arXiv subjects

C. Austen Angell

Publications and source records attributed to C. Austen Angell.

At least 19 recordsLinked to original sources

arXiv2020

Ionic liquids of ultralow proton activity

The route to super acidic ionic liquids by proton transfer from molecular super acids to weak molecular bases like pentafluoropyridine, has been described in recent work. Here we consider the process of making super basic ionic liquids by a similar procedure, and encounter the problem of finding molecular bases of high enough basicity to accept protons from weak acids to form ionic liquids of both high basicity and high iconicity. The consequence is that no ionic liquid with proton activity outside the aqueous base limit of pH =14, has yet been made. The problem is resolved by considering the six possible types of proton transfer processes, selecting the process of transfer from weak molecular acids to very basic ionic anions - partnered with alkali metal cations, and then replacing the alkali metals cations with reduction-resistant organic cations by metathesis. In many cases the alkali metal salts of target anions are commercially available, and only the metathesis reaction is needed to obtain the basic ionic liquid. A thermodynamic route to the assessment of basicity levels on the water scale, i.e. relative to the H3O+/H2O donor/acceptor level, is described, and is verified for the case of pure H2SO4. It is then applied to show how to extend the possible proton activity range down, by some 25 orders of magnitude, to reach pKa values of order 40, with even higher values possible in principle.

physics.chem-ph↗

arXiv2018

Breakdown of the Stokes-Einstein Relation Above the Melting Temperature in a Liquid Phase-Change Material

The dynamic properties of liquid phase-change materials (PCMs), such as viscosity $η$ and atomic self-diffusion coefficients D, play an essential role in ultrafast phase switching behavior of novel non-volatile phase-change memory applications, as they are intimately related to crystallization kinetics and phase stabilities. To connect $η$ to D, the Stokes-Einstein relation (SER) is commonly assumed to be valid at high temperatures near or above the melting temperature $T_{m}$ and is frequently employed for assessing liquid fragility (or crystal growth velocity) of technologically important PCM compositions. However, using quasi-elastic neutron scattering (QENS), we give here experimental evidence for a breakdown of the SER even at temperatures above $T_{m}$ in the high-atomic-mobility state of a typical PCM, Ge$_{1}$Sb$_{2}$Te$_{4}$, where the decay of density correlation functions still remains exponential. The origin of the breakdown is thus unlikely the result of dynamical heterogeneities, as is usually postulated for viscous liquids. Rather, we discuss its possible connections to a metal-semiconductor and fragile-strong transition hidden below $T_{m}$.

cond-mat.mtrl-sci↗

arXiv2017

Glass transitions, semiconductor-metal (SC-M) transitions and fragilities in Ge-V-Te (V=As, or Sb) liquid alloys: the difference one element can make

Glass transition temperatures (Tg) and liquid fragilities are measured along a line of constant Ge content in the system Ge-As-Te, and contrasted with the lack of glass-forming ability in the twin system Ge-Sb-Te at the same Ge content. The one composition established as free of crystal contamination in the latter system shows a behavior opposite to that of more covalent system. Comparison of Tg vs bond density in the three systems Ge-As-chalcogen differing in chalcogen i.e. S, Se, or Te, shows that as the chalcogen becomes more metallic, i.e. in the order S = 2.3. When the more metallic Sb replaces As at greater than 2.3, incipient metallicity rather than directional bond covalency apparently gains control of the physics. This leads us to an examination of the electronic conductivity and, then, semiconductor-to-metal (SC-M) transitions, with their associated thermodynamic manifestations, in relevant liquid alloys. The thermodynamic components, as seen previously, control liquid fragility and cause fragile-to-strong transitions during cooling. We tentatively conclude that liquid state behavior in phase change materials (PCMs) is controlled by liquid-liquid (SC-M) transitions that have become submerged below the liquidus surface. In the case of the Ge-Te binary, a crude extrapolation to GeTe stoichiometry indicates that the SC-M transition lies about 20% below the melting point, suggesting a parallel with the intensely researched "hidden liquid-liquid (LL) transition", in supercooled water. In the water case, superfast crystallization initiates in the high fragility domain some 4% above the TLL which is located at ~15% below the (ambient pressure) melting point.

cond-mat.mtrl-sci↗

arXiv2016

Potential tuning in the S-W system. (i) Bringing Tc,2 to ambient pressure, and (ii) colliding Tc,2 with the liquid-vapor spinodal

Following Vasisht et al's identification of the second critical point (Tc2,Pc2) for liquid silicon in the Stillinger-Weber (S-W) model for silicon, we study the variation of Tc2,Pc2 with tetrahedral repulsion parameter in an extension of the earlier "potential tuning" study of this system. We use the simple isochoric cooling approach to identify the location of the second critical point as a function of the "tuning" or "tetrahedrality", parameter λ, and identify two phenomena of high interest content. The first is that the second critical point pressure Pc2, becomes less negative as λ decreases from the silicon value (meaning the drive to high tetrahedrality is decreased) and reaches zero pressure at the same value of lambda as earlier found to mark the onset of glassforming ability in an earlier study of this tunable system. The second is that, as the Tc,2 approaches the temperature of the liquid-gas spinodal, λ>22, the behavior of the temperature of maximum density TMD switches from the behavior seen in most current water pair potential models (locus of TMDs has a maximum), to the behavior seen in empirical engineering multiparameter equations of state (EoS) (and also by two parameter Speedy isothermal expansion EoS) for water, according to which the locus of TMDs of HDL phase has no maximum, and the EoS for HDL has no second critical point. At λ= 23 the behavior is isomorphic with that of the mW model of water, which is now seen to conform to the "critical point free" scenario for water.

cond-mat.dis-nn↗

arXiv2016

Liquid-liquid transition in supercooled aqueous solution involving a low-temperature phase similar to low-density amorphous water

The striking anomalies in physical properties of supercooled water that were discovered in the 1960-70s, remain incompletely understood and so provide both a source of controversy amongst theoreticians, and a stimulus to experimentalists and simulators to find new ways of penetrating the "crystallization curtain" that effectively shields the problem from solution. Recently a new door on the problem was opened by showing that, in ideal solutions, made using ionic liquid solutes, water anomalies are not destroyed as earlier found for common salt and most molecular solutes, but instead are enhanced to the point of precipitating an apparently first order liquid-liquid transition. The evidence was a spike in apparent heat capacity during cooling that could be fully reversed during reheating before any sign of ice crystallization appeared. Here, we use decoupled-oscillator infrared spectroscopy to define the structural character of this phenomenon using similar down and upscan rates as in the calorimetric study. Thin-film samples also permit slow scans (1 K/min) in which the transition has a width of less than 1 K, and is fully reversible. The OH spectrum changes discontinuously at the phase-transition temperature, indicating a discrete change in hydrogen-bond structure. The spectral changes show that the low-temperature liquid is more strongly hydrogen bonded and less disordered as compared to the high-temperature liquid. The spectrum of the low-temperature liquid is essentially that seen in low-density amorphous water. This similarity suggests that the liquid-liquid transition observed here also exists in neat undercooled water, providing a unified explanation for many of its anomalies.

physics.chem-ph↗

arXiv2014

Search for a liquid-liquid critical point in models of silica

Previous research has indicated the possible existence of a liquid-liquid critical point (LLCP) in models of silica at high pressure. To clarify this interesting question we run extended molecular dynamics simulations of two different silica models (WAC and BKS) and perform a detailed analysis of the liquid at temperatures much lower than those previously simulated. We find no LLCP in either model within the accessible temperature range, although it is closely approached in the case of the WAC potential near 4000 K and 5 GPa. Comparing our results with those obtained for other tetrahedral liquids, and relating the average Si-O-Si bond angle and liquid density at the model glass temperature to those of the ice-like beta-cristobalite structure, we conclude that the absence of a critical point can be attributed to insufficient "stiffness" in the bond angle. We hypothesize that a modification of the potential function to mildly favor larger average bond angles will generate a LLCP in a temperature range that is accessible to simulation. The tendency to crystallize in these models is extremely weak in the pressure range studied, although this tendency will undoubtedly increase with increasing stiffness.

cond-mat.soft↗

arXiv2014

Water and its relatives: the stable, supercooled and particularly the stretched, regimes

While the water molecule is simple, its condensed phase liquid behavior is so complex that no consensus description has emerged despite three centuries of effort. Here we identify features of its behavior that are the most peculiar, hence suggest ways forward. We examine the properties of water at the boundaries of common experience, including stable states at high pressure, the supercooled state at normal and elevated pressure, and the stretched ("negative pressure") state, out to the limits of mechanical stability. The familiar anomalies at moderate pressures (viscosity and density (TMD) behavior, etc.), are not explained by H-bond breaking, according to common bond-breaking criteria. A comparison of data on the TMD, at both positive and negative pressures, with the predictions of popular pair potential models, shows dramatic discrepancies appearing in the stretched liquid domain. This prompts questions on the second critical point (TC2) hypothesis that has been guiding much current thinking. We turn to related systems for guidance, reviewing a hierarchy of water-like anomalies. We conclude that water models are far from complete and that proper understanding of water will depend on success in mastering the measurement of liquid behavior in the negative pressure domain - which we discuss.

physics.chem-ph↗

arXiv2013

Slow but complete, two state unfolding/refolding of lysozyme in "tuned" ~6M guanidinium carboxylate solutions

We using differential scanning calorimetry of the unfolding process to identify conditions in which the pseudo two-state refolding of thermally denatured lysozyme can be observed to occur on time scales of hours, in solutions that approach 6M in guanidinium cation, Gdm+. Remarkably, the fraction of lysozyme re-folded at 25 C reaches, and remains at 1.0. The refolded fraction is linear in log(waiting time), where the waiting time is the time at ambient temperature after an initial thermal denaturing (details in text) and immediate rapid cool to ambient. The favorable refolding conditions are achieved by tuning the solution anion composition to be comparable in pKa to, but somewhat smaller than, the pKa values of the carboxylates residues, aspartic acid and glutamic acid in the heteropolymer chain. To date we have used mixtures of guanidinium formate and acetate in equal proportions, with sufficient water to achieve the Gdm+ concentrations 5.36 and 4.39M. Reducing the concentration increases the folding rate without changing the final 100% refolded state. We suggest the slowdown occurs because we have chemically pre-empted the critical links that nucleate the folding process and determine its all-or-nothing character.

cond-mat.soft↗

arXiv2013

Glass Transitions and Critical Points in Orientationally Disordered Crystals and Structural Glassformers: "Strong" Liquids are More Interesting Than We Thought

When liquids are classified using Tg -scaled Arrhenius plots of relaxation times (or relative rates of entropy increase above Tg) across a "strong-fragile" spectrum of behaviors, the "strong" liquids have always appeared rather uninteresting [1, 2]. Here we use updated plots of the same type for crystal phases of the "rotator" variety [3] to confirm that the same pattern of behavior exists for these simpler (center of mass ordered) systems. However, in this case we can show that the "strong" systems owe their behavior to the existence of lambda-type order-disorder transitions at higher temperatures (directly observable in the cases where observations are not interrupted by prior melting). Furthermore, the same observation can be made for other systems in which the glass transition, at which the ordering is arrested, occurs in the thermodynamic ground state of the system. This prompts an enquiry into the behavior of strong liquids at high temperatures. Using the case of silica itself, we again find strong evidence from extended ion dynamics simulations, for a lambda transition at high temperatures, but only if pressure is adjusted to a critical value. In this case the lambda point is identifiable as a liquid-liquid critical point of the type suggested for supercooled water. We recognize the possibility of exploring, a postiori, the consequences of rapid cooling of laboratory liquid SiO2 from >5000K and multi-GPa pressures, using the phenomenology of damage-induced plasmas in optical fibers. The ramifications of these considerations will be explored to establish a "big picture"2 of the relation of thermodynamic transitions to supercooled liquid phenomenology [4, 5].

cond-mat.soft↗

arXiv2012

Physics of the Jagla Model as the Liquid-Liquid Coexistence Line Approaches Horizontal

The slope of the coexistence line of the liquid-liquid phase transition (LLPT) can be positive, negative, or zero. All three possibilities have been found in Monte-Carlo simulations of a modified spherically symmetric two-scale Jagla model. Since the liquid-liquid critical point (LLCP) frequently lies in a region of the phase diagram that is difficult to access experimentally, it is of great interest to study critical phenomena in the supercritical region. We therefore study the properties of the Widom line, which is defined in the one-phase region above the critical point as the locus of maximum correlation length as function of the ordering field at constant thermal field. Asymptotically close to the critical point, the Widom line coincides with the loci of the response function extrema, because all response functions can be asymptotically expressed as functions of the diverging correlation length. We find that the method of identifying the Widom line as the loci of heat capacity maxima becomes unfruitful when the slope of the coexistence line approaches zero in the $T$-$P$ plane. In this case the specific heat displays no maximum in the one-phase region because for a horizontal phase coexistence line, according to the Clapeyron equation, the enthalpy difference between the coexisting phases is zero, and thus there can be no contribution to enthalpy fluctuations from the critical fluctuations. The extension of the coexistence line beyond the critical point into the one-phase region must in this case be performed using density fluctuations; the line of compressibility maxima remains well defined, though it bifurcates into a symmetrical pair of lines. These findings agree well with the linear scaling theory of the LLCP by Anisimov and collaborators.

cond-mat.stat-mech↗

arXiv2011

Potential-tuning molecular dynamics studies of fusion, and the question of ideal glassformers: (I) The Gay-Berne model

The ability of some liquids to vitrify during supercooling is usually seen as a consequence of the rates of crystal nucleation (and/or crystal growth) becoming small- thus a matter of kinetics. However there is evidence, dating back to the empirics of coal briquetting for maximum trucking efficiency, that ellipsoids pack efficiently when disordered. Noting that key studies of non-spherical object packing have never been followed from hard ellipsoids or spherocylinders (diatomics excepted) into the world of molecules with attractive forces, we have made a molecular dynamics MD study of crystal melting and glass formation on the Gay- Berne (G-B) model of ellipsoidal objects across the aspect ratio range of the hard ellipsoid studies. Here we report that, in the aspect ratio range of maximum ellipsoid packing efficiency, various G-B crystalline states, that cannot be obtained directly from the liquid, disorder spontaneously near 0 K and transform to liquids without any detectable enthalpy of fusion. Without claiming to have proved the existence of single component examples, we use the present observations, together with our knowledge of non-ideal mixing effects, to discuss the probable existence of "ideal glassformers" - single or multicomponent liquids that vitrify before ever becoming metastable with respect to crystals. The existence of crystal-free routes to the glassy state removes any precrystalline fluctuation perspective from the "glass problem". Unexpectedly we find that liquids with aspect ratios in the "crystallophobic" range also behave in an unusual (non-hysteritic) way during temperature cycling through the glass transition. We link this to the highly volume fraction-sensitive ("fragile") behavior observed in recent hard dumbbell studies at similar length/diameter ratios.

cond-mat.soft↗

arXiv2010

On the decoupling of relaxation modes in a molecular liquid caused by isothermal introduction of 2nm structural inhomogeneities

To support a new interpretation of the origin of the dynamic heterogeneity observed pervasively in fragile liquids as they approach their glass transition temperatures Tg, we demonstrate that the introduction of ~2 nm structural inhomogeneities into a homogeneous glassformer leads to a decoupling of diffusion from viscosity similar to that observed during the cooling of orthoterphenyl OTP below TA, where Arrhenius behavior is lost. Further, the decoupling effect grows stronger as temperature decreases (and viscosity increases). The liquid is cresol and the ~2nm inhomogeneities are cresol-soluble asymmetric derivatized tetrasiloxy-based (POSS) molecules. The decoupling is the phenomenon predicted by Onsager in discussing the approach to a liquid-liquid phase separation with decreasing temperature. In the present case the observations support the notion of a polyamorphic transition in fragile liquids that is hidden below the glass transition. A similar decoupling can be expected as a globular protein is dissolved in dilute aqueous solutions or in protic ionic liquids.

cond-mat.dis-nn↗

arXiv2010

Glass transition with decreasing correlation length during cooling of Fe50Co50 superlattice and strong liquids

The glass transition GT is usually thought of as a structural arrest that occurs during the cooling of a liquid, or sometimes a plastic crystal, trapping a metastable state of the system before it can recrystallize to stabler forms1. This phenomenon occurs in liquids of all classes, most recently in bulk metallic glassformers2. Much theoretical interest has been generated by the dynamical heterogeneity observed in cooling of fragile liquids3, 4, and many have suggested that the slow-down is caused by a related increasing correlation length 5-9. Here we report both kinetics and thermodynamics of arrest in a system that disorders while in its ground state, exhibits a large !Cp on arrest (!Cp = Cp,mobile - Cp,arrested), yet clearly is characterized by a correlation length that is decreasing as GT is approached from above. We show that GT kinetics in our system, the disordering superlattice Fe50Co50, satisfy the kinetic criterion for ideally 'strong' glassformers10, and since !Cp behavior through Tg also correlates10, we propose that very strong liquidsand very fragile liquids exist on opposite flanks of an order-disorder transition - one that is already known for model systems.

physics.chem-ph↗

arXiv2007

Insights into glass formation and glass transition in supercooled liquids, by study of related phenomena in crystals

We divide glass and viscous liquid sciences into two major research areas, the first dealing with how to avoid crystals and so access the viscous liquid state, and the second dealing with how liquids behave when no crystals form. We review some current efforts to elucidate each area, looking at strategies for vitrification of monatomic metals in the first, and the origin of the property fragility in the second. Essential here is the non- trivial behavior of the glassformer thermodynamics. We explore the findings on nonexponential relaxationand dynamic heterogeneities in viscous liquids, emphasizing the way in which direct excitation of the configurational modes has helped differentiate configurational from nonconfigurational contributions to the excess heat capacity. We then propose a scheme for understanding the relation between inorganic network and non-network glassformers which includes the anomalous case of water as an intermediate. In a final section we examine the additional insights to be gained by study of the ergodicity-breaking, glass-like, transitions that occur in disordering crystals. Here we highlight systems in which the background thermodynamics is understood because the ergodic behavior is a lambda transition. Water and the classical network glassformers appear to be attenuated versions of these.

cond-mat.dis-nn↗

arXiv2007

Thermodynamics and Dynamics of a Monoatomic Glass-Former. Constant Pressure and Constant Volume Behavior

We report constant-volume and constant-pressure simulations of the thermodynamic and dynamic properties of the low-temperature liquid and crystalline phases of the modified Stillinger-Weber (mSW) model. We have found an approximately linear increase of the effective Gaussian width of the distribution of inherent structures. This effect comes from non-Gaussianity of the landscape and is consistent with the predictions of the Gaussian excitations model representing the thermodynamics of the configurational manifold as an ensemble of excitations, each carrying an excitation entropy. The mSW model provides us with both the configurational and excess entropies, with the difference mostly attributed to vibrational anharmonicity. We therefore can address the distinction between the excess thermodynamic quantities often used in the Adam-Gibbs (AG) equation. We find a new break in the slope of the constant pressure AG plot when the excess entropy is used in the AG equation. The simulation diffusivity data are equally well fitted by applying a new equation, derived within the Gaussian excitations model, that emphasizes enthalpy over entropy as the thermodynamic control variable for transport in viscous liquids.

cond-mat.soft↗

arXiv2007

Directed destabilization of lysozyme in protic ionic liquids reveals a compact, low energy, soluble, reversibly-unfolding (pre-fibril) state

Recent demonstrations of extraordinary stabilization of proteins in mobile protic [1] and aprotic [2] ionic liquid solutions at ambient temperatures have raised hopes of new biopreservation and drug transportation technologies. Here we examine the relation of folded protein stability to the state of the transferred proton [1], as determined by the N-H proton chemical shift, d(N-H). We identify a range of d(N-H) in which the unfolded lysozyme refolds 97%. Exceeding the stability range in the acid direction leads to the sudden formation and stabilization of a small, soluble, amyloid form of lysozyme which has its own stability range and which can again unfold/refold many times before an irreversible process, fibrillization, occurs. The tightly bound amyloid form of the lysozyme molecule, identified by circular dichroism spectra and dynamic light scattering, must be of very low energy since the unfolding process absorbs almost three times the enthalpy of normal lysozyme unfolding. alpha-lactalbumin shows similar behavior.

physics.bio-ph↗

arXiv2006

Tuning of tetrahedrality in a silicon potential yields a series of monatomic (metal-like) glassformers of very high fragility

We obtain monatomic glass formers in simulations by modifying the tetrahedral character in a silicon potential to explore a triple point zone between potentials favoring diamond (dc) and bcc crystals. dc crystallization is always preceded by a polyamorphic transformation of the liquid, and is frustrated when the Kauzmann temperature of the high temperature liquid intersects the liquid-liquid coexistence line. The glass forming liquids are extraordinarily fragile. Our results suggest that Si and Ge liquids may be vitrified at a pressure close to the diamond-beta-tin-liquid triple point.

cond-mat.soft↗

arXiv2006

Thermodynamics and Dynamics of the Two-Scale Spherically-Symmetric Jagla Model of Anomalous Liquids

Using molecular dynamics simulations, we study a liquid model which consists of particles interacting via a spherically-symmetric two-scale Jagla ramp potential with both repulsive and attractive ramps. The Jagla potential displays anomalies similar to those found in liquid water, namely expansion upon cooling and an increase of diffusivity upon compression, as well as a liquid-liquid (LL) phase transition in the region of the phase diagram accessible to simulations. The LL coexistence line, unlike in tetrahedrally-coordinated liquids, has a positive slope, because of the Clapeyron relation, corresponding to the fact that the high density phase (HDL) is more ordered than low density phase (LDL). When we cool the system at constant pressure above the critical pressure, the hydrodynamic properties rapidly change from those of LDL-like to those of HDL-like upon crossing the Widom line. The temperature dependence of the diffusivity also changes rapidly in the vicinity of the Widom line, namely the slope of the Arrhenius plot sharply increases upon entering the HDL domain. The properties of the glass transition are different in the two phases, suggesting that the less ordered phase (LDL) is fragile, while the more ordered phase (HDL) is strong, which is consistent with the behavior of tetrahedrally-coordinated liquids such as water silica, silicon, and BeF$_2$.

cond-mat.soft↗