified various hydrogen-bonding networks [12] from density functional theory (DFT) calculations. These calculations show that there is no significant charge transfer during the dehydration process, which opened up the possibility of performing a largescale non-reactive molecular dynamics (MD) simulation.…”