A Crystal Dries Out Like a Lake ✛

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A lake in a drought doesn't dry up all at once. First, islands of dry land appear on the water's surface, they expand and merge, and only then does the water recede from the exposed bed. A mirabilite crystal behaves similarly when it loses water: round patches of anhydrous salt appear on its surface, growing outward and then beginning to deepen.

Chemists call this dehydration: sodium sulfate decahydrate (Na₂SO₄·10H₂O) gives up water and turns into anhydrous thenardite (Na₂SO₄). Previously, it was thought that the entire process could be described by a single simple model, but now it turns out that behind the averaged mass-loss curve hide two distinct regimes.

The transition between regimes depends on the proximity of patches: isolated islands grow outward for longer, while those surrounded by neighbors start growing inward sooner. After complete water loss, the crystal shrinks by about a third and acquires a layered structure with two types of pores: large ones between layers and small ones within them.

Air humidity controls the size of thenardite nanocrystals. At low humidity, they are small and elongated; at high humidity, they are larger and more isotropic. Comparison with other crystal hydrates showed that if the crystal system changes upon drying, round patches appear on the surface, whereas if the system remains the same, linear cracks form.

This link between crack shape and dehydration mechanism allows one to predict, from a single glance at the crystal surface, which process limits the reaction. For developing thermal storage devices based on salt hydrates, this offers a quick way to screen out unsuitable materials without lengthy kinetic experiments.