Deliquescence is a first-order phase transition, happening when a salt absorbs water vapor. This has a major impact on the stability of crystalline powders that are important for example in pharmacology, food science and for our environment and climate. Here we show that during deliquescence, the abundant salt sodium sulfate decahydrate, mirabilite (Na₂SO₄·10H₂O), behaves differently than anhydrous salts. Using various microscopy techniques combined with Raman spectroscopy, we show that mirabilite crystals not only lose their facets but also become soft and deformable. As a result, microcrystals of mirabilite simultaneously behave crystalline-like in the core bulk and liquid-like at the surface. Defects at the surface can heal at a speed much faster than the deliquescence rate by the mechanism of visco-capillary flow over the surface. While magnesium sulfate hexahydrate (MgSO₄⋅6H₂O) behaves similarly during deliquescence, a soft and deformable state is completely absent for the anhydrous salts sodium chloride (NaCl) and sodium sulfate thenardite (Na₂SO₄). The results highlight the effect of crystalline water, and its mobility in the crystalline structure on the observed softness during deliquescence. Controlled hydrated salts have potential applications such as thermal energy storage, where the key parameter is relative humidity rather than temperature.

潮解是一级相变，发生在盐吸收水蒸气时。这对结晶粉末的稳定性有重大影响，例如在药理学、食品科学以及我们的环境和气候中很重要。我们在这里表明，在潮解过程中，丰富的十水硫酸钠、芒硝 (Na ₂ SO ₄ ·10H ₂O), 行为不同于无水盐。我们使用各种显微技术与拉曼光谱相结合，表明芒硝晶体不仅失去了它们的刻面，而且变得柔软且可变形。结果，芒硝的微晶同时在核心块体中表现为结晶状，在表面表现为液体状。表面的缺陷可以通过表面上的粘毛细管流动机制以比潮解速率快得多的速度愈合。虽然六水硫酸镁 (MgSO ₄ ⋅ 6H ₂ O) 在潮解过程中表现相似，但无水盐氯化钠 (NaCl) 和硫酸钠芒硝 (Na ₂ SO ₄). 结果突出了结晶水及其在晶体结构中的流动性对潮解过程中观察到的柔软度的影响。受控水合盐具有潜在的应用，例如热能储存，其中关键参数是相对湿度而不是温度。