Glass-to-cryogenic-liquid transitions in aqueous solutions suggested by crack healing

Abstract

Observation of theorized glass-to-liquid transitions between lowdensity amorphous (LDA) and high-density amorphous (HDA) water states had been stymied by rapid crystallization below the homogeneous water nucleation temperature (∼235 K at 0.1 MPa). We report optical and X-ray observations suggestive of glass-toliquid transitions in these states. Crack healing, indicative of liquid, occurs when LDA ice transforms to cubic ice at 160 K, and when HDA ice transforms to the LDA state at temperatures as low as 120 K. X-ray diffraction study of the HDA to LDA transition clearly shows the characteristics of a first-order transition. Study of the glass-toliquid transitions in nanoconfined aqueous solutions shows them to be independent of the solute concentrations, suggesting that they represent an intrinsic property of water. These findings support theories that LDA and HDA ice are thermodynamically distinct and that they are continuously connected to two different liquid states of water.glass-to-liquid transition | high-density amorphous ice | low-density amorphous ice | quenched HDA | first-order phase transition W ater has glassy states, including low-density amorphous (LDA) and high-density amorphous (HDA) ice (1-3). The glass-to-liquid transition in these polyamorphic forms of ice is the focus of theories proposed to explain anomalous properties of supercooled water (4-8). Although supporting experimental evidence exists (9-15), it remains controversial as the direct observation of a glass-to-liquid transition has been stymied by rapid crystallization below the homogeneous nucleation temperature (∼235 K at 0.1 MPa).In general, glasses are nonergodic, noncrystalline solids, in which atoms are fixed to their initial positions for macroscopically long periods of time (16). As is well known, when stress is applied, glasses can be cracked. Above the glass transition temperature, T g , the ergodic liquid is restored, and the stress-induced cracks in glasses can be healed by the diffusive motions of liquids. Indeed, it has been shown that the crack-healing process is reproducible and correlated with the glass transition temperatures, independent of liquid fragility (17, 18). ResultsBulk Water. In this study, crack healing is used to probe the molecular mobility in cryogenic transitions between glassy states of water or between glassy and crystalline states. Cracks do not heal for a sample held within any particular cryogenic solid state, indicating a low mobility in each of these states. Crack healing observed during phase transformations requires a high molecular mobility and is suggestive of an intermediate glass-to-liquid transition in the pathway between solid states. Fig. 1 shows the paths in a schematic phase diagram of water that were used to form and probe LDA and HDA ice. To facilitate vitrification of the bulk state of water, either NaK tartrate [sodium potassium tartrate, 0.9 M; mole fraction (moles salt/total moles) of 0.016 or hydration number R (moles H 2 O/moles salt) of 62] or NaCl (sodium chloride, 1.5 M; m...

Diffusive dynamics during the high-to-low density transition in amorphous ice

“…The HDA-to-LDA transition has been proposed to be a first-order phase transition (4)(5)(6)(7)(8) and could occur in the ultraviscous liquid state at temperatures above the glass transition (9). Similar observations have been reported by studying water confined in protein crystals and aqueous solutions (10,11), which indicate the occurrence of the transition between two viscous liquid states of water. However, there is contradicting evidence that the transition from HDA to LDA involves several intermediate forms (12,13) and it is questioned whether the amorphous ices are thermodynamically linked with supercooled water (14,15).…”

Glass polymorphism and liquid–liquid phase transition in aqueous solutions: experiments and computer simulations

“…By contrast, samples vitrified at ambient pressure do not show crack healing until the LDA water crystallizes. 281 As crack healing requires rather high molecular mobility this strongly reinforces the argument that HDA turns into a mobile liquid at 120 K while LDA turns into a mobile liquid just before crystallization near 150 K. [90][91][92][93] This is consistent with calorimetric glass transition temperatures for HDA 83 and LDA. 100 Unexpectedly, the amorphous solutions do not crystallize to ice I but rather to host-guest compounds (type I clathrates) in which the tartrate is trapped in ice cages.…”

“…The mobility of water molecules at the polyamorphic transition was further probed in pressure-vitrified sodium potassium tartrate solution 278 by manually cracking the frozen phase and monitoring its evolution. 281 The cracks persist as long as water is in the HDA state but crack healing is observed once the HDA -LDA transition is induced upon heating at E120 K and 1 bar. By contrast, samples vitrified at ambient pressure do not show crack healing until the LDA water crystallizes.…”

“…While not accessible in bulk water the FSDC at ambient pressure was inferred for protein hydration water. 247,249,250 Going beyond this dynamic transition, the work by Kim et al 248,[277][278][279][280][281] tackles structural transformations induced by pressure. When rapidly cooling a pressurized (E0.2 GPa) protein crystal, ice formation can be entirely avoided as indicated by X-ray studies.…”

“…248 This finding is striking, as such transitions have previously been observed only around 180-240 K. 284,285 Interestingly, at 110 K also the polyamorphic transition of solvent water occurs. 248,281 However, when avoiding the formation of HDA-type hydration water by cooling an identical protein crystal at ambient pressure, no increase in protein mobility is visible up to 180-240 K. Thus, it seems that the HDA -LDA transition in pressure-vitrified hydration water is accountable for the protein dynamical transition at cryogenic temperatures, whereas the mobility in LDA-type hydration water is not sufficient to trigger the transition at 110 K. One could speculate that this results from the water molecules being more mobile in the HDA than in the LDA state. Indeed, it has been shown that the dynamics in HDA near 110 K are two orders of magnitude faster than in LDA, 286 reaching the relaxation times of an ultraviscous liquid.…”