Hydration level dependent dynamics of supercooled water confined in water harvester metal organic framework-303

Abstract

Crystallization of bulk water cannot be avoided in the temperature range 150–235 K, which is known as no man’s land in the literature focused on the dynamics of the supercooled water. Dynamics of the supercooled water in this temperature range can be accessed by confining water in the nanopores. In the present study, we have investigated dynamics of the supercooled water confined in the pore network of a water harvester metal organic framework (MOF-303) at two different hydration levels (63.4 and 37.0 %). The framework was chosen as a confining host, because the hydrogen bonded–structure of the confined water molecules during adsorption–desorption (i.e. at different hydration levels) is explicitly known. All the relaxation processes observed in the supercooled region are comparatively slower than the bulk–like water dynamics due to the modified hydrogen-bonded structure and the interactions with the pore walls. The confined water, not directly interacting with the pore walls, undergoes a liquid-to-liquid transition (Ts) at ∼186 K followed by its glassy freezing with glass transition temperature Tg ∼ 108 K which is consistent with high density liquid phase of amorphous ice. Both Ts as well as Tg are observed to increase with the decrease in hydration level. It confirmed that the evolved hydrogen-bonded network and the interactions with the pore walls play deterministic roles in the glassy freezing of the confined water. No crystallization as well as the glassy freezing of the water molecules directly interacting with the pore walls through the hydrogen bonding is observed at both the hydration level. The observed relaxation processes corresponding to these molecules in the supercooled region can be attributed to rearrangement of their hydrogen-bonded structure.