Linking amorphous ice and supercooled liquid water

Water has fascinated humans for centuries, and, despite its central importance in biology, chemistry, and industry, we are still refining our understanding of its properties. One area of vigorous research over the last ∼50 y has been water’s supercooled liquid state—water cooled below the temperature at which ice would normally form, but maintained instead as a metastable liquid (1). Exploration of supercooled liquid water is particularly challenging because water’s propensity to crystallize increases dramatically as the temperature is decreased. One way that researchers have attempted to circumvent this issue is to study water in its amorphous solid or “glassy” states, as a way to obtain an approximate window into the supercooled liquid (2). However, a key open question in this body of work was whether the most common forms of amorphous ice (traditionally obtained by processes whose first step involves pressurizing crystalline ice at low temperature until the water molecules assume an amorphous structure; Fig. 1 A ) could be considered equivalent to a more traditional glass (most often prepared by rapidly quenching a liquid sample to low temperatures; Fig. 1 B ). Put another way, is amorphous ice actually thermodynamically connected to the liquid state, or is it, instead, just a mechanically collapsed crystal (3)? New work by Bachler et al. (4) in PNAS shows just that thermodynamic connection, by using state-of-the-art methods to prepare “hyperquenched glassy water” (HGW) by rapidly vitrifying samples of liquid water with minimal crystallization. They then performed calorimetric and X-ray diffraction experiments to demonstrate that HGW behaves identically to amorphous ices obtained by more traditional routes, strengthening the thermodynamic link between glassy water and the supercooled liquid state. Fig. 1. Amorphous ice preparation routes overlaid on a schematic metastable and glassy water phase diagram. ( A ) Crystalline ice Ih is pressurized at low temperature (solid arrow) to form HDA, which … [↵][1]1Email: tgartner{at}princeton.edu. [1]: #xref-corresp-1-1