Supercooled water confined in a metal-organic framework

Within the so-called “no-man’s land” between about 150 and 235 K, crystallization of bulk water is inevitable. The glass-like freezing and a liquid-to-liquid transition of water, predicted to occur in this region, can be investigated by confining water in nanometer-sized pores. Here, we report the molecular dynamics of water within the pores of a metal-organic framework using dielectric spectroscopy. The detected temperature-dependent dynamics of supercooled water matches that of bulk water as reported outside the borders of the no-man’s land. In confinement, a different type of water is formed, nevertheless still undergoing a glass transition with considerable molecular cooperativity. Two different length scales seem to exist in water: a smaller one, of the order of 2 nm, being the cooperativity length scale governing glassy freezing, and a larger one (> 2 nm), characterizing the minimum size of the hydrogen-bonded network needed to create “real” water with its unique dynamic properties. When water is held in a porous material and cooled below its freezing point the typical molecular interactions leading to crystallisation can be inhibited leading to a supercooled liquid state. Here, the authors use dielectric spectroscopy to analyse a metal organic framework with a symmetrical nanometer sized porous structure and reveal the temperature dependent processes and how they compare with those of bulk water.