The glass transition of confined water.

What is it about?

Water can turn into a glass-like state (vitrification) when cooled below its glass transition temperature (Tg). Understanding this behaviour is vital for preserving organs, freezing food, protecting buildings from frost damage, and studying atmospheric water. However, specifying the exact temperature at which the glass transition occurs for water is challenging. Our study investigated how water behaves when confined in tiny spaces (with sizes between 0.3 and 2.5 nanometers) to determine its Tg. We found that water in these confined spaces transitions to a glassy state between 170 K and 200 K, depending on the confinement size and how the water interacts with its surroundings. We also clarified that a previous observation near 136 K is not the actual Tg but a related phenomenon called a "shadow glass transition." By connecting these findings to the behaviour of bulk (unconfined) water, we provide new insights into how water vitrifies. This could lead to better technologies and methods like cryopreservation and climate science.

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Why is it important?

The current understanding of the glass transition temperature (Tg) in confined water is highly debated. This study resolves this controversy by identifying three distinct contributions to the calorimetric response of supercooled confined water. We demonstrate that confined water has a Tg in the 170 to 200 K range, depending on the confinement size and wall interactions.