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ScienceWeek
PHYSICAL CHEMISTRY: ON THE GLASS TRANSITION OF WATER
The following points are made by Y. Yue and C.A Angell (Nature 2004 427:717):
1) The formation of glasses is normal for substances that remain liquid over a wide temperature range (the "good glassformers") and can be induced for most liquids if cooling is fast enough to bypass crystallization. During reheating but still below the melting point, good glassformers exhibit glass transitions as they abruptly transform into supercooled liquids, whereas other substances transform directly from the glassy to the crystalline state. Whether water exhibits a glass transition before crystallization has been much debated over five decades(1-5). For the last 20 years, the existence of a glass transition at 136 K (ref. 3) has been widely accepted(2-4), but the transition exhibits qualities difficult to reconcile with our current knowledge of glass transitions(2,5).
2) Glassy water is the most abundant form of water in the Universe, found as thin films condensed on the interstellar dust particles that constitute the major component of comets. This material, amorphous solid water (ASW), like hyperquenched glassy water (HQGW), is in a configurationally excited state (or state of high fictive temperature), relative to glasses formed by standard (20 K/min) cooling. To release the excitation enthalpy, the glass is usually annealed at temperatures where it does not crystallize. There is controversy about the state to which it is able to relax before crystallization occurs(1-5).
3) Until recently it was believed that ASW, HQGW and low-density amorphous water (LDA) all reach the internally equilibrated (or metastable liquid) state, so that it is a viscous liquid that is crystallizing. Observation of a weak endothermic effect at 136 K attributed to a glass transition(3), penetration of millimeters-thick samples of LDA water (a state close in character to ASW) by a blunt probe at a temperature 14 K above the LDA glass transition temperature Tg of 129 K, and liquid-like diffusion at 150 K support this view. Water, it was concluded, is a very fragile liquid, not only near its melting point, but also near the glass transition temperature.
4) In summary: The authors report detailed calorimetric characterizations of hyperquenched inorganic glasses that, when heated, do not crystallize before reaching their glass transition temperatures. The authors compare their results to the behavior of glassy water and find that small endothermic effects, such as the one attributed to the glass transition of water, are only a "shadow" of the real glass transition occurring at higher temperatures, thus substantiating the conclusion that the glass transition of water cannot be probed directly.
References (abridged):
1. Pryde, J. A. & Jones, G. O. Properties of vitreous water. Nature 170, 635 639 (1952)
2. Angell, C. A. Liquid fragility and the glass transition in water and aqueous solutions. Chem. Rev. 102, 2627 2649 (2002)
3. Johari, G. P., Hallbrucker, A. & Mayer, E. The glass transition of hyperquenched glassy water. Nature 330, 552 553 (1987)
4. Johari, G. P. Does water need a new Tg? J. Chem. Phys. 116, 8067 8073 (2002)
5. MacFarlane, D. R. & Angell, C. A. Nonexistent glass transition for amorphous solid water. J. Phys. Chem. 88, 759 762 (1984)
Nature http://www.nature.com/nature
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CAGE REARRANGEMENTS AND THE COLLOIDAL GLASS TRANSITION
The following points are made by E.R. Weeks and D.A. Weitz (Phys. Rev. Lett. 2002 89:095704):
1) Many liquids undergo a glass transition when rapidly cooled, where their viscosity grows by orders of magnitude for only modest decreases in temperature. This drastic increase in viscosity is unaccompanied by significant structural changes; instead, the dynamics slow dramatically. Physically, this slowing of the dynamics reflects the confinement of any given particle by a "cage" formed by its neighbors; it is the rearrangement of the cage which leads to the final structural relaxation, allowing the particle to diffuse through the sample [1]. The dynamics of cages have been studied with scattering measurements, which probe a spatial and temporal average of their behavior, and with computer simulations; however, in real systems, the actual motion of the individual particles involved in cage dynamics and breakup are still poorly understood [1-5].
2) The authors report a study of the motion of the individual particles and their neighbors during cage breakup, and provide the first direct experimental visualization of this process. The authors use confocal microscopy to study the motion of colloidal particles in a dense suspension, an excellent model for the glass transition. The authors use sterically stabilized poly-methylmethacrylate) particles with a radius of 1.18 microns. The rearrangement of cages involves the cooperative motion of neighboring particles [2-5]. While most neighboring particles move in similar directions, a significant minority move in opposite directions, resulting in local changes in topology. The authors also find that the more mobile particles are located in regions with a lower local volume fraction, and higher disorder. The authors suggest these measurements provide a direct, quantitative physical picture of the nature of cage rearrangements.
References (abridged):
1. M.D. Ediger, C. A. Angell, S.R. Nagel, J. Phys. Chem. 100, 13 200 (1996); C. A. Angell, J. Phys. Condens. Matter 12, 6463 (2000).
2. W.K. Kegel and A. van Blaaderen, Science 287, 290 (2000).
3. E. R. Weeks, J. C. Crocker, A. C. Levitt, A. Schofield, and D. A. Weitz, Science 287, 627 (2000).
4. W. Kob, C. Donati, S.J. Plimpton, P.H. Poole, S.C. Glotzer, Phys. Rev. Lett. 79, 2827 (1997).
5. C. Donati et al, Phys. Rev. Lett. 80, 2338 (1998).
Phys. Rev. Lett. http://prl.aps.org
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PHYSICAL CHEMISTRY: THE GLASS TRANSITION OF WATER
The following points are made by Dennis D. Klug (Science 2001 294:2305):
1) Few known simple molecular systems can rival the complexity of the water phase diagram. Water boasts numerous solid phases and may even form two different liquid phases at low temperatures. But pinning down the exact nature of the different phases and the transitions between them has proved difficult.
2) One of the intractable properties of water is the temperature at which water changes from a liquid to a glassy state. The glass transition temperature is usually defined as the temperature at which the liquid becomes very viscous and essentially a quenched liquid upon cooling, or as the temperature at which the solid-like glass transforms to a liquid upon heating. This is an operational rather than a thermodynamic definition, because the glass transition temperature depends, for example, on the rate at which the liquid is cooled. On timescales of a picosecond, even liquid water at room temperature is quite hard.
3) Experimental studies of the liquid and amorphous phases of water indicate a highly complex behavior. Several theories suggest the possible existence of two distinct liquid water phases, a liquid-liquid phase transition upon cooling, and a liquid-liquid critical point in the low-temperature region of the phase diagram. The glass transition may occur in one of these liquid forms of water if the theories are correct. The location of the glass transition defines the region where one can search for the low-temperature liquid, the liquid-liquid transition, and the proposed second critical point. The glass transition of water is also of interest in the context of cryoprotection processes and biological organisms at low temperature.
Science http://www.sciencemag.org
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ON THE DYNAMIC GLASS TRANSITION
The following points are made by T.S. Grigera et al (Phys. Rev. Lett. 2002 88:055502):
1) Despite a large number of investigations, there is still much to understand about the dynamic glass transition in supercooled liquids. The basic problem is that, strictly speaking, there is no dynamic transition at all. In systems known as fragile liquids, experiment reveals a sharp rise of the viscosity in a very narrow interval of temperature upon cooling. The shear relaxation time increases by several orders of magnitude within a few degrees, and it becomes impossible to perform an equilibrium experiment. Nevertheless, sharp as this behavior may be, it is not a genuine dynamic singularity. At the other extreme of the experimental spectrum, one finds strong liquids that experience a gentle increase of the relaxation time, often according to the Arrhenius law. Even in such systems, however, when the viscosity becomes too large, equilibrium can no longer be achieved on experimental timescales.
2) The glass transition temperature is conventionally defined as that temperature at which the value of the viscosity is 10^(13) poise. Below the glass transition temperature, equilibrium experiments become difficult to perform and a sample can be considered to be in its glass phase. However, the glass transition temperature is merely a conventional experimental temperature defined out of the need to mark the onset of glassy dynamics. The attempt to give a theoretical description of such an ill-defined transition may therefore seem pointless. On the one hand, this conclusion is correct for the strongest liquids: here nothing peculiar happens close to the glass transition temperature , and the glass transition fully displays its purely conventional nature. On the other hand, the most fragile systems resist such an objection, simply by virtue of the extremely steep increase of relaxation time within a small interval of temperature around the glass transition temperature. This fact suggests that some kind of new physical mechanism is indeed responsible for the onset of the glassy phase in fragile supercooled liquids.
3) The authors report they numerically studied the potential energy landscape of a fragile glassy system and found that the dynamic crossover corresponding to the glass transition is actually the effect of an underlying geometric transition caused by the vanishing of the instability index of saddle points of the potential energy. Furthermore, the authors demonstrate that the potential energy barriers connecting local glassy minima increase with decreasing energy of the minima, and they relate this behavior to the fragility of the system. Finally, the authors analyze the real space structure of activated processes by studying the distribution of particle displacements for local minima connected by simple saddles.
References (abridged):
1. C. A. Angell, J. Phys. Chem. Solids 49, 863 (1988)
2. M. Goldstein, J. Chem. Phys. 51, 3728 (1969)
3. F.H. Stillinger and T.A. Weber, Phys. Rev. A 25, 978 (1982)
4. A. Cavagna, Europhys. Lett. 53, 490 (2001)
5. J. Kurchan and L. Laloux, J. Phys. A 29, 1929 (1996)
Phys. Rev. Lett. http://prl.aps.org
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