Observation of Glass-to-liquid Transition of Water at Cryogenic Temperatures

Abstract

Water has more than two glassy states, including low-density amorphous (LDA) and high-density amorphous (HDA) ice. The glass-to-liquid transition in these polyamorphic forms of ice is the focus of theories proposed to explain anomalous properties of supercooled water. We studied phase behavior of HDA ice at cryogenic temperatures (80 – 160 K). The HDA ice was induced by rapidly cryocooling water (either in bulk state or in confined state such as in protein crystals) from room temperature to 77K under hydrostatic pressure (200 MPa). Using optical observation of crack-healing and in situ X-ray diffraction, we show that the glass-to-liquid transition of HDA ice in bulk state occurs before HDA transforms to LDA state (as low as 120 K). Using time-resolved X-ray diffraction, we show that the high-density state undergoes a first order phase transition to the low-density state. The same phase behavior was observed from the HDA ice confined in the high-pressure cryocooled protein crystals. Our findings support theories suggesting that LDA and HDA ice are thermodynamically distinct and they are continuously connected to two different liquid states of water.