Role of Coordination Motifs and Cluster Sizes in the Dehydration Thermodynamics of MgSO4·nH2O.

Dehydration of MgSO4·7H2O to anhydrous MgSO4 proceeds through nonstoichiometric or amorphous intermediates, making finite-cluster models essential for describing stepwise dehydration thermodynamics. Here, we present a cluster-based density functional theory (DFT) study of MgSO4·nH2O (n = 0-7) that directly links local coordination motifs to dehydration energetics. Low-energy cluster isomers were generated by global optimization and refined at the DFT level. Proton transfer from water to sulfate is observed in both small clusters (n = 1-2) and bulk-derived high-hydration clusters (n = 6-7) lacking direct Mg2+···SO4 2- contact. Reaction energies were calculated and equilibrium PH2O-T relationships were then constructed. Successive dehydration steps (MgSO4·7H2O ⇌ MgSO4·6H2O ⇌ lower hydrates ⇌ MgSO4) show a systematic increase in dehydration free energy with decreasing hydration, due to an increase of the ion-ion (Mg2+···SO4 2-) interactions over water···water hydrogen bonding. Monomeric clusters overestimate dehydration energies at low hydration due to undercoordinated Mg2+ centers, while dimer clusters partially restore Mg coordination and improve agreement with experimental trends. These results demonstrate that cluster-based DFT essentially captures structure-thermodynamics relationships in MgSO4 dehydration thermodynamics and clarify the strengths and limitations of the cluster approach.