Biomedical subjects

C Austen Angell

Publications and source records attributed to C Austen Angell.

12 recordsLinked to original sources

PubMed2006

Fragility and thermodynamics in nonpolymeric glass-forming liquids.

For nonpolymeric supercooled liquids, the empirical correlation m = 56Tg DeltaCp(Tg)/DeltaHm provides a reliable means of correlating dynamic and thermodynamic variables. The dynamics are characterized by the fragility or steepness index m and the glass transition temperature Tg, while thermodynamics enter in terms of the heat capacity step DeltaCp at Tg and the melting enthalpy DeltaHm. The combination of the above correlation with the 23 rule for the Tg/Tm ratio yields an expression, m = 40DeltaCp(Tg)/DeltaSm, which was rationalized as the correlation of the thermodynamic and kinetic fragilities. Defining a thermodynamic fragility via DeltaCp(Tg)/DeltaSm also reveals that the slopes in Kauzmann's original DeltaS(T)/DeltaSm versus T/Tm plot reflect the fragility concept [Chem. Rev. 43, 219 (1948)], so long as Tm/Tg = 1.5. For the many liquids whose excess heat capacity is a hyperbolic function of temperature, we deduce that the fragility cannot exceed m = 170, unless the Tg/Tm = 2/3 rule breaks down.

Chemical Phenomena↗

PubMed2005

Structural relaxation in the glass transition region of water.

We perform molecular dynamics (MD) simulations using the extended simple-point-charge (SPC/E) model for water to study the structural relaxation through the glass transition region. We follow the same standard protocol used in differential scanning calorimetry experiments. Specifically, we cool liquid configurations at different cooling rates to produce a glass, and then we heat the glass back to the liquid state. We also study aging effects in the glass before heating. We find that MD simulations can reproduce the phenomenology observed in experiments. We review the Tool-Narayanaswamy-Moynihan (TNM) phenomenological approach, introduced to describe the behavior of the specific heat upon heating glass to the liquid phase. The TNM approach requires, as an ansatz, an expression for the temperature dependence of the relaxation time. We compare the simulation results with the prediction of the TNM approach supplemented with two well-known expressions for the relaxation time: the Narayanaswamy-Moynihan (NM) and the Adam-Gibbs-Scherer (AGS) expressions. We find that, in the case of slow cooling rates, our simulations are well represented by the TNM approach, but only if the AGS expression is adopted. We also find that the TNM approach fails in the case of fast cooling rates for both NM and AGS expressions. Several attempts to provide more freedom to the fitting procedure by allowing the fitting parameters to depend on temperature, cooling, and/or heating rates do not improve the agreement between the simulation data and the TNM predictions.

Journal Article↗

PubMed2005

Energy landscapes for cooperative processes: nearly ideal glass transitions, liquid-liquid transitions and folding transitions.

We describe basic phenomenology in the physics of supercooling liquids at constant volume (most simulations), and at constant pressure (most laboratory experiments) before focusing attention on the exceptional cases that exhibit liquid-liquid phase transitions on constant-pressure cooling. We give evidence for point defects in glasses and liquids near T(g). Models based on defects predict transitions with density gaps in constant-pressure systems. We describe the energy landscape representation of such systems. Water, in these terms, is post-critical, and its nearly ideal glass formation can be related to nucleation-free protein 'funnel-folding'. For nucleated folding of proteins, a pseudo-gap should be present. Experimental methods of distinguishing between alternative folding scenarios are described.

Computer Simulation↗

PubMed2004

Dielectric studies deny existence of ultraviscous fragile water.

The glass transition, a relaxation phenomenon, sets the low temperature limit to the liquid state. Glassy water that forms only under extreme quenching conditions is unstable against crystallization. Opinions differ on whether the glass transition can be observed at all. Here we measure the dielectric tan(delta for easily glassforming waterlike aqueous solutions, H2O-H2O2 and H2O-N2H4, to characterize the behavior of such systems during passage through their glass transitions. All show unambiguous Tg values of 136-140 K, the value generally assigned to pure water. However, the behavior of epsilon''/epsilon' is quite different from that in amorphous water in the same temperature range. Our findings eliminate "ultraviscous fragile liquid" as a possible description of water between 136 K and crystallization, but leave "ultraviscous strong liquid" a possibility to be considered.

Journal Article↗

PubMed2004

Glass-transition temperature of water: a simulation study.

We report a computer simulation study of the glass transition for water using the extended simple point charge potential. To mimic the difference between standard and hyperquenched glass, we generate glassy configurations with different cooling rates, and we calculate the temperature dependence of the specific heat on heating. The absence of crystallization phenomena allows us, for properly annealed samples, to detect in the specific heat the simultaneous presence of a weak prepeak ("shadow transition") and an intense glass transition peak at higher temperature. Our results support the view-point that the glass transition temperature is higher than the conventionally accepted value 136 K. We also compare our simulation results with the Tool-Narayanaswamy-Moynihan phenomenological model.

Journal Article↗

PubMed2004

Clarifying the glass-transition behaviour of water by comparison with hyperquenched inorganic glasses.

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. For the last 20 years, the existence of a glass transition at 136 K (ref. 3) has been widely accepted, but the transition exhibits qualities difficult to reconcile with our current knowledge of glass transitions. Here we report detailed calorimetric characterizations of hyperquenched inorganic glasses that, when heated, do not crystallize before reaching their glass transition temperatures. We compare our results to the behaviour 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.

Journal Article↗

PubMed2004

Amorphous water.

After providing some background material to establish the interest content of this subject, we summarize the many different ways in which water can be prepared in the amorphous state, making clear that there seems to be more than one distinct amorphous state to be considered. We then give some space to structural and spectroscopic characterization of the distinct states, recognizing that whereas there seems to be unambiguously two distinct states, there may be in fact be more, the additional states mimicking the structures of the higher-density crystalline polymorphs. The low-frequency vibrational properties of the amorphous solid states are then examined in some detail because of the gathering evidence that glassy water, while difficult to form directly from the liquid like other glasses, may have some unusual and almost ideal glassy features, manifested by unusually low states of disorder. This notion is pursued in the following section dealing with thermodynamic and relaxational properties, where the uniquely low excess entropy of the vitreous state of water is confirmed by three different estimates. The fact that the most nearly ideal glass known has no properly established glass transition temperature is highlighted, using known dielectric loss data for amorphous solid water (ASW) and relevant molecular glasses. Finally, the polyamorphism of glassy water, and the kinetic aspects of transformation from one form to the other, are reviewed.

Journal Article↗

PubMed2003

Ionic liquids by proton transfer: vapor pressure, conductivity, and the relevance of DeltapKa from aqueous solutions.

We describe the behavior of the conductivity, viscosity, and vapor pressure of various binary liquid systems in which proton transfer occurs between neat Brönsted acids and bases to form salts with melting points below ambient. Such liquids form an important subgroup of the ionic liquid (IL) class of reaction media and electrolytes on which so much attention is currently being focused. Such "protic ionic liquids" exhibit a wide range of thermal stabilities. We find a simple relation between the limit set by boiling, when the total vapor pressure reaches one atm, and the difference in pK(a) value for the acid and base determined in dilute aqueous solutions. For DeltapK(a) values above 10, the boiling point elevation becomes so high (>300 degrees C) that preemptive decomposition prevents its measurement. The completeness of proton transfer in such cases is suggested by the molten salt-like values of the Walden product, which is used to distinguish good from poor ionic liquids. For the good ionic liquids, the hydrogen bonding of acid molecules to the proton-transfer anion is strong enough that boiling points, but not melting points, may maximize at the hydrogen-bonded dianion composition. High boiling liquids of this type constitute an interesting class of high-temperature protonic acid that may have high-temperature fuel cell applications.

Journal Article↗

PubMed2003

Solvent-free electrolytes with aqueous solution-like conductivities.

Aqueous solutions are generally assumed to be superior electrolytic conductors because of the unique dielectric and fluid properties of water. We show that their conductivities can be matched by liquid electrolytes that contain no solvent. These are proton transfer salts that are liquid at ambient temperature. The high conductivities are due to the high fluidity and ionicity rather than some sort of Grotthus mechanism, although in certain cases a mobile proton population may make a non-negligible contribution. The highest conductivities have been obtained when both cations and anions contain protons. At 25 degrees C, values of >150 millisiemens per centimeter (mS x cm(-1)) appear possible; at 100 degrees C, 470 mS x cm(-1) has been measured. Because of the combination of high ionicity and proton exchange kinetics with low vapor pressure, the systems we describe also make excellent fuel cell electrolytes.

Journal Article↗

PubMed2003

Liquid-liquid phase transition in supercooled silicon.

Silicon in its liquid and amorphous forms occupies a unique position among amorphous materials. Obviously important in its own right, the amorphous form is structurally close to the group of 4-4, 3-5 and 2-6 amorphous semiconductors that have been found to have interesting pressure-induced semiconductor-to-metal phase transitions. On the other hand, its liquid form has much in common, thermodynamically, with water and other 'tetrahedral network' liquids that show density maxima. Proper study of the 'liquid-amorphous transition', documented for non-crystalline silicon by both experimental and computer simulation studies, may therefore also shed light on phase behaviour in these related materials. Here, we provide detailed and unambiguous simulation evidence that the transition in supercooled liquid silicon, in the Stillinger-Weber potential, is thermodynamically of first order and indeed occurs between two liquid states, as originally predicted by Aptekar. In addition we present evidence to support the relevance of spinodal divergences near such a transition, and the prediction that the transition marks a change in the liquid dynamic character from that of a fragile liquid to that of a strong liquid.

Computer Simulation↗

PubMed2003

Recent results on the connection between thermodynamics and dynamics in supercooled water.

We review recent results on the connection between thermodynamics and dynamics in a model for water. We verify the Adam-Gibbs relation between entropy and dynamic properties using computer simulations, which allow direct access to the relevant properties. We combine experimental measurements of entropy with the Adam-Gibbs hypothesis to predict dynamic properties in deeply supercooled states, which are difficult to access experimentally. We find evidence suggesting that the glass transition temperature of water may be significantly higher than previously reported, but is still consistent with recent measurements. Finally, we discuss the hypothesis that the dynamical behavior of deeply supercooled water undergoes a crossover from 'fragile' to 'strong' behavior.

Cold Temperature↗

PubMed2003

Hyperquenching and cold equilibration strategies for the study of liquid--liquid and protein folding transitions.

In this paper we consider the extension of the recent quantitative studies of hyperquenched glassformers to include (1). systems that exhibit first order liquid-liquid phase transitions, and (2). systems that contain molecules, which, during normal cooling, undergo internal structural changes above the glass temperature. The general aim of these studies is to trap-in a high enthalpy, high entropy, state of the system and then observe it evolving in time at low temperatures during a controlled annealing procedure. In this manner events that normally occur during change of temperature may be observed occurring during passage of time, at much lower temperatures. At such low temperatures the smearing effects of vibrations are greatly reduced. While the case of most interest in the second class is the refolding of thermally denatured protein molecules, any reconstructive molecular or chemical exchange process is a potential subject for investigation. Processes that occur in stages can be studied in greater detail, and any stage of interest can be frozen when desired, by drop of temperature, for more detailed spectroscopic examination. We review an electrospray method for hyperquenching liquids at approximately 10(5) K/s, and discuss some results of such experiments in order to illustrate a calorimetric approach to exploiting the hyperquenching-and- cold-equilibration strategy. To apply the idea to the study of proteins, the following protein solvent requirements must be met: (1). the solvents must not crystallize ice on cooling or heating, yet must not denature the proteins; (2). the solvents must support thermally denatured molecules without permitting aggregation. We describe two solvent systems, the first of which meets the first requirement, but the second only partially. The second solvent system apparently meets both. Preliminary results, only at the proof of concept stage, are reported for cold refolding of lysozyme, which, it seems, can be trapped in our solvent in the unfolded but refoldable state, with only moderate (approx. 120 K/s) quenching rates.

Cold Temperature↗