We ascertain the properties of liquid water through the no man's land, which are key to understanding its anomalies. For bulk water, crystallization hampers measurements in this temperature range. Therefore, we confine water to silica pores or mix it with alcohols or salts to reduce crystallization propensity in experiments and we exploit fast cooling available in simulations. As for dynamics of water, previous experimental and computational studies reported dynamic crossovers but the origin, in particular, the relation to a phase transition between high-density and low-density liquids, is still controversial. In experimental work, we combine nuclear magnetic resonance (NMR), broadband dielectric spectroscopy (BDS), and differential scanning calorimetry to investigate the effect. For confined water, NMR and BDS results indicate a dynamic crossover at ~220 K, which is independent of the pore diameter. Also, the rotational correlation times agree with that of bulk water above and below the no man's land. However, various findings indicate that there is a homogeneous phase in the weakly cooled regime but not in the deeply cooled one. Explicitly, we attribute the dynamic crossover at ~220 K to an emergence of very small and highly distorted ice nuclei, which are unstable and exist at equilibrium with liquid water. Consistently, NMR field-gradient diffusometry informs that confined water continues to show long-range translational motion below 220 K. A universal nature of water dynamics at low temperatures is indicated by NMR and BDS results for aqueous alcohol and salt solutions, which show that the same process exists also in these mixtures and decouples from the structural relaxation when the glass transition is approached. Molecular dynamics simulations (MDS) show that pore walls, molecular surfaces, and ice nuclei cause strongly distorted structures and slowed dynamics of neighboring water. The range of the interface effects is small but increases upon cooling. Thus, bulk-like behavior is probed at sufficiently large distances and high temperatures, while interfaces alter water properties in intimately mixed systems at low temperatures.

Dynamics of cooled water in bulk, mixtures, and confinements: Combined experimental and computational studies

2019 Fall National Meeting

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