Hyperquenched Glassy Bulk Water: A Comparison with Other Amorphous Forms of Water, and with Vitreous but Freezable Water in a Hydrogel and on Hydrated Methemoglobin

Abstract

The calorimetric glass→liquid transition behaviour of the amorphous forms of water is reviewed: for a heating rate of 30 K min− 1 the onset temperature, or Tg, of the glass→liquid transition is 136±1 K for glassy “bulk” water prepared by hyperquenching of micrometer-sized droplets and for annealed vapour-deposited amorphous solid water, and 129±1 K for the low-density form of pressure-amorphized hexagonal ice. The increase in heat capacity in the glass→liquid transition region is between 1.6–2 J K− 1 mol− 1 for the three amorphous forms which is similar to those of the typical network liquids SiO2 and GeO2. The differences in Tgs are not recognizable by diffraction which shows similar local environment for the three amorphous forms. Studies of water in confined geometries show that the calorimetric glass→liquid transition behaviour is a sensitive indicator for deviation from “bulk” structure: for both water embedded in a PHEMA hydrogel and for hydrated methemoglobin (for hydration range from ≈0.4-≈0.7 g water/g protein), the vitreous but freezable water fraction has a Tg of between 163–170 K, and an increase in heat capacity at Tg of ≈30 J K− 1 mol− 1. This suggests similar perturbation by the synthetic hydrogel and the protein.