Liquid-like relaxation in hyperquenched water at ≤140 K

Abstract

Micrometre-sized water droplets were hyperquenched on a solid substrate held at selected temperatures between 150 and 77 K. These samples were characterized by differential scanning calorimetry (DSC) and X-ray diffraction. 140 K is the upper temperature limit to obtain mainly amorphous samples on deposition within 16–37 min. DSC scans of glassy water prepared at 140 K exhibit on heating an endothermic step assignable to glass → liquid transition, with an onset temperature (T_g) of 136 ± 2 K on heating at 30 K min⁻¹. For T_g of ≈136 K, water relaxes during deposition at 140 K for 16 min, moving towards metastable equilibrium. The apparent increase in heat capacity (ΔC_p) depends, for a given rate of heating, on the rate of prior cooling, and a so-called overshoot develops. 140 K deposits cooled at a rate of 5, 2 or 0.2 K min⁻¹ show on subsequent reheating at a rate of 30 K min⁻¹ ΔC_p values of 0.7, 1.1 and 1.7 J K⁻¹ mol⁻¹. This is consistent with liquid-like relaxation at 140 K, and it indicates that different limiting structures are obtained. When these 140 K deposits are in addition annealed at 130 K for 90 min, after slow-cooling at 5, 2 or 0.2 K min⁻¹, their ΔC_p values on subsequent reheating are similar to those of hyperquenched glassy water (HGW) deposits made at 77 K and annealed at 130 K. Thus, the previous ΔC_p value of 1.6 J K⁻¹ mol⁻¹ obtained with glassy water samples annealed at 130 K (A. Hallbrucker, E. Mayer and G. P. Johari, Philos. Mag. B, 1989, 60, 179) must be an upper-bound limit because it contains a contribution from an overshoot. The T_g value of 140 K deposits, which had relaxed during deposition towards metastable equilibrium, is within experimental error the same as that of 140 K deposits annealed in addition at 130 K. This contradicts Yue and Angell’s (Y. Yue and C. Angell, Nature, 2004, 427, 717) claim for assigning the endothermic step to a sub-T_g peak or a “shadow” T_g. Our new data further support the proposed fragile-to-strong transition on cooling liquid water from ambient temperature into the deeply supercooled and glassy state. We also describe in detail experimental aspects to obtain HGW specimens, show the ultrastructure of the deposits using electron microscopy, and discuss the mechanism of our hyperquenching method.