Contrasting dynamics of fragile and non-fragile polyalcohols through the glass, and dynamical, transitions: A comparison of neutron scattering and dielectric relaxation data for sorbitol and glycerol.

BACKGROUND: Glycerol and sorbitol are glass-forming hydrogen-bonded systems characterized by intriguing properties which make these systems very interesting also from the applications point of view. The goal of this work is to relate the hydrogen-bonded features, relaxation dynamics, glass transition properties and fragility of these systems, in particular to seek insight into their very different liquid fragilities. METHODS: The comparison between glycerol and sorbitol is carried out by collecting the elastic incoherent neutron scattering (EINS) intensity as a function of temperature and of the instrumental energy resolution. RESULTS: Intensity data vs temperature and resolution are analyzed in terms of thermal restraint and Resolution Elastic Neutron Scattering (RENS) approaches. CONCLUSIONS: The number of OH groups, which are related to the connecting sites, is a significant parameter both in the glass transition and in the dynamical transition. On the other hand, the disordered nature of sorbitol is confirmed by the existence of different relaxation processes. GENERAL SIGNIFICANCE: From the applications point of view, glycerol and sorbitol have remarkable bioprotectant properties which make these systems useful in different technological and industrial fields. Furthermore, polyols are rich in glassforming liquid phenomenology and highly deserving of study in their own right. The comparison of EINS and calorimetric data on glycerol and sorbitol helps provide a connection between structural relaxation, dynamical transition, glass transition, and fragility. The evaluation of the inflection point in the elastic intensity behavior as a function of temperature and instrumental energy resolution provides a confirmation of the validity of the RENS approach. This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo.

Vitrification of a monatomic metallic liquid.

Although the majority of glasses in use in technology are complex mixtures of oxides or chalcogenides, there are numerous examples of pure substances-'glassformers'-that also fail to crystallize during cooling. Most glassformers are organic molecular systems, but there are important inorganic examples too, such as silicon dioxide and elemental selenium (the latter being polymeric). Bulk metallic glasses can now be made; but, with the exception of Zr50Cu50 (ref. 4), they require multiple components to avoid crystallization during normal liquid cooling. Two-component 'metglasses' can often be achieved by hyperquenching, but this has not hitherto been achieved with a single-component system. Glasses form when crystal nucleation rates are slow, although the factors that create the slow nucleation conditions are not well understood. Here we apply the insights gained in a recent molecular dynamics simulation study to create conditions for successful vitrification of metallic liquid germanium. Our results also provide micrographic evidence for a rare polyamorphic transition preceding crystallization of the diamond cubic phase.

On the dynamics of liquids in their viscous regime approaching the glass transition.

Recently, Mallamace et al. (Eur. Phys. J. E 34, 94 (2011)) proposed a crossover temperature, T(×), and claimed that the dynamics of many supercooled liquids follow an Arrhenius-type temperature dependence between T(×) and the glass transition temperature T(g). The opposite, namely super-Arrhenius behavior in this viscous regime, has been demonstrated repeatedly for molecular glass-former, for polymers, and for the majority of the exhaustively studied inorganic glasses of technological interest. Therefore, we subject the molecular systems of the Mallamace et al. study to a "residuals" analysis and include not only viscosity data but also the more precise data available from dielectric relaxation experiments over the same temperature range. Although many viscosity data sets are inconclusive due to their noise level, we find that Arrhenius behavior is not a general feature of viscosity in the T(g) to T(×) range. Moreover, the residuals of dielectric relaxation times with respect to an Arrhenius law clearly reveal systematic curvature consistent with super-Arrhenius behavior being an endemic feature of transport properties in this viscous regime. We also observe a common pattern of how dielectric relaxation times decouple slightly from viscosity.

Relaxation dynamics and ionic conductivity in a fragile plastic crystal.

We report a thorough characterization of the dielectric relaxation behavior and the ionic conductivity in the plastic-crystalline mixture of 60% succinonitrile and 40% glutaronitrile. The plastic phase can be easily supercooled and the relaxational behavior is investigated by broadband dielectric spectroscopy in the liquid, plastic crystalline, and glassy crystal phases. The α-relaxation found in the spectra is characterized in detail. A well-pronounced secondary and faint indications for a third relaxation process were found. The latter most likely is of Johari-Goldstein type. From the temperature dependence of the α-relaxation time, a fragility parameter of 62 was determined. Thus, together with Freon112, this material stands out among all other plastic crystals by being a relatively fragile glass former. This finding provides strong support for an energy-landscape related explanation of the fragility of glass formers. In addition, unusually strong conductivity contributions were detected in the spectra exhibiting the typical features of ionic charge transport making this material a good basis for solid-state electrolytes.

Reorientation Times for Solid-State Electrolyte Solvents and Electrolytes from NMR Spin-Lattice Relaxation Studies.

Ionic and molecular plastic crystals have been studied recently as solid electrolytes or solvents, but the specific role of molecular reorientation has not been clarified. We use NMR spin-lattice relaxation times (T1 minima) to compare the time scale for magnetic fluctuations in a plastic crystal solvent to the molecular reorientation times, as established by dielectric spectroscopy. We focus on a mixture of succinonitrile and glutaronitrile, in which the rotationally disordered phase is stabilized against crystallization. Reorientation times can then be studied over 13 orders of magnitude, down to the glass transition temperature at 144 K. For each nucleus, 1H and 13C, the most probable magnetic fluctuation time is found to be slightly shorter than the reorientation time, but with practically indistinguishable temperature dependence. This facilitates investigation of the relation of solvent reorientation to ion conductivity relaxation times in ionic conducting systems in which the conductivity swamps the dielectric signature of solvent reorientation.

Formation of glasses from liquids and biopolymers.

Glasses can be formed by many routes. In some cases, distinct polyamorphic forms are found. The normal mode of glass formation is cooling of a viscous liquid. Liquid behavior during cooling is classified between "strong" and "fragile," and the three canonical characteristics of relaxing liquids are correlated through the fragility. Strong liquids become fragile liquids on compression. In some cases, such conversions occur during cooling by a weak first-order transition. This behavior can be related to the polymorphism in a glass state through a recent simple modification of the van der Waals model for tetrahedrally bonded liquids. The sudden loss of some liquid degrees of freedom through such first-order transitions is suggestive of the polyamorphic transition between native and denatured hydrated proteins, which can be interpreted as single-chain glass-forming polymers plasticized by water and cross-linked by hydrogen bonds. The onset of a sharp change in ddT( is the Debye-Waller factor and T is temperature) in proteins, which is controversially indentified with the glass transition in liquids, is shown to be general for glass formers and observable in computer simulations of strong and fragile ionic liquids, where it proves to be close to the experimental glass transition temperature. The latter may originate in strong anharmonicity in modes ("bosons"), which permits the system to access multiple minima of its configuration space. These modes, the Kauzmann temperature T(K), and the fragility of the liquid, may thus be connected.

Density maxima in high-pressure supercooled water and liquid silicon dioxide.

With glass capillary pressure vessels it has been possible to study the effect of pressure on the temperature of maximum density (TMD) and on the "sharpness" of the density maximum in liquid H(2)O and D(2)O in the important but little-studied supercooled regime. A pressure of 1200 bars produces a 33 degrees C depression of the TMD in these liquids and a considerable reduction in sharpness. Comparison with the rather flat density maximum for liquid SiO(2) supports the notion that the presence or absence of density anomalies in "tetrahèdral" liquids depends on the average bridge-bond angle, which is evidently unusually large in water at normal pressure.

Supercooling of Water to -92{degrees}C Under Pressure.

The temperature at which "clean" supercooled water freezes has been determined as a function of pressure up to 3 kilobars, using a differential thermal analysis technique on subdivided water samples. The supercooling limit of such samples, -38 degrees C at normal pressure, is lowered by initial increase of pressure, reaching a minimum value of -92 degrees C at 2.00 kilobars.

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.

Anomalous heat capacities of supercooled water and heavy water.

Emulsification makes it possible to supercool water to the homogeneous nucleation temperature. Accordingly, the heat capacities of water and deuterium oxide have been determined from the respective equilibrium melting points to -38 degrees and to -34 degrees C, respectively. Two methods, drift calorimetry and differential scanning calorimetry, have been used. Both methods reveal a striking rise in the constant-pressure heat capacity below -20 degrees C. This indication of an apparently cooperative behavior should serve to test current theories of water, most notably perhaps, the pair potential model of Ben-Naim and Stillinger. Some implications of possible meteorological significance are mentioned.

Pressure enhancement of ion mobilities in liquid silicates from computer simulation studies to 800 kilobars.

Ion dynamics computer simulation methods show that for many liquid silicates, like silica itself, the component diffusion coefficients show anomalous pressure dependences. This implies that their viscosities have negative pressure dependences. Overall, there is an interesting degree of analogy between the fundamental binary solutions of geochemistry and the aqueous solutions of common experience; however, due to the stronger bonding in silicate systems, the anomalies are much more persistent. Diffusivity maxima occur at pressures of 200 to 300 kilobars and are correlated with a prevalence of fivefold coordination of silicon ions. The relevance of these findings to planetary dynamics and thermophysical modeling problems is briefly considered.

Liquids at large negative pressures: water at the homogeneous nucleation limit.

An isochoric cooling method for obtaining unprecedented tensions on liquids was used to determine the homogeneous nucleation limit for stretching of water at a variety of water densities. At densities in the range 0.55 to 0.68 gram per milliliter (g/ml), the data agree with the homogeneous nucleation temperatures measured by Skripov for superheated water at positive pressures. At densities between 0.68 and 0.93 g/ml, cavitation occurred only at negative pressures (that is, under tension). The cavitation tensions measured were in excellent agreement with those predicted by Fisher's 1948 vapor nucleation theory. A maximum tension of 140 megapascals (=1400 bars) was reached at 42 degrees C, which lies on an extrapolation of the line of isobaric density maxima. At higher densities, cavitation of droplets that survived heterogeneous nucleation failed to occur at all unless provoked, at much lower temperatures, by freezing. This observation confirms the existence of a density maximum at 42 degrees C and -140 megapascals and hence greatly strengthens the basis for Speedy's conjecture of a reentrant spinodal for water.

Water and solutions at negative pressure: Raman spectroscopic study to -80 megapascals.

Microscopic inclusions of aqueous fluids trapped in interstices in quartz and other crystals provide novel systems for the deliberate study of liquids under tension. Liquids under tension should differ in interesting ways from those at ambient pressure or compressed liquids because attractive, rather than repulsive, forces should dominate their behavior. Static tensions in excess of 100 megapascals (~1000 atmospheres) have been obtained reproducibly. Video-recorded observations of the final liquid rupture process, coupled with extrapolations of data at positive pressure, suggest that the homogeneous vapor nucleation point was reached in two of the cases studied. Raman spectra of the fluids at -80 megapascals show that an isothermal volume stretch of -5 percent by volume has only a weak effect on the spectral features and is similar to the effect of isobaric heating.

Depressed luteinizing hormone response to estradiol in vivo and gonadotropin-releasing hormone in vitro in experimentally diabetic swine.

The influence of the acute withdrawal of insulin therapy in streptozocin-diabetic female swine was examined for changes in 1) the in vivo pulsatile secretion of luteinizing hormone (LH), 2) the preovulatory-like gonadotropin patterns after exogenous estradiol, and 3) the in vitro LH secretion by cultured pituitary cells. In Experiment 1, ovariectomized diabetic pigs (n = 4) were maintained with insulin therapy until 4 d before estradiol benzoate (EB; 7 micrograms/kg body weight; subcutaneous) was administered. Four normal ovariectomized pigs, matched for age and weight, served as controls. The diabetic state was confirmed by the measurement of glucose and insulin concentrations during a glucose tolerance test. Pulsatile LH secretion was not influenced by experimental diabetes mellitus. However, the expected surge in LH was not induced by EB in diabetic gilts. In contrast, three of four normal gilts had a preovulatory-type surge in LH. Concentrations of follicle-stimulating hormone in serum were not affected by diabetes mellitus. Estradiol concentrations in serum after ER were influenced by diabetes mellitus (treatment by time interaction; P < 0.001). In individual estradiol profiles, maximum concentrations were similar (104 +/- 10.4 and 91 +/- 12.0 ng/ml for normal and diabetic pigs, respectively), but the interval to maximum concentration was delayed in diabetic pigs (27.5 vs. 9.0 h; SE = 3.0; P < 0.05). However, the duration of standing estrus (2.2 +/- .3 d) and the interval from EB to estrus (3.6 +/- 0.3 d) were not influenced by diabetes mellitus. In Experiment 2, LH secretion by cultured cells and residual cellular LH content were greater in the pituitaries of normal than diabetic pigs (P < 0.05), and only cells from normal pigs responded to gonadotropin-releasing hormone (GnRH), with increased production of LH (P < 0.05). In conclusion, diabetes mellitus did not affect pulsatile LH secretion but did lower the ability of exogenous estradiol to stimulate a surge in vivo and of GnRH to increase LH in vitro, suggesting that the pituitary response to estradiol and GnRH is more severely affected by diabetes than is the GnRH pulse generator.

Entropy and Fragility in Supercooling Liquids.

We review the Kauzmann paradox and what it implies about the configuration space energy hypersurface for "structural glassformers." With this background, we then show how the relaxation expression of Adam and Gibbs qualitatively accounts for most of the phenomenology of liquid and polymeric glassformers including the strong/fragile liquid pattern, and the behavior of non-ergodic systems. Extended temperature range relaxation studies are consistent with a relaxation time pre-exponent on the quasi-lattice vibration time scale. When this boundary condition is imposed on Vogel-Fulcher-Tammann fittings, correspondence of T0 with TK is found for liquids with Tg ranging over 1000 K. When it is imposed on the WLF equation C1 is obliged to become ~16, and the corresponding force-fitted C2 provides a measure of the polymer fragility which is generally not available from thermodynamic studies. Systems which exhibit discontinuous changes in configurational entropy on temperature increase, which include unfolding proteins, are briefly reviewed.

Measurement of conductivity and permittivity on samples sealed in nuclear magnetic resonance tubes.

We present a broadband impedance spectroscopy instrument designed to measure conductivity and∕or permittivity for samples that are sealed in glass tubes, such as the standard 5 mm tubes used for nuclear magnetic resonance experiments. The calibrations and corrections required to extract the dielectric properties of the sample itself are outlined. It is demonstrated that good estimates of the value of dc-conductivity can be obtained even without correcting for the effects of glass or air on the overall impedance. The approach is validated by comparing data obtained from samples sealed in nuclear magnetic resonance tubes with those from standard dielectric cells, using glycerol and butylmethylimidazolium-hexafluorophosphate as respective examples of a molecular and an ionic liquid. This instrument and approach may prove useful for other studies of permittivity and conductivity where contact to the metal electrodes or to the ambient atmosphere needs to be avoided.

Unusual phase behavior of one-component systems with two-scale isotropic interactions.

We study the phase behavior of systems of particles interacting through pair potentials with a hard core plus a soft repulsive component. We consider several different forms of soft repulsion, including a square shoulder, a linear ramp and a quasi-exponential tail. The common feature of these potentials is the presence of two repulsive length scales, which may be the origin of unusual phase behaviors such as polyamorphism both in the equilibrium liquid phase and in the glassy state, water-like anomalies in the liquid state and anomalous melting at very high pressures.

Protein unfolding, and the "tuning in" of reversible intermediate states, in protic ionic liquid media.

Protic ionic liquids (PILs) are currently being shown to be as interesting and valuable to chemical manipulations as the well-known aprotic ionic liquids (APIL). PILs have the additional advantage that the proton activity (PA) can be adjusted by the choice of Bronsted base and Bronsted acid used in their formation. In the absence of solvent, the PA plays the role of pH in ordinary solutions. Previously, we have shown that solution of proteins in ionic-liquid-rich solutions conveys surprising stabilization against hydrolysis and aggregation, permitting multiple unfold/refold cycles without loss to aggregation. Here, we show that the denaturing temperatures of both hen egg white lysozyme and ribonuclease A are sensitive to the PA of the PIL as much as they are to pH in aqueous solutions. A maximum stability for more basic solutions is found, and the unfolding process is well described by the two-state (cooperative) model. Finally, we show that, by PA tuning, the PILs can select folding pathways featuring the postulated intermediates so that they are fully populated during the unfolding process. The intermediates are themselves capable of multiple unfold/refold cycles with little loss per cycle to aggregation process.

Two-Gaussian excitations model for the glass transition.

We develop a modified "two-state" model with Gaussian widths for the site energies of both ground and excited states, consistent with expectations for a disordered system. The thermodynamic properties of the system are analyzed in configuration space and found to bridge the gap between simple two-state models ("logarithmic" model in configuration space) and the random energy model ("Gaussian" model in configuration space). The Kauzmann singularity given by the random energy model remains for very fragile liquids but is suppressed or eliminated for stronger liquids. The sharp form of constant-volume heat capacity found by recent simulations for binary mixed Lennard-Jones and soft-sphere systems is reproduced by the model, as is the excess entropy and heat capacity of a variety of laboratory systems, strong and fragile. The ideal glass in all cases has a narrow Gaussian, almost invariant among molecular and atomic glassformers, while the excited-state Gaussian depends on the system and its width plays a role in the thermodynamic fragility. The model predicts the possibility of first-order phase transitions for fragile liquids. The analysis of laboratory data for toluene and o-terphenyl indicates that fragile liquids resolve the Kauzmann paradox by a first-order transition from supercooled liquid to ideal-glass state at a temperature between T(g) and Kauzmann temperature extrapolated from experimental data. We stress the importance of the temperature dependence of the energy landscape, predicted by the fluctuation-dissipation theorem, in analyzing the liquid thermodynamics.