Floppy hydrated salt microcrystals

TL;DR

This study reveals that hydrated salt microcrystals can become soft and deformable at their deliquescence point, exhibiting a unique combination of crystalline and liquid-like properties due to water molecules within their structure.

Contribution

It demonstrates the unexpected floppy, deformable behavior of hydrated salt microcrystals at deliquescence, highlighting the role of entrapped water molecules in their mechanical properties.

Findings

Hydrated salt microcrystals become soft and deformable at deliquescence.

The elasticity results from a balance between capillary energy and viscous flow.

This behavior suggests new applications in pharmaceuticals and energy storage.

Abstract

The crystalline structure of minerals due to the highly ordered assembly of its constituent atoms, ions or molecules confers a considerable hardness and brittleness to the materials. As a result, they are generally subject to fracture. Here we report that microcrystals of natural inorganic salt hydrates such as sodium sulfate decahydrate (Na2SO4.10H2O) and magnesium sulfate hexahydrate (MgSO4.6H2O) can behave remarkably differently: instead of having a defined faceted geometrical shape and being hard or brittle, they lose their facets and become soft and deformable when in contact with their saturated salt solution at their deliquescence point. As a result, the hydrated crystals simultaneously behaves as crystalline and liquid-like. We show that the observed elasticity is a consequence of a trade-off between the excess capillary energy introduced by the deformation from the equilibrium shape and viscous flow over the surface of the microcrystals. This surprising, unusual mechanical properties reported here reveals the role of the water molecules present in the crystalline structure. Although many compounds can incorporate water molecules in their crystalline frameworks, the relationship between the different hydrates and anhydrate crystal forms are still poorly investigated. Our results on the floppy behaviour of such crystalline structure reveals some unexplored properties of water molecules entrapped in the crystalline structure and can open novel routes for their application in various fields such as pharmaceutical sciences, thermal energy storage and even the traceability of water on Mars.