Hyperquenched Water Research Articles

Structural relaxation of water during rapid cooling from ambient temperatures.

Experiments investigating the properties of deeply supercooled liquid water are needed to develop a comprehensive understanding of water's anomalous properties. One approach involves transiently heating nanoscale water films into the supercooled region for several nanoseconds at a time and then interrogating the water films after they have quenched to cryogenic temperatures. To relate the results obtained with this approach to other experiments and simulations on supercooled water, it is important to understand how closely the quenched structure tracks the (metastable) equilibrium structure of water as a function of the transient heating temperature. A key step involves quantifying the extent to which water that is transiently heated to ambient temperatures [hyperquenched water (HQW)] subsequently relaxes toward the structure of low-density amorphous (LDA) ice as it cools. We analyzed the infrared reflection-absorption spectra of LDA, HQW, and crystalline ice films to determine their complex indices of refraction. With this information, we estimate that HQW retains ∼50%-60% of a structural motif characteristic of water at high temperatures with the balance comprised of a low-temperature motif. This result, along with results from x-ray diffraction experiments on water and amorphous ices, allows one to quantify the fraction of the high-temperature motif at approximately zero pressure as a function of temperature from 150 to 350K.

Comment on “Hyperquenched Glassy Water and Hyperquenched Glassy Ethanol Probed by Single Molecule Spectroscopy”

Comment on “Hyperquenched Glassy Water and Hyperquenched Glassy Ethanol Probed by Single Molecule Spectroscopy”

Hyperquenched Glassy Water and Hyperquenched Glassy Ethanol Probed by Single Molecule Spectroscopy

It is still unclear whether hyperquenched water (i.e., amorphous glassy water) heated to about 140-150 K remains glassy until it crystallizes near 154 K or whether instead it turns into a supercooled and very viscous liquid. It has been proposed that the glass transition temperature (T(g)) for water is 165 K and not, as previously thought, 136 K [V. Velikov et al., Science, 294, 2335 (2001)]. Support for both interpretations exists in the literature, since the T(g) of water is difficult to measure due to the formation of metastable cubic ice (I(c)) near 154 K. To address the nature of water in the 110-160 K temperature range, a confocal microscopy approach is used to study whether single-probe molecules (i.e., Rhodamine 700, Rh-700) embedded in hyperquenched glassy water (HGW) rotate in the temperature range of 110-160 K. If T(g) is 136 K and the liquid above this temperature is fragile (or strong with the fragility index m > 7), then rotation of the Rh-700 molecules should be observed several degrees above T(g). It is shown that no anticorrelated fluorescence intensity changes of single molecules in HGW (when excited at orthogonal polarizations) were observed up to temperatures of 160 K, although such changes were detected in control experiments performed for hyperquenched glassy ethanol (fragile liquid) at 99 K, that is, at T(g) + 2 K. The viscosity at which single Rh-700 molecules rotate in ethanol at 99 K is estimated to be about 10(12) poise. Since single-molecule spectroscopy did not reveal any rotation of probe molecules in HGW above 136 K, we conclude that water above 136 K is most likely a solid (i.e., glass), supporting the assignment that water remains glassy until it crystallizes near T = 154 K. It cannot be excluded, of course, that the value of m for water is smaller than 7 (i.e., water above 136 K could be an extremely strong liquid), but this possibility is considered unlikely as this would make water the strongest liquid ever known.

Glass transition in hyperquenched water? (reply)

The alternating support for and denial of a glass transition for amorphous water at 136 K has resumed after a hiatus of 20 years, during which it seemed secure. We revived the alternative interpretation1 by looking again at the calorimetric signal that previously provided the most direct evidence for the glass transition2,3 — and now Kohl . 4 present new data to support the original interpretation. We show here that their results are also consistent with our conclusions.

Glass transition in hyperquenched water?

It has been unclear whether amorphous glassy water heated to around 140-150 K remains glassy until it crystallizes or whether instead it turns into a supercooled and very viscous liquid. Yue and Angell compare the behaviour of glassy water under these conditions to that of hyperquenched inorganic glasses, and claim that water stays glassy as it heats up to its crystallization point; they also find a 'hidden' glass-to-liquid transition at about 169 K. Here we use differential scanning calorimetry (DSC) heating to show that hyperquenched water deposited at 140 K behaves as an ultraviscous liquid, the limiting structure of which depends on the cooling rate--as predicted by theoretical analysis of the liquid-to-glass transition. Our findings are consistent with a glass-to-liquid transition-onset temperature (T(g)) in the region of 136 K (refs 3,4), and they indicate that measurements of the liquid's properties may clarify the anomalous properties of supercooled water.

Liquid-like relaxation in hyperquenched water at ≤140 K

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 approximately 136 K, water relaxes during deposition at 140 K for 16 min, moving towards metastable equilibrium. The apparent increase in heat capacity (deltaC(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) deltaC(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 deltaC(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 deltaC(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.

The glass transition of water, based on hyperquenching experiments.

The glass transition temperature (Tg) in water is still uncertain, with conflicting values reported in the literature. As with other hyperquenched glasses, water exhibits a large relaxation exotherm on reheating at the normal rate of 10 kelvin (K) per minute. This release of heat indicates the transformation of a high enthalpy state to a lower one found in slow-cooled glasses. When the exotherm temperature is scaled by Tg, the good glass-formers show a common pattern. However, for hyperquenched water, when this analysis is performed using the commonly accepted Tg = 136 K, its behavior appears completely different, but this should not be the case because enthalpy relaxation is fundamental to the calorimetric glass transition. With Tg = 165 +/- 5 K, normal behavior is restored in comparison with other hyperquenched glasses and with the binary solution behavior of network-former systems (H2O, ZnCl2, or BeF2 plus a second component). This revised value has relevance to the understanding of water- biomolecule interactions.

Measurement of glass transition temperatures of freeze-concentrated solutes by differential scanning calorimetry.

Thermal analysis of aqueous solutions in which the solute does not crystallize immediately upon freezing was carried out to define the effects of experimental parameters on thermograms in the glass transition region. The intensity of enthalpy relaxations in the glass transition region is related to both the rate of cooling and the rate of heating through the glass transition region--slow cooling or slow heating increases the extent of structural relaxation in the glassy state and increases the intensity of the endotherm. Plots of the logarithm of heating rate versus 1/Tg' are linear, and activation enthalpies for structural relaxation are in the range of 210-350 kJ/mol. For polymeric solutes, both the activation enthalpies for structural relaxation and the heat capacity change accompanying the glass transition increase with increasing molecular weight of the solute. Molecular weight dependence of the observed midpoint of the glass transition agrees with the Fox-Flory relationship. Results are compared and contrasted with glass transitions in solid polymers and with the glass transition of hyperquenched water. Practical implications for characterization of formulations intended for freeze-drying are discussed.

A defect theory for the glass transition and residual entropy of hyperquenched water

The ground state structure of thermodynamically equilibrated metastable amorphous water at 0 K is considered as a continuous random network of H-bonded water molecules. At higher temperatures, this structure contains a fixed number of broken H bonds, or frozen-in fluctuations of local density, termed defects, where molecules have a considerable degree of configurational freedom to allow viscous flow. Their concentration has been calculated from the known change in the heat capacity at its Tg of 136 K, and statistical thermodynamics. The configurational enthalpy and entropy of a thermodynamically equilibrated glassy water, according to this theory, become zero at 0 K, and not at 20%–30% below its Tg, and lead to a residual entropy of 0.20 J/K/mol for the 0.23% defects containing structure. This is in addition to the entropy of R ln(3/2) due to the orientational disorder, as in ice. Any attempt to include an extra configurational enthalpy leads to a further lower value for residual entropy. The metastable equilibrium at different temperatures and the relaxation towards this equilibrium structure have been expressed in terms of free energy-defect concentration diagrams for glassy water.

Characterizing amorphous and microcrystalline solids by calorimetry

A two-step calorimetric test can be used to distinguish a truly amorphous from a microcrystalline material, when diffractograms do not allow a distinction between them. Results on hyperquenched water and a metal alloy illustrate the usefulness of this method.

The glass–liquid transition of hyperquenched water

Non-crystalline solid forms of water prepared by the usual two methods of vapour-deposition on a substrate1,2 and by compression of hexagonal ice in a piston cylinder apparatus at 77 K (refs 3,4) do not seem to undergo a glass–liquid transition on heating2,5–7. Neither of these two, or possibly three, non-crystalline forms seem to be interconvertible by a single thermodynamic path involving only temperature or pressure. Also, their molecular structures are thermodynamically discontinuous8, with the structure of bulk water above 273 K and of emulsified water in the supercooled state near 230 K (see also refs 2 and 9). It has been difficult, therefore, to resolve whether or not water supercools to a glassy state in a thermodynamically reversible manner. We now report the results of a calorimetric study of the glassy state of water obtained by rapid cooling or by hyperquenching, demonstrate the reversibility of its glass–liquid transition in the temperature range 113–148 K, and consider the implication to our understanding of its other non-crystalline solid forms.