Softness of hydrated salt crystals under deliquescence

Deliquescence is an important phenomenon that occurs when a salt absorbs water vapor and slowly dissolves. Investigating the behavior of the salt sodium sulfate decahydrate (mirabilite) during deliquescence using different microscopy and spectroscopy techniques, we found something really remarkable: the mirabilite crystals didn’t just dissolve, they also got all soft and squishy. Next to that the surface of the crystal could actually heal itself if it got any indentations caused by impurities. We found that not all salts can do this trick – only the ones with a lot of water molecules trapped inside. This behavior is not observed in anhydrous salts like sodium chloride and sodium sulfate thenardite. These findings highlight the importance of crystalline water and its mobility in the behavior of hydrated salts, which could have potential applications in for example thermal energy storage.

Microscopy images made with cross polarizers of the deliquescence process of the sodium sulfate decahydrate (Na2SO4 · 10 H2O). The colors that are seen with cross-polarized light highlight the fact that what we are seeing is not a liquid structure.
Non-faceted mirabilite microcrystals are deformable and adapt their shape to neighboring crystals in the sodium sulfate solution

Evolution of surface defects. During deliquescence, while the entrapped thenardite dissolves, the indentation at the surface of the hydrated crystal is seen to level out rapidly.

Publication:

Wijnhorst, R., Demmenie, M., Jambon-Puillet, E., Ariese, F., Bonn, D., & Shahidzadeh, N. (2023). Softness of hydrated salt crystals under deliquescence. Nature Communications, 14(1), 1090.

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