A multi-scale study to unravel the dehydration mechanism of hydrated salts

Abstract

Understanding the dehydration mechanism of hydrated salts remains fundamentally important in solid state chemistry, as their behavior affect fields ranging from heat storage to heritage conservation and pharmaceutical crystallization. Combining Raman confocal microscopy, dynamic weight loss measurements, SEM, and micro CT, we show that dehydration kinetics of sodium sulfate decahydrate (mirabilite) unfold through two regimes: an initial nucleation controlled phase, where atomic rearrangement drives two dimensional lateral growth following an exponential law, and a subsequent phase boundary controlled regime, where growth advances into the crystal depth, limited by water vacancy formation at the hydrated dehydrated interface. The resulting thenardite product shows around 35 percent shrinkage and forms a porous, layered dual porosity nanocrystalline structure, with relative humidity directly governing crystal size. Extending this analysis to other sulfate hydrates reveals that crystallographic symmetry changes between hydrate and anhydrous phases dictate surface morphology, linking microstructure to mechanism. This multiscale framework uncovers dynamics hidden from bulk measurements and suggests structural indicators could predict dehydration pathways in other hydrated salt families, and opening route toward designing thermal energy storage materials without exhaustive experimental screening.