Pressure-Driven Water Release from Magnesium Sulfate Hydrates: Thermodynamic and Mechanistic Insights

Understanding the behavior of hydrated salts under pressure is essential for interpreting geochemical processes in planetary interiors and for developing (de)hydration-based technologies. In this study, we use density functional theory calculations to investigate the thermodynamics of pressure-induced dehydration in magnesium sulfate hydrates (MgSO₄·nH₂O, n = 11 and 7), where compression drives the release of water as dense ice polymorphs (such as ice II and VI) and the formation of hydrates with fewer water molecules. Our results show that dehydration becomes thermodynamically favorable at 0.8 GPa for MgSO₄·11H₂O and 1.1 GPa for MgSO₄·7H₂O, with ice VI emerging as the dominant crystallization product. Interaction energy analysis identifies interstitial, rather than metal-coordinated, water molecules as the dehydration initiation sites. Unlike thermal dehydration of MgSO₄·7H₂O, which proceeds via MgSO₄·6H₂O and water vapor, our calculations indicate that pressure-induced dehydration yields MgSO₄·5H₂O and dense ice. These results highlight distinct mechanisms of dehydration under temperature and pressure and provide insight into hydrate behavior relevant to both thermochemical technologies and planetary environments.