Dynamics of Deeply Cooled Confined Water

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 experimental studies in this temperature range. Therefore, we confine water to silica pores or mix it with alcohols or salts to reduce the crystallization propensity in experiments and we exploit high cooling rates available in simulations. As for the dynamics of cooled water, previous experimental and computational studies reported dynamic crossovers but the origin, in particular, the relation to a phase transition between high-density (HDL) and low-density (LDL) liquids, is still controversial. In experimental work, we combine nuclear magnetic resonance (NMR), broadband dielectric spectroscopy (BDS), and differential scanning calorimetry (DSC) to investigate the effect. For confined water, NMR and BDS results indicate a dynamic crossover at ~220 K, which is largely independent of the pore diameter. Moreover, 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. NMR field-gradient diffusion measurements inform that confined water shows long-range translational motion above and below the crossover temperature. 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 temperature is approached. Molecular dynamics simulations (MDS) further add to our understanding of cooled water in bulk, confinement, and mixtures. The results show that pore walls and molecular surfaces cause strongly distorted structures and slowed dynamics of water in the neighborhood. The range of these effects is small but increases upon cooling. As a consequence, bulk-like behavior is probed at sufficiently large distances and high temperatures, while such interfaces significantly alter the behavior of intimately mixed systems at low temperatures.