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

Salt crystals are not merely static rocks; they are dynamic structures that can breathe, holding water molecules within their very lattice. In the world of chemistry, these are known as hydrated salts, compounds where water is an essential, built-in component of the crystal itself, rather than just liquid trapped in a pore. This ability to absorb and release water is a powerful natural mechanism. When these salts absorb moisture, they release heat; when they dry out, they absorb it. This cycle makes them promising candidates for storing thermal energy, potentially capturing the sun's warmth during the day to heat a home at night. However, for this technology to work reliably over many years, scientists must understand exactly how these crystals lose their water. If the process damages the crystal structure or makes it impossible to rehydrate, the material fails. For decades, researchers have watched these crystals dry out from the outside, measuring how much weight they lose over time, but the internal story of how the water actually escapes has remained hidden.

A team of researchers at the University of Amsterdam has now peeled back the layers of this mystery by studying a common salt called sodium sulfate decahydrate, known in its hydrated form as mirabilite. Instead of just weighing the crystals, they used a combination of high-resolution cameras, lasers, and X-ray scanners to watch the process happen in real time, from the scale of the whole crystal down to the individual atoms. They discovered that the drying process is not a single, uniform event as previously thought. Instead, it unfolds in two distinct stages. It begins with a quiet, microscopic phase where tiny, circular spots of dry material suddenly appear on the surface of the wet crystal. These spots grow outward in two dimensions, like ripples spreading on a pond, before the drying front pushes deep into the crystal's interior. Only after this initial burst of surface activity does the process slow down and become controlled by the movement of water vapor escaping from the deep layers of the crystal.

The researchers found that the speed of this drying process is dictated by the humidity of the air surrounding the crystal. When the air is very dry, the water leaves quickly, but the resulting dry material forms a chaotic, jagged structure of tiny, needle-like crystals. When the air is more humid, the drying happens more slowly, allowing the dry crystals to grow into larger, rounder, and more orderly spheres. This difference in the final shape of the dry material is crucial because it determines how easily the crystal can absorb water again in the next cycle. If the dry crystals are too small and the pores between them are too tight, water vapor cannot penetrate easily, and the material becomes less efficient over time. The study revealed that the dry crystal shrinks by about 35 percent as it loses its water, creating a complex, layered internal structure with two different sizes of pores.

By comparing sodium sulfate to other salts, the team uncovered a rule that links the crystal's internal geometry to how it breaks apart. When a salt changes from a hydrated form to a dry form and the internal arrangement of its atoms shifts into a more symmetrical pattern, the crystal tends to break open in circular spots, as seen with sodium sulfate. However, if the symmetry remains the same, the crystal simply cracks in straight lines. This observation suggests that the way a crystal fractures during drying is a direct clue to the mechanism driving the reaction. The researchers propose that this connection between the crystal's shape and its breaking pattern could serve as a guide for predicting how other hydrated salts will behave, potentially helping engineers design better materials for energy storage without needing to test every single possibility in the lab.

The study also challenged a long-held belief about how these reactions proceed. For years, models assumed that the drying of a salt crystal was a steady, uniform process where the boundary between wet and dry moved inward at a constant pace. The new observations show that this view is incomplete. The initial, rapid formation of those circular dry spots is a critical step that happens so quickly and on such a small scale that it is invisible in standard weight-loss experiments. It is only by zooming in with advanced imaging techniques that this hidden nucleation phase becomes clear. The researchers confirmed that the rate-limiting step—the slowest part of the process that controls the overall speed—is the creation of empty spaces, or vacancies, where water molecules used to be. Once these vacancies form at the interface between the wet and dry sections, the water can escape, and the dry layer advances.

This work provides a clearer picture of the physical changes that occur when a salt crystal dries, moving beyond simple measurements of weight to a detailed understanding of the structural transformation. The findings suggest that the environment in which a salt dries fundamentally alters the architecture of the resulting material, which in turn affects its ability to function in a thermal storage system. By understanding that the process starts with a specific type of surface nucleation and is governed by the symmetry of the crystal structure, scientists can now look for these specific signs when evaluating new materials. The research does not solve every problem regarding thermal energy storage, but it offers a new lens through which to view the behavior of hydrated salts, turning a previously opaque process into a visible, understandable sequence of events.