Heterogeneous slow dynamics and the interaction potential of glass-forming liquids

The role of the intermolecular interaction potential on the dynamic and thermodynamic properties of model glass-forming mixtures is investigated through molecular dynamics simulations. Variations of the repulsive exponent m in the well-studied Lennard-Jones Kob-Andersen mixture are shown to have a negligible effect on the fragility and dynamic correlation volumes when quenches are performed at constant pressure. The number of dynamically correlated particles, estimated from the temperature derivative of a two-point dynamic correlation function, is approximately invariant to m at any fixed relaxation time. Further, the density scaling property of a model tetrahedral network glass-former, based on inverse power law and Lennard-Jones potentials, is investigated. The optimal scaling exponent γ is close to zero and does not superpose the data well. The breakdown of density scaling is consistent with the absence of correlation between fluctuations of the virial and the potential energy. These results emphasize the crucial role of structural many-body correlations in glass-forming systems and show the need of investigations of more complex and realistic model liquids.     

Elastic models for the non-Arrhenius viscosity of glass-forming liquids

This paper first reviews the shoving model for the non-Arrhenius viscosity of viscous liquids. According to this model the main contribution to the activation energy of a flow event is the energy needed for molecules to shove aside the surrounding, an energy which is proportional to the instantaneous shear modulus of the liquid. Data are presented supporting the model. It is shown that the fractional Debye–Stokes–Einstein relation, which quantitatively expresses the frequently observed decoupling of, e.g., conductivity from viscous flow, may be understood within the model. The paper goes on to review several related explanations for the non-Arrhenius viscosity. Most of these are also ‘elastic models’, i.e., they express the viscosity activation energy in terms of short-time elastic properties of the liquid. Finally, two alternative arguments for elastic models are given, a general solid-state defect argument and an Occam’s razor type argument.     

Sound velocity in alumino-silicate liquids determined up to 2550 K from Brillouin spectroscopy: glass transition and crossover temperatures

Hypersonic longitudinal sound velocities in five silicate and alumino-silicate liquids have been measured between 293 and 2550 K by Brillouin spectroscopy. Together with previous observations for four other glasses and liquids of the system SiO₂-Al₂O₃-CaO-MgO, these results are used to discuss changes in hypersonic velocities in three adjacent temperature domains, i.e., below, in, and above the glass transformation range. The temperature dependence of Brillouin velocities is consistent with the observed variations with temperature of viscosity, density, and mean heat capacity for the same three temperature domains. These variations of physical properties of alumino-silicate liquids are qualitatively in agreement with the Inherent Structure Theory for liquids.     

Synergetic model of the formation of non-crystalline structures

The peculiarity of the transition to the non-crystalline state (i.e. the dependence of the transition temperature on the control parameter as well as an anomalous increase of the life time and thermodynamic properties in the transition region) and the formation of dissipative structures were considered from the standpoint of synergetics. The dissipative structures in non-crystalline state (structured liquids, glasses, amorphous substances) are determined in the organizing mode of a specific system with a minimum of energy dissipation.     

High Temperature Ionic Liquids: Thermal Properties and Dimorphism in [1-(Methoxycarbonyl) Ethyl] Triphenylphosphonium <Emphasis Type="Italic">p</Emphasis>-Toluenesulfonate

Abstract  Investigation of the thermal properties of triphenylphosphonium p-toluenesulfonates, which are high temperature ionic liquids, reveals that they solidify to a noncrystalline material on fast cooling from the melt. During the study, a second crystalline form of [1-(methoxycarbonyl) ethyl] triphenylphosphonium p-toluenesulfonate (form II) has been obtained. Form II is monoclinic P2₁/c, with a = 11.002(5)?, b = 12.793(5)?, c = 18.805(5)?, and β = 102.498(5)°. Comparison with form I reveals the existence of a common robust structural feature involving interactions between cations and anions. Graphical Abstract  Investigation of thermal properties of ionic liquids has revealed a second crystalline form of [1-(methoxycarbonyl) ethyl] triphenylphosphonium p-toluenesulfonate.      

Kinetic vs. thermodynamic control of crystal nucleation and growth in molten silicates

A series of calcium aluminosilicate liquids have been experimentally heat-treated at high but variable states of undercooling, from just above the glass transition to the vicinity of the solidus. The mineralogy and chemistry of crystalline phases which appear in these experiments have been quantified using a combination of Transmission Electron Microscopy and Raman spectroscopy. The results show that mineral compositions are highly variable as a function of temperature, but that changes are governed by the contrasting and strongly temperature-dependent mobilities of network-modifying and network-forming cations. Whereas equilibrium crystals form near the liquidus, disordered and non-stoichiometric phases precipitate near the glass transition. Despite this apparently complex situation, the relative importance of thermodynamic and kinetic factors is found to be a single function of T − T_g (where T is temperature and T_g the glass transition temperature), regardless of the silicate composition. The existence of this mastercurve may be used to control the composition of novel composite materials such as glass ceramics.     

‘Intra-cluster rearrangement' model for the α-process in supercooled liquids, as opposed to ‘cooperative rearrangement of whole molecules within a cluster'

A new model for the α-relaxation process in supercooled liquids and glasses is proposed which distinguishes between a structurally correlated region (cluster) of molecules and a unit of molecules for rather independent, correlated rearrangement motion. The essential aspects of the model are that the α-process is due to rearrangement of one or a few molecules within the cluster, while essentially the same motion in the space between the clusters is responsible for the β-process. The model leads to the following expectations: (i) absence of divergent behavior of α-relaxation times at non-zero temperature (e.g., Kauzmann temperature), (ii) close agreement between the glass transition temperatures, T_g, for the α-relaxation in liquid and crystalline phases of the same composition and (iii) possibility of crystal nucleation proceeding much below the T_g, and evidence for the latter two is presented.     

Why is the osmotic second virial coefficient related to protein crystallization?

A molecular basis is presented for characterizing the osmotic second virial coefficient, B₂₂, of dilute protein solutions, which provides a measure of the nature of protein–protein interactions and has been shown to be correlated with crystallization behavior. Experimental measurements of the second virial coefficient of lysozyme and bovine α-chymotrypsinogen A were performed by static light scattering, as a function of pH and electrolyte concentration. Although some of the trends can be explained qualitatively by simple colloidal models of protein interactions, a more realistic interpretation based on protein crystallographic structures suggests a different explanation of experimental trends. The interactions accounted for are solute–solute excluded volume (steric), electrostatic and short-range (mainly van der Waals) interactions. The interactions depend strongly on orientation, and this profoundly affects calculated second virial coefficients. We find that molecular configurations in which complementary surfaces are apposed contribute disproportionately to the second virial coefficient, mainly through short-range interactions; electrostatic interactions play a secondary role in many of these configurations. Thus molecular recognition events can play a role in determining the solution thermodynamic properties of proteins, and this provides a plausible basis for explaining the observed relationship with crystallization behavior.     

Low frequency relaxation in liquid crystals in relation to structural relaxation in glass-formers

Liquid crystals (LC) are the state of matter intermediate between isotropic liquids and the crystalline state. LC-forming molecules have strongly anisotropic shapes (rod-like in most cases). This leads to an interaction potential that consists of distance-dependent and orientation-dependent parts. Rotational dynamics of LC molecules falls into two frequency regions. Rotations about the short axes are strongly hindered by the potential barrier and thus coupled to fluctuations of the molecular centers of mass. This in turn causes these longitudinal or “flip-flop” motions, characterized by a relatively large relaxation time τ_||, to exhibit considerable temperature, pressure and volume dependences. Experimental relaxation times determined to date for various LC phases (nematic, smectic A, C, and E) for different thermodynamic conditions (isobaric, isothermal and isochoric) are discussed herein, adopting the formulae applied for characterization of the structural relaxation times of glass-formers (GF). This analysis appears fruitful; in particular, the strength parameter characterizing the steepness of the interaction potential can be determined from the relaxation times, and τ_|| is independent of temperature and pressure along the nematic-isotropic transition line, similar to the behavior of the structural relaxation time along certain transitions in GFs.     

Thermodynamical properties of glass forming systems: A Nuclear Magnetic Resonance analysis

We present a Nuclear Magnetic Resonance (NMR) study of the thermal evolution of the magnetic properties of three different glass formers: glycerol, o-terphenyl and salol. In particular, we analyze how the response of these liquids to the applied magnetic field changes with temperature. We focus on the total magnetization and on the chemical shift of each protonated group. By means of these quantities we account that the dynamics of the glass forming materials, on decreasing the temperature, is dominated by the onset of well defined local inhomogeneities due to precise microscopic cooperative processes. Just these “dynamical heterogeneities” and their energetic topology determine the dynamic crossover from fragile (super Arrhenius) to strong (pure Arrhenius) glass forming behavior. The specific heat changes evaluable from the measured NMR chemical shift associate this phenomenon, and all the related ones, to local configurational changes.     

The Electro-Optical Properties of Doping Polycyclic Aromatic Hydrocarbon Liquids in Twisted Nematic Liquid Crystals

In this paper, doping liquid materials to enhance the electro-optical (EO) properties of twisted nematic liquid crystals (NLCs) was presented. Two polycyclic aromatic hydrocarbon (PAH) liquids, toluene and 1-methylnaphthalene, were chosen as dopants in order to lower the driving voltage and response time of the NLCs. We find that the main reason of this phenomenon is due to a large amount of reduction in the rotational viscosity of PAH liquids doped NLCs. Without the drawbacks of aggregation that the solid nanoparticles could have, the method of doping liquids provides a more reliable choice for applications in various LC display systems.     

Atomic dynamics in different regions of the potential energy surface

The atomic dynamics in two (bulk) metallic glasses, Ni₄₀Pd₄₀P₂₀ and Zr₅₅Cu₃₀Al₁₀Ni₅, were investigated by neutron inelastic scattering in different regions of the potential energy landscape, which are reached by slow cooling the bulk glasses and by hyper-quenching the same alloys. The results prove that the atomic dynamics depends also on the fictive temperature, i.e. the region of the potential energy surface, in which the glass is frozen in. Obviously the shapes of the basins or inherent structures are not the same everywhere on the potential energy surface, and the glass with a higher fictive temperature has more low energy modes than has the same glass with a lower fictive temperature. As results from computer simulation have suggested already, on moving to regions of lower mean potential energy (aging), part of theses low energy modes are transferred to the energy region of the calculated Debye cut-off energy. The difference between the vibrational entropies, calculated from the generalized vibrational density-of-states, which have been determined for both fictive temperatures, shows that the contribution from the vibrational entropy to the total entropy change, when moving through the potential energy landscape, is small for the two metallic glasses investigated. Structural relaxation of the hyper-quenched glass removes part of the additional low energy modes, but quantitatively possibly only at the low and perhaps also at the high-energy limit of the density-of-states. The wavelength dependence of the dynamics suggests that the additional low energy modes in the glass with the higher fictive temperature are not dominated by extended but more likely by localized modes.     

Composition and polyamorphism in supercooled yttria-alumina melts

By extending recent work on liquid-liquid transitions in supercooled yttria-alumina AYx liquids we draw attention to the compositional dependence of the structure factor of the high density liquid, arguing that this is sufficiently sensitive to discriminate between liquids at the level of a few %. Comparing structure factor differences between liquids of different compositions and in the same liquid AY20 between high and low temperatures straddling the transition at 1788 K between a high density liquid (HDL) and a low density liquid (LDL) enables compositional phase separation to be ruled out. It points instead to kinetic changes in polyhedral configurational order being the drivers for this polyamorphic transformation. Rotor behaviour observed in levitated liquid drops used in the high temperature experiments enables the reversibility of the LLT transition (LLT) and the associated changes in entropy and density to be identified. Evidence for critical-like behaviour in the structural relaxation time and in the fluctuation correlation length is presented. By re-examining recent work which failed to find the structural and thermal signatures for the LLT in liquid AY20 at 1788 K we present evidence for the LLT occurring instead in liquid AY15 at 1940 K, suggesting that the liquid-liquid transition temperature in AYx liquids decreases with increasing yttria content.     

The defect diffusion model and the properties of glasses and liquids

Several previously unexplained relaxation phenomena are interpreted quantitatively in terms of the defect diffusion model (DDM). These include widely different pre-exponentials, exponents and consequently fragility. In addition, the DDM is used to give a qualitative explanation of many properties that glasses and liquids exhibit. This includes interpretations of the glass transition (T_g), the liquid–liquid transition (T_B), the crystalline melting temperature (T_m), and the different values of the characteristic temperature, T_C, that some materials exhibit for different types of physical measurements. Next, qualitative explanations of the origin of secondary relaxations such as the excess wing and the β relaxation are given. In addition, the boson peak is discussed in terms of the DDM. Finally, it is pointed out that in the DDM, universality is embodied in the defects i.e. free volume.     

离子液体对硫化镉粒径和光催化性能调控研究

以氯化镉和硫代乙酰胺为原料,分别以水,离子液以及两者的混合液为溶剂,用超声法成功合成了不同粒径的硫化镉纳米粒子.用XRD,SEM,IR以及UV-Vis等手段对所制备样品的晶型、形貌和光学特性进行了表征;以紫外光为光源、甲基橙为目标降解物评价了硫化镉纳米粒子的光催化活性.结果表明,离子液的引入使硫化镉纳米粒子的粒径明显变小,带隙能量变大,光催化性能测试表明,离子液介质中合成的CdS纳米粒子具有更高的光催化活性.离子液体对比实验还表明,三乙烯四胺型离子液体中合成的硫化镉具有更高的光催化活性.     

The Effective Temperature of Surfaces of Liquid Metals and Some Non-Metals

An estimation of decrease of atomic bond energy by raising temperature for surfaces of liquid metals and some non-metals had been made. The calculation values of the derivative du_sdu_b where u_s and u_b, are bond energy of atoms in surface and bulk of a liquid, are significantly above 1 and in limits from 2 to 10. It was supposed that this results from anomalously intensive thermal expansion of surface layers of liquids – the effect which is well known for crystal surfaces. This expansion was explained by the effective temperature of these surfaces which, thus, was found to be by several times above those for bulk of liquids.     

Theoretical and experimental study of excess quantities in glass-forming systems

The present work deals with the correlation among a macroscopic quantity, i.e. kinematic viscosity, and some microscopic quantities, namely the mean-square displacement and the free volume. In primis, some definitions of ‘fragility’, a quantity which allows to rank different liquids on the basis of the temperature dependence of kinematic and/or thermodynamic quantities, are recalled. Successively, the temperature dependence of viscosity, mean-square displacement and free volume is taken into account for three glass-forming systems and is analyzed in order to highlight the linear relationships between the logarithm of viscosity and the excess mean-square displacement and between the logarithm of viscosity and the excess free volume. Finally, on the basis of the observed correlations, two ‘fragility’ parameters, operatively evaluated by elastic neutron scattering and positron annihilation lifetime spectroscopy, are discussed and compared.     

Key experiments in highly viscous liquids

The flow of highly viscous liquids is still a theoretical no-man's-land, with many contradictory theories competing for the explanation of the flow process. On the other hand, a rapid experimental development, in particular in the field of broadband dielectric spectroscopy, supplies more and more information on the nature of the highly viscous flow. The flow process itself (the so-called primary or alpha-process) seems to be intimately related to faster precursor processes, which appear either as part of the wing of the flow process itself or as a secondary relaxation peak, the “Johari-Goldstein” peak, often five orders of magnitude faster than the primary process. An aging experiment shows that the secondary process occurs between strongly asymmetric states, with an energy difference of about 4 k_BT. The paper describes some of these key experiments and their possible theoretical interpretation.