Translational diffusion in supercooled water at and near the glass transition temperature-136 K

The properties of amorphous solid water at and near the calorimetric glass transition temperature, Tg, of 136 K have been debated for years. One hypothesis is that water turns into a "true" liquid at Tg (i.e., it becomes ergodic) and exhibits all the characteristics of an ergodic liquid, including translational diffusion. A competing hypothesis is that only rotational motion becomes active at Tg, while the "real" glass transition in water is at a considerably higher temperature. To address this dispute, we have investigated the diffusive mixing in nanoscale water films, with thicknesses up to ∼100 nm, using infrared (IR) spectroscopy. The experiments used films that were composed of at least 90% H2O with D2O making up the balance and were conducted under conditions where H/D exchange was essentially eliminated. Because the IR spectra of multilayer D2O films (e.g., thicknesses of ∼3-6 nm) embedded within thick H2O films are distinct from the spectrum of isolated D2O molecules within H2O, the diffusive mixing of (initially) isotopically layered water films could be followed as a function of annealing time and temperature. The results show that water films with total thicknesses ranging from ∼20 to 100 nm diffusively mixed prior to crystallization for temperatures between 120 and 144 K. The translational diffusion had an Arrhenius temperature dependence with an activation energy of 40.8 ± 3.5 kJ/mol, which indicates that water at and near Tg is a strong liquid. The measured diffusion coefficient at 136 K is 6.25 ± 1.4 × 10-21 m2/s.