Structural Relaxation Dynamics in Binary Glass-Forming Molecular Liquids with Ideal and Complex Mixing Behavior

Ionic Conductivity of a Lithium-Doped Deep Eutectic Solvent: Glass Formation and Rotation-Translation Coupling

Deep eutectic solvents with admixed lithium salts are considered as electrolytes in electrochemical devices, such as batteries or supercapacitors. Compared to eutectic mixtures of hydrogen-bond donors and lithium salts, their raw-material costs are significantly lower. Not much is known about glassy freezing and rotational-translation coupling of such systems. Here, we investigate these phenomena by applying dielectric spectroscopy to the widely studied deep eutectic solvent glyceline, to which 1 and 5 mol % LiCl were added. Our study covers a wide temperature range, including a deeply supercooled state. The temperature dependences of the detected dipolar reorientation dynamics and ionic direct current (dc) conductivity reveal the signatures of glassy freezing. In comparison to pure glyceline, the lithium admixture leads to a reduction of ionic conductivity, which is accompanied by a reduction of the rotational dipolar mobility. However, this reduction is much smaller than that for deep eutectic solvents (DESs), where one main component is lithium salt, which we trace back to the lower glass-transition temperatures of lithium-doped DESs. In contrast to pure glyceline, the ionic and dipolar dynamics become increasingly decoupled at low temperatures and obey a fractional Debye-Stokes-Einstein relation, as previously found in other glass-forming liquids. The obtained results demonstrate the relevance of decoupling effects and glass transition to the enhancement of the technically relevant ionic conductivity in such lithium-doped DESs.

Rotational dynamics, ionic conductivity, and glass formation in a ZnCl2-based deep eutectic solvent

Glass formation and reorientational motions are widespread but often-neglected features of deep eutectic solvents although both can be relevant for the technically important ionic conductivity at room temperature. Here, we investigate these properties for two mixtures of ethylene glycol and ZnCl2, which were recently considered superior electrolyte materials for application in zinc-ion batteries. For this purpose, we employed dielectric spectroscopy performed in a broad temperature range, extending from the supercooled state at low temperatures up to the liquid phase around room temperature and beyond. We find evidence for a relaxation process arising from dipolar reorientation dynamics, which reveals the clear signatures of glassy freezing. This freezing also governs the temperature dependence of the ionic dc conductivity. We compare the obtained results with those for deep eutectic solvents that are formed by the same hydrogen-bond donor, ethylene glycol, but by two different salts, choline chloride and lithium triflate. The four materials reveal significantly different ionic and reorientational dynamics. Moreover, we find varying degrees of decoupling of rotational dipolar and translational ionic motions, which can partly be described by a fractional Debye-Stokes-Einstein relation. The typical glass-forming properties of these solvents strongly affect their room-temperature conductivity.

Deuteron magnetic resonance study of glyceline deep eutectic solvents: Selective detection of choline and glycerol dynamics

Glyceline, a green solvent considered for various electrochemical applications, represents a multi-component glass former. Viewed from this perspective, the choline cation and the hydrogen bond donor glycerol, the two major constituents forming this deep eutectic solvent, were studied using nuclear magnetic resonance in a selective manner by means of suitably deuteron-labeled isotopologues. Carried out from far above to far below the glass transition temperature, measurements and analyses of the spin-lattice and spin-spin relaxation times reveal that the reorientational dynamics of the components, i.e., of glycerol as well as of chain deuterated choline chloride are slightly different. Possible implications of this finding regarding the hydrogen-bonding pattern in glyceline are discussed. Furthermore, the deuterated methyl groups in choline chloride are exploited as sensitive probes of glyceline's supercooled and glassy states. Apart from spin relaxometry, a detailed line shape analysis of the CD₃ spectra yields valuable insights into the broad intermolecular and intramolecular energy barrier distributions present in this binary mixture.

Quantifying Concentration Fluctuations in Binary Glass-Forming Systems by Small- and Wide-Angle X-ray Scattering

Functionality of amorphous multicomponent systems largely depends upon the miscibility among components, especially in systems such as amorphous drugs and electrolytes. An in-depth understanding of mixing behaviors of various constituents is necessitated. Here, we applied the small- and wide-angle X-ray scattering (SWAXS) technique to monitor the mixing behaviors in three typical glass-forming binary systems imposed by varied heat of mixing. It is found that the Porod invariant (Q) determined at the glass transition temperature is remarkably enhanced as the concentration fluctuation becomes intensified. Meanwhile, the deviation of Q from the ideal mixing law is markedly weaken at elevated temperatures. The results unambiguously suggest that the degree of concentration fluctuations in mixing systems can be accurately quantified by the structural property, allowing the link to mixing thermodynamics.

Lithium-salt-based deep eutectic solvents: Importance of glass formation and rotation-translation coupling for the ionic charge transport

Lithium-salt-based deep eutectic solvents, where the only cation is Li⁺, are promising candidates as electrolytes in electrochemical energy-storage devices, such as batteries. We have performed broadband dielectric spectroscopy on three such systems, covering a broad temperature and dynamic range that extends from the low-viscosity liquid around room temperature down to the glassy state approaching the glass-transition temperature. We detect a relaxational process that can be ascribed to dipolar reorientational dynamics and exhibits the clear signatures of glassy freezing. We find that the temperature dependence of the ionic dc conductivity and its room-temperature value also are governed by the glassy dynamics of these systems, depending, e.g., on the glass-transition temperature and fragility. Compared to the previously investigated corresponding systems, containing choline chloride instead of a lithium salt, both the reorientational and ionic dynamics are significantly reduced due to variations in the glass-transition temperature and the higher ionic potential of the lithium ions. These lithium-based deep eutectic solvents partly exhibit significant decoupling of the dipolar reorientational and the ionic translational dynamics and approximately follow a fractional Debye-Stokes-Einstein relation, leading to an enhancement of the dc conductivity, especially at low temperatures. The presented results clearly reveal the importance of decoupling effects and of the typical glass-forming properties of these systems for the technically relevant room-temperature conductivity.

Vitrification of octonary perylene mixtures with ultralow fragility

Strong glass formers with a low fragility are highly sought-after because of the technological importance of vitrification. In the case of organic molecules and polymers, the lowest fragility values have been reported for single-component materials. Here, we establish that mixing of organic molecules can result in a marked reduction in fragility. Individual bay-substituted perylene derivatives display a high fragility of more than 70. Instead, slowly cooled perylene mixtures with more than three components undergo a liquid-liquid transition and turn into a strong glass former. Octonary perylene mixtures display a fragility of 13 ± 2, which not only is a record low value for organic molecules but also lies below values reported for the strongest known inorganic glass formers. Our work opens an avenue for the design of ultrastrong organic glass formers, which can be anticipated to find use in pharmaceutical science and organic electronics.

Unveiling the strong dependence of the α-relaxation dispersion on mixing thermodynamics in binary glass-forming liquids

Structural α-relaxation dispersion in binary molecular glass forming mixtures with distinct mixing enthalpy ΔHmix was investigated using enthalpic and dielectric relaxation measurements across the entire composition range. This study focused on the dependence of the relaxation dispersion on the mixing thermodynamics by determining the non-exponential exponent β, and its composition dependence. The β values determined by the enthalpic and dielectric relaxations agree well. Remarkably, it is found that the systems with positive enthalpy of mixing (exothermic, ΔHmix >0) have positive deviations in the composition dependence of β from the linear averaging of the two β values of the pure components, while negative deviations are observed for the systems with negative enthalpy of mixing (endothermic, ΔHmix <0). Furthermore, the relation between the non-exponential behaviors and entropy of mixing is discussed, revealing that the positive or negative deviation of β in its composition dependence on mixing is accompanied by the same sign of the excess entropy of mixing relative to the ideal one.

Theory for Glassy Behavior of Supercooled Liquid Mixtures

We present a model for glassy dynamics in supercooled liquid mixtures. Given the relaxation behavior of individual supercooled liquids, the model predicts the relaxation times of their mixtures as temperature is decreased. The model is based on dynamical facilitation theory for glassy dynamics, which provides a physical basis for relaxation and vitrification of a supercooled liquid. This is in contrast to empirical linear interpolations such as the Gordon-Taylor equation typically used to predict glass transition temperatures of liquid mixtures. To understand the behavior of supercooled liquid mixtures we consider a multicomponent variant of the kinetically constrained East model in which components have a different energy scale and can also diffuse when locally mobile regions, i.e., excitations, are present. Using a variational approach we determine an effective single component model with a single effective energy scale that best approximates a mixture. When scaled by this single effective energy, we show that experimental relaxation times of many liquid mixtures all collapse onto the "parabolic law" predicted by dynamical facilitation theory. The model can be used to predict transport properties and glass transition temperatures of mixtures of glassy materials, with implications in atmospheric chemistry, biology, and pharmaceuticals.

Dynamical Correlations for Statistical Copolymers from High-Throughput Broad-Band Dielectric Spectroscopy

Broad-band dielectric spectroscopy (BDS) provides a powerful method of characterizing relaxation dynamics in diverse materials. Here we describe and employ a novel instrument for high-throughput broad-band dielectric spectroscopy (HTBDS) that accelerates this capability, enabling simultaneous measurements of 48 samples. This capability is based around a coaxial switching system for rapid scanning between multiple samples on the same sample stage, coordinated with shared environmental control. We validate the instrument by measuring dielectric response in three polymers, distributed across 48 sample sites, and comparing results to measurements via a standard BDS instrument. Results are found to be reproducible and are in agreement with relaxation times from traditional BDS. We then employ HTBDS to establish mixing rules for glass transition temperatures, kinetic fragility indices, and segmental stretching exponents in a series of acrylate copolymers, a matter of considerable technological interest in a variety of technological applications. Results are consistent with the empirical Fox rule for the glass transition temperature T_g averaging in polymer blends, while they reveal a linear mixing rule for kinetic fragility indices. Finally, we test several proposed correlations between these distinct dynamical properties. These results demonstrate that HTBDS enables measurements of polymer relaxation at a throughput approximately 10 times higher than that of standard BDS approaches, opening the door to high-throughput materials design of dynamic response across a broad range of frequencies.

Ionic conductivity of deep eutectic solvents: the role of orientational dynamics and glassy freezing

Dielectric spectroscopy reveals that the ionic conductivity of deep eutectic solvents is closely coupled to their reorientational dipolar relaxation dynamics.

Relaxation dynamics in the strong chalcogenide glass-former of Ge22Se78

The enthalpy relaxation is performed in the glassy Ge₂₂Se₇₈ to understand the dynamic behaviors. The structure of the glass is examined by X-ray diffraction and Raman spectra. The dynamic parameters such as the fragility, stretching exponent and non-linear factor are determined. A low fragility of m = 27 is exhibited for the chalcogenide, however, the stretching exponent is found not to have a larger value. The enthalpy relaxation spectra are constructed for various glass formers, and a relationship between the fragility and the symmetry of the spectra is demonstrated. The dynamic results are used to evaluate the structure of the Ge₂₂Se₇₈ glass.

Proton Conductivity in Phosphoric Acid: The Role of Quantum Effects

Phosphoric acid has one of the highest intrinsic proton conductivities of any known liquids, and the mechanism of this exceptional conductivity remains a puzzle. Our detailed experimental studies discovered a strong isotope effect in the conductivity of phosphoric acids caused by (i) a strong isotope shift of the glass transition temperature and (ii) a significant reduction of the energy barrier by zero-point quantum fluctuations. These results suggest that the high conductivity in phosphoric acids is caused by a very efficient proton transfer mechanism, which is strongly assisted by quantum effects.

Relaxation and physical aging in network glasses: a review

Recent progress in the description of glassy relaxation and aging are reviewed for the wide class of network-forming materials such as GeO2, Ge x Se1-x , silicates (SiO2-Na2O) or borates (B2O3-Li2O), all of which have an important usefulness in domestic, geological or optoelectronic applications. A brief introduction of the glass transition phenomenology is given, together with the salient features that are revealed both from theory and experiments. Standard experimental methods used for the characterization of the slowing down of the dynamics are reviewed. We then discuss the important role played by aspects of network topology and rigidity for the understanding of the relaxation of the glass transition, while also permitting analytical predictions of glass properties from simple and insightful models based on the network structure. We also emphasize the great utility of computer simulations which probe the dynamics at the molecular level, and permit the calculation of various structure-related functions in connection with glassy relaxation and the physics of aging which reveal the non-equilibrium nature of glasses. We discuss the notion of spatial variations of structure which leads to the concept of 'dynamic heterogeneities', and recent results in relation to this important topic for network glasses are also reviewed.

Revisiting the glass transition and dynamics of supercooled benzene by calorimetric studies

The glass transition and dynamics of benzene are studied in binary mixtures of benzene with five glass forming liquids, which can be divided into three groups: (a) o-terphenyl and m-xylene, (b) N-butyl methacrylate, and (c) N,N-dimethylpropionamide and N,N-diethylformamide to represent the weak, moderate, and strong interactions with benzene. The enthalpies of mixing, ΔH(mix), for the benzene mixtures are measured to show positive or negative signs, with which the validity of the extrapolations of the glass transition temperature T(g) to the benzene-rich regions is examined. The extrapolations for the T(g) data in the mixtures are found to converge around the point of 142 K, producing T(g) of pure benzene. The fragility m of benzene is also evaluated by extrapolating the results of the mixtures, and a fragility m ∼ 80 is yielded. The obtained T(g) and m values for benzene allow for the construction of the activation plot in the deeply supercooled region. The poor glass formability of benzene is found to result from the high melting point, which in turn leads to low viscosity in the supercooled liquid.

Unveiling the dependence of glass transitions on mixing thermodynamics in miscible systems

The dependence of the glass transition in mixtures on mixing thermodynamics is examined by focusing on enthalpy of mixing, ΔH_mix with the change in sign (positive vs. negative) and magnitude (small vs. large). The effects of positive and negative ΔH_mix are demonstrated based on two isomeric systems of o- vs. m- methoxymethylbenzene (MMB) and o- vs. m- dibromobenzene (DBB) with comparably small absolute ΔH_mix. Two opposite composition dependences of the glass transition temperature, T_g, are observed with the MMB mixtures showing a distinct negative deviation from the ideal mixing rule and the DBB mixtures having a marginally positive deviation. The system of 1, 2- propanediamine (12PDA) vs. propylene glycol (PG) with large and negative ΔH_mix is compared with the systems of small ΔH_mix, and a considerably positive T_g shift is seen. Models involving the properties of pure components such as T_g, glass transition heat capacity increment, ΔC_p, and density, ρ, do not interpret the observed T_g shifts in the systems. In contrast, a linear correlation is revealed between ΔH_mix and maximum T_g shifts.

Anomalously large isotope effect in the glass transition of water

We present the discovery of an unusually large isotope effect in the structural relaxation and the glass transition temperature Tg of water. Dielectric relaxation spectroscopy of low-density as well as of vapor-deposited amorphous water reveal Tg differences of 10 ± 2 K between H2O and D2O, sharply contrasting with other hydrogen-bonded liquids for which H/D exchange increases Tg by typically less than 1 K. We show that the large isotope effect and the unusual variation of relaxation times in water at low temperatures can be explained in terms of quantum effects. Thus, our findings shed new light on water's peculiar low-temperature dynamics and the possible role of quantum effects in its structural relaxation, and possibly in dynamics of other low-molecular-weight liquids.

Calorimetric determination of fragility in glass forming liquids: T(f) vs. T(g-onset) methods

The calorimetric determination of the fragility m-index is compared using the T f and T g-onset methods for typical metallic and molecular glass forming systems of Pd39Ni10Cu30P21, glycerol, triacetin and propylene carbonate. The results are evaluated by referring to the standard m-values determined from the kinetic measurements of the viscosity or structural relaxation time in the supercooled liquid regimes. The m-indexes derived from the T f method are found to generally agree well with the kinetic measurements for all the systems. However, a large deviation is shown between the m-indexes calculated with the T g-onset method and the kinetic results for the fragile liquids of triacetin and propylene carbonate, indicating the calorimetric determination of the fragility m-indexes in terms of the T f method produces less uncertainty.

Effects of 2 nm size added heterogeneity on non-exponential dielectric response, and the dynamic heterogeneity view of molecular liquids

To investigate how non-exponential response could vary under different conditions, we studied the effects of adding 2 nm size polyhedral oligomeric silsesquioxane (POSS) to diglycidyl ether of bisphenol-A, whose molecules have the same terminal (epoxide) dipoles as the tentacle-like side chains attached to the silsesquioxane core of the POSS molecule. Dielectric relaxation spectra show that, on initial addition, the POSS nano-heterogeneity decreases the non-exponential response parameter β, which is consistent with the dynamic heterogeneity view, but it also decreases the relaxation time τ(m), which is inconsistent with that view. The variations in β and τ(m) with the composition have a thermal equivalence. Despite the lack of translational diffusion required for dynamic heterogeneity, plastic crystals show non-exponential response and non-Arrhenius dynamics. Measurements of β and τ(m) seem more appropriate than using probe molecules or modeling nonlinear response data as a sum of linear responses for testing the dynamic heterogeneity view. Data on molecular liquid mixtures is not generally consistent with this view, and adding a solute does not always decrease β. Studies of mixtures of different size rigid molecules with identical dipolar groups, including polymers, may be useful for comparing the relative effects of temperature and molecular size on β and τ(m).

Kinetic fragility of binary and ternary glass forming liquid mixtures

The experimental studies of liquid fragility in miscible binary and ternary glass forming mixtures reveal a general observation of the negative deviation in fragility upon mixing from the linear average of those of the components. Further analyses from ideal, near ideal to non-ideal mixing modes show that the deviation magnitude does not increase monotonically with mixing enthalpy, and a moderate intermolecular interaction would generate a largest reduction in fragility. Four eutectic systems, methyl-o-toluate-methyl-p-toluate, ZnCl(2)-AlCl(3), glycerol-water, and fructose-water, are studied to locate the composition where the largest fragility deviation occurs in phase diagrams. It is found that the compositions with the fragility minima do not coincide with the eutectic points. The results partly explain the experimental observation that the best glass forming region is not located at the eutectic composition.

Component dynamics in miscible mixtures of water and methanol

In binary mixtures with hydrophilic substances, water is usually the more mobile component and its relaxation time is shorter than those of the other components. An exception is the case of the mixture of 1-propanol with 45 mol % water, where the α-relaxation of water is slower than the α-relaxation of 1-propanol and even slower than the local relaxation of water confined in various spaces of nanometer size. This unusual result, so far obtained in a mixture of 1-propanol with water at a single composition, deserves confirmation by experiments in another mixture at more than one composition. Toward this goal, we have chosen mixtures of methanol with water at concentrations of water ranging from 10 to 40 mol % and investigated the dynamics of the slower water and the faster methanol components by broad-band dielectric relaxation measurements. The α-relaxation time of the water component becomes shorter with increasing content of the faster methanol component in the mixture as expected and is much shorter than in the mixture of 1-propanol with 45 mol % water. In mixtures with lower water contents of 10-20 mol %, the α-relaxation of the methanol component has a narrow frequency dispersion and no resolved Johari-Goldstein β-relaxation, indicating a low degree of intermolecular coupling or cooperativity of methanol. An increase of the content of the slower water component effectively enhances intermolecular coupling of the methanol component. Consequently, the α-relaxation of the methanol component becomes more cooperative, as evidenced by broadening of its frequency dispersion and the appearance of a resolved Johari-Goldstein β-relaxation of methanol when the water concentration is higher than 30 mol %. The observations are rationalized by application of the coupling model.

An upper limit to kinetic fragility in glass-forming liquids

The kinetic fragility of a liquid is correlated to the magnitude of enthalpy hysteresis in various glass-forming materials during thermal cycling across the glass transition. While the lower bound of liquid fragility is well known, there has been little research into the possibility of an inherent upper limit to fragility. In this paper, we present a theoretical argument for the existence of a maximum fragility and show that the correlation between fragility and enthalpy hysteresis allows for an empirical evaluation of the upper limit of fragility. This upper limit occurs as the enthalpy hysteresis involved in thermal cycling about the glass transition approaches zero, leading to m(max)≈175. This result agrees remarkably well with our previous estimate. The dynamics of maximum fragility liquids are discussed, and a critical temperature of ∼1.5 T(g) (where T(g) is the glass transition temperature) is revealed where a transition from nonexponential to exponential structural relaxation occurs.

Prediction of favored and optimized compositions for Cu-Zr-Ni metallic glasses by interatomic potential

For the Cu-Zr-Ni system, an interatomic potential was constructed under the newly proposed formulism named smoothed and long-range second-moment approximation of tight-binding. Applying the constructed potential, molecular dynamics simulations were carried out to compare the relative stability of crystalline solid solution versus its disordered counterpart. Simulations not only reveal that the origin of metallic glass formation is the crystalline lattice collapsing while the solute concentrations exceed critical values, but also determine a quadrilateral region, within which the metallic glass formation is energetically favored. Moreover, the energy differences between the crystalline solid solutions and the disordered states were considered as the driving force for amorphization and were computed by molecular static calculations. The calculation results located an optimized composition area with the driving force much greater than those outside. In addition, the alloys around the composition of Cu(16)Zr(60)Ni(24) were identified to have maximum driving force, and the atomic configurations were also analyzed by the Voronoi tessellation method.

Dielectric relaxation dynamics in glass-forming mixtures of propanediol isomers

The relaxation dynamics of 1,2-propanediol-1,3-propanediol mixtures is studied in supercooled liquid regions across a wide composition range. The composition dependences of liquid fragility and nonexponential parameter β(KWW) are presented in the hydrogen-bonded mixtures with ideal mixing. The fragility index and glass transition temperature are shown to develop inversely with β(KWW), in analogy to the dynamic behaviors in mixtures of van der Waals liquids. Negative mixing effects on liquid fragility and β(KWW) are observed, and the strongest dependence of β(KWW on relaxation dynamics is revealed at the equimolar concentration. The glass formation in isomeric liquids is also addressed.