The dielectric response of simple organic glass formers

On the spectral shape of the structural relaxation in supercooled liquids

Structural relaxation in supercooled liquids is non-exponential. In susceptibility representation, χ″(ν), the spectral shape of the structural relaxation is observed as an asymmetrically broadened peak with a ν1 low- and ν-β high-frequency behavior. In this perspective article, we discuss common notions, recent results, and open questions regarding the spectral shape of the structural relaxation. In particular, we focus on the observation that a high-frequency behavior of ν-1/2 appears to be a generic feature in a broad range of supercooled liquids. Moreover, we review extensive evidence that contributions from orientational cross-correlations can lead to deviations from the generic spectral shape in certain substances, in particular in dielectric loss spectra. In addition, intramolecular dynamics can contribute significantly to the spectral shape in substances containing more complex and flexible molecules. Finally, we discuss the open questions regarding potential physical origins of the generic ν-1/2 behavior and the evolution of the spectral shape toward higher temperatures.

Manifestation of Rouse and Entanglement Dynamics in Non-Cross-Linked and Cross-Linked Polymers Studied by Field-Cycling and Multiple Quantum NMR

Rubbers prepared from technical poly(butadiene) and natural poly(isoprene) are studied by field-cycling (FC) 1H NMR relaxometry to elucidate the changes of the relaxation spectrum. Starting with the non-cross-linked polymer successively cross-links are introduced via sulfur or peroxide vulcanization. Applying an advanced home-built relaxometer allows one to probe entanglement dynamics in addition to Rouse dynamics. We show that entanglement dynamics evidenced in terms of a characteristic power-law in the NMR susceptibility is still observed with an exponent identical to that in non-cross-linked linear polymers. Yet, the entanglement regime disappears more and more from the accessible frequency window upon increasing the cross-link density and a spectrally enlarged Rouse regime is revealed. Adding a swelling agent, the manifestation of the Rouse and entanglement regimes virtually does not change, yet, the apparent power-law exponents increase. Concomitant multiple-quantum (MQ) 1H NMR experiments provide information on the structure of the rubber network in terms of the residual dipolar coupling and the fraction of the network defects, i.e., persisting entangled or nonentangled chains, introduced upon cross-linking and swelling.

Model for the Shape of the Relaxation Spectrum in Glass Formers

Traditionally the broadness of the spectrum of the relaxation times observed in glass-forming materials has been rationalized by local heterogeneity, where a variety of atomistic environments leads to spectrum of single-exponential relaxation responses. However, the assumption of heterogeneity can break down when tested against the shape of the relaxation spectrum. An alternative homogeneous scenario assumes that the relaxation is inherently multiexponential. A recently developed switchback model [Medvedev, G. A. Phys. Rev. E 2023, 107 (3), 034122] naturally results in a multiexponential wedge-like spectrum that is consistent with the dielectric relaxation, light scattering, and the NMR data. As a particular case the switchback model allows for the spectrum to become single-exponential; under the heterogeneous scenario this would require the heterogeneities to completely vanish, which is hard to justify. Using data from photobleaching experiments and molecular dynamic simulations, it is shown that the relaxation spectrum may become single-exponential under large anisotropic deformation. This is interpreted as an argument in favor of the homogeneous scenario and specifically the switchback model for the relaxation of the glass formers.

Dipolar Order Controls Dielectric Response of Glass-Forming Liquids

The dielectric response of liquids reflects both reorientation of single molecular dipoles and collective modes, i.e., dipolar cross-correlations. A recent theory predicts the latter to produce an additional slow peak in the dielectric loss spectrum. Following this idea we argue that in supercooled liquids the high-frequency power law exponent of the dielectric loss β should be correlated with the degree of dipolar order, i.e., the Kirkwood correlation factor g_{K}. This notion is confirmed for 25 supercooled liquids. While our findings support recent theoretical work the results are shown to violate the earlier Kivelson-Madden theory.

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.

Glass spectrum, excess wing phenomenon, and master curves in molecular glass formers: A multi-method approach

The relaxation spectra of glass formers solely displaying an α-peak and excess wing contribution collected by various methods are reanalyzed to pin down their different spectral evolution. We show that master curve construction encompassing both α-peak and emerging excess wing works for depolarized light scattering (DLS) and nuclear magnetic resonance (NMR) relaxometry. It reveals the self-part of the slow dynamics' spectrum. Master curves are to be understood as a result of a more extensive scaling covering all temperatures instead of strict frequency-temperature superposition. DLS and NMR display identical relaxation spectra; yet, comparing different systems, we do not find a generic structural relaxation at variance with recent claims. Dielectric spectroscopy (DS) spectra show particularities, which render master curve construction obsolete. The DS α-peak is enhanced or suppressed with respect to that of DLS or NMR, yet, not correlated to the polarity of the liquid. Attempting to single out the excess wing from the overall spectrum discloses a stronger exponential temperature dependence of its amplitude compared to that below Tg and a link between its exponent and that of the fast dynamics' spectrum. Yet, such a decomposition of α-peak and excess wing appears to be unphysical. Among many different glasses, the amplitude of the excess wing power-law spectrum is found to be identical at Tg, interpreted as a relaxation analog to the Lindemann criterion.

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.

Molecular Dynamics and Near-Tg Phenomena of Cyclic Thioethers

This article presents the synthesis and molecular dynamics investigation of three novel cyclic thioethers: 2,3-(4'-methylbenzo)-1,4,7,10-tetrathiacyclododeca-2-ene (compound 1), 2,3,14,15-bis(4',4″(5″)-methylbenzo)-1,4,7,10,13,16,19,22,25-octathiacyclotetracosa-2,14-diene (compound 2), and 2,3,8,9-bis(4',4″(5″)-methylbenzo)-1,4,7,10-tetrathiacyclododeca-2,8-diene (compound 3). The compounds exhibit relatively high glass transition temperatures (Tg), which range between 254 and 283 K. This characteristic positions them within the so-far limited category of crown-like glass-formers. We demonstrate that cyclic thioethers may span both the realms of ordinary and sizeable molecular glass-formers, each featuring distinct physical properties. Furthermore, we show that the Tg follows a sublinear power law as a function of the molar mass within this class of compounds. We also reveal multiple dielectric relaxation processes of the novel cyclic thioethers. Above the Tg, their dielectric loss spectra are dominated by a structural relaxation, which originates from the cooperative reorientation of entire molecules and exhibits an excess wing on its high-frequency slope. This feature has been attributed to the Johari-Goldstein (JG) process. Each investigated compound exhibits also at least one intramolecular secondary non-JG relaxation stemming from conformational changes. Their activation energies range from approximately 19 kJ/mol to roughly 40 kJ/mol. Finally, we analyze the high-pressure molecular dynamics of compound 1, revealing a pressure-induced increase in its Tg with a dTg/dp coefficient equal to 197 ± 8 K/GPa.

Orientational dynamics in supercooled glycerol computed from MD simulations: self and cross contributions

The orientational dynamics of supercooled glycerol is probed using molecular dynamics simulations for temperatures ranging from 323 K to 253 K, through correlation functions of first and second ranks of Legendre polynomials, pertaining respectively to dielectric spectroscopy (DS) and depolarized dynamic light scattering (DDLS). The self, cross, and total correlation functions are compared with relevant experimental data. The computations reveal the low sensitivity of DDLS to cross-correlations, in agreement with what is found in experimental work, and strengthen the idea of directly comparing DS and DDLS data to evaluate the effect of cross-correlations in polar liquids. The analysis of the net static cross-correlations and their spatial decomposition shows that, although cross-correlations extend over nanometric distances, their net magnitude originates, in the case of glycerol, from the first shell of neighbouring molecules. Accessing the angular dependence of the static correlation allows us to get a microscopic understanding of why the rank-1 correlation function is more sensitive to cross-correlation than its rank-2 counterpart.

Phenol, the simplest aromatic monohydroxy alcohol, displays a faint Debye-like process when mixed with a nonassociating liquid

Solvated in propylene carbonate, viscous phenol is studied using dielectric spectroscopy and shear rheology. In addition, several oxygen-17 and deuteron nuclear magnetic resonance (NMR) techniques are applied to specifically isotope labeled equimolar mixtures. Quantum chemical calculations are used to check the electrical field gradient at phenol's oxygen site. The chosen combination of NMR methods facilitates the selective examination of potentially hydrogen-bond related contributions as well as those dominated by the structural relaxation. Taken together the present results for phenol in equimolar mixtures with the van der Waals liquid propylene carbonate provide evidence for the existence of a very weak Debye-like process that originates from ringlike supramolecular associates.

Benzothiadiazole-based rotation and possible antipolar order in carboxylate-based metal-organic frameworks

By modifying organic ligands of metal-organic framework with dipolar units, they turn suitable for various applications, e.g., in the field of sensor systems or switching of gas permeation. Dipolar linkers in the organic ligand are capable to rotate in certain temperature and frequency ranges. The copper-bearing paddlewheel shaped metal-organic frameworks ZJNU-40 and JLU-Liu30 possess such a polarizable dipole moment due to their benzothiadiazole moiety in the organic ligands. Here, we investigate the molecular rotor behavior of benzothiadiazole units of the two carboxylate-based MOFs by dielectric spectroscopy and computational simulation. Our dielectric results provide clear evidence for significant reorientational relaxation dynamics of these rotors, revealing various characteristics of glasslike freezing upon cooling. The calculated rotational energy barriers are consistent with experimentally determined barriers for single-dipole dynamics. Moreover, for JLU-Liu30 we find hints at antipolar ordering below about 300 K.

Non-simple flow behavior in a polar van der Waals liquid: Structural relaxation under scrutiny

The non-exponential character of the structural relaxation is considered one of the hallmarks of the glassy dynamics, and in this context, the relatively narrow shape observed by dielectric techniques for polar glass formers has attracted the attention of the community for long time. This work addresses the phenomenology and role of specific non-covalent interactions in the structural relaxation of glass-forming liquids by the study of polar tributyl phosphate. We show that dipole interactions can couple to shear stress and modify the flow behavior, preventing the occurrence of the simple liquid behavior. We discuss our findings in the general framework of glassy dynamics and the role of intermolecular interactions.

Dynamical Susceptibilities of Confined Water from Room Temperature to the Glass Transition

We confine water to narrow silica pores, where crystallization is suppressed, and determine the dynamical susceptibilities of the liquid from room temperature down to the glass transition by combining broadband dielectric spectroscopy (BDS) with ¹H and ²H nuclear magnetic resonance (NMR), in particular, by establishing NMR field-cycling relaxometry. For the correlation times, derivative analysis reveals Vogel-Fulcher-Tammann and Arrhenius regimes at T ≥ 215 K and T ≤ 160 K, respectively, which are separated by a broad crossover region. The continuous transition in the temperature dependence is accompanied by a gradual change from asymmetric high-temperature shapes of the dynamical susceptibilities to symmetric low-temperature ones and by a steady decrease of the dielectric relaxation strength. In the Arrhenius regime (E_a = 0.48 eV) at T ≤ 160 K, 2D ²H NMR spectra reveal quasi-isotropic water reorientation. We rationalize these results in terms of a crossover to an interface-affected, noncooperative relaxation involving both rotational and translational motions.

Random walk model of mobility in glass formers

A mechanism responsible for the generic features of the mean squared displacement and the decay of the orientational autocorrelator of a molecule in a glass forming liquid is poorly understood, where such a mechanism would be critical for creating the theory of glass transition. A discrete random walk model is proposed where, instead of a straight line, a walk is along a tortuous path consisting of blocks of switchback ramps. Subdiffusive regime, short-term dynamic heterogeneity, and existence of the α- and β-relaxation processes emerge naturally from the model. The model suggests that slowing of the rate of relaxation may be due to an increase in the number of the switchback ramps per block rather than due to growth of an energy barrier as typically assumed.

Simple Rules for Complex Near-Glass-Transition Phenomena in Medium-Sized Schiff Bases

Glass-forming ability is one of the most desired properties of organic compounds dedicated to optoelectronic applications. Therefore, finding general structure–property relationships and other rules governing vitrification and related near-glass-transition phenomena is a burning issue for numerous compound families, such as Schiff bases. Hence, we employ differential scanning calorimetry, broadband dielectric spectroscopy, X-ray diffraction and quantum density functional theory calculations to investigate near-glass-transition phenomena, as well as ambient- and high-pressure molecular dynamics for two structurally related Schiff bases belonging to the family of glycine imino esters. Firstly, the surprising great stability of the supercooled liquid phase is shown for these compounds, also under high-pressure conditions. Secondly, atypical self-organization via bifurcated hydrogen bonds into lasting centrosymmetric dimers is proven. Finally, by comparing the obtained results with the previous report, some general rules that govern ambient- and high-pressure molecular dynamics and near-glass transition phenomena are derived for the family of glycine imino esters. Particularly, we derive a mathematical formula to predict and tune their glass transition temperature (T_g) and its pressure coefficient (dT_g/dp). We also show that, surprisingly, despite the presence of intra- and intermolecular hydrogen bonds, van der Waals and dipole–dipole interactions are the main forces governing molecular dynamics and dielectric properties of glycine imino esters.

NMR Relaxometry Accessing the Relaxation Spectrum in Molecular Glass Formers

It is a longstanding question whether universality or specificity characterize the molecular dynamics underlying the glass transition of liquids. In particular, there is an ongoing debate to what degree the shape of dynamical susceptibilities is common to various molecular glass formers. Traditionally, results from dielectric spectroscopy and light scattering have dominated the discussion. Here, we show that nuclear magnetic resonance (NMR), primarily field-cycling relaxometry, has evolved into a valuable method, which provides access to both translational and rotational motions, depending on the probe nucleus. A comparison of ¹H NMR results indicates that translation is more retarded with respect to rotation for liquids with fully established hydrogen-bond networks; however, the effect is not related to the slow Debye process of, for example, monohydroxy alcohols. As for the reorientation dynamics, the NMR susceptibilities of the structural (α) relaxation usually resemble those of light scattering, while the dielectric spectra of especially polar liquids have a different broadening, likely due to contributions from cross correlations between different molecules. Moreover, NMR relaxometry confirms that the excess wing on the high-frequency flank of the α-process is a generic relaxation feature of liquids approaching the glass transition. However, the relevance of this feature generally differs between various methods, possibly because of their different sensitivities to small-amplitude motions. As a major advantage, NMR is isotope specific; hence, it enables selective studies on a particular molecular entity or a particular component of a liquid mixture. Exploiting these possibilities, we show that the characteristic Cole-Davidson shape of the α-relaxation is retained in various ionic liquids and salt solutions, but the width parameter may differ for the components. In contrast, the low-frequency flank of the α-relaxation can be notably broadened for liquids in nanoscopic confinements. This effect also occurs in liquid mixtures with a prominent dynamical disparity in their components.

A Combined Atomic and Molecular Probe Characterization of Aromatic Hydrocarbons via PALS and ESR: Methylbenzene

A combined study of one of the simplest aromatic hydrocarbons, i.e., methylbenzene (toluene) (TOL), via the annihilation of an ortho-positronium (o-Ps) probe via positron annihilation lifetime spectroscopy (PALS) and the rotation dynamics of nitroxide spin probe 2,2,6,6-tetramethyl-piperidinyl-1-oxy (TEMPO) using electron spin resonance (ESR) over a wide temperature range, 10–300 K, is reported. The o-Ps lifetime, τ₃, and the relative o-Ps intensity, I₃, as a function of temperature exhibit changes defining several characteristic PALS temperatures in the slowly and rapidly cooled samples. Similarly, the spectral parameter of TEMPO mobility in TOL, 2A_zz‘, and its correlation time, τ_c, reveal several effects at a set of the characteristic ESR temperatures, which were determined and compared with the PALS results. Finally, the physical origins of the changes in free volume expansion and spin probe mobility are revealed. They are reflected in a series of the mutual coincidences between the characteristic PALS and ESR temperatures and appropriate complementary thermodynamic and dynamic techniques.

Microscopic observation of hidden Johari-Goldstein-β process in glycerol

The Johari-Goldstein-β (JG-β) process is widely observed in a variety of glass-forming systems and recognized as an intrinsic process in deeply supercooled and glassy states. However, in some systems, e.g., glycerol, a clear sign of the JG-β process is often not apparent; for example, an isolated JG-β peak may not be observed in the dielectric relaxation spectrum. In this study, we directly investigated the angstrom-scale dynamics of glycerol through quasielastic scattering experiments using time-domain interferometry. The relaxation times of the local motions start to decouple from the timescale of the diffusion process and follow the established behavior of the JG-β process. This finding microscopically indicates the existence of the hidden JG-β process in glycerol. In addition, we succeeded in determining the decoupling temperature of the JG-β process by using the spatial-scale selectivity of the quasielastic scattering technique.

Nuclear Spin Relaxation in Viscous Liquids: Relaxation Stretching of Single-Particle Probes

Spin-lattice relaxation rates R₁(ω,T), probed via high-field and field-cycling nuclear magnetic resonance (NMR), are used to test the validity of frequency-temperature superposition (FTS) for the reorientation dynamics in viscous liquids. For several liquids, FTS is found to apply so that master curves can be generated. The susceptibility spectra are highly similar to those obtained from depolarized light scattering (DLS) and reveal an excess wing. Where FTS works, two approaches are suggested to access the susceptibility: (i) a plot of deuteron R₁(T) vs the spin-spin relaxation rate R₂(T) and (ii) a plot of R₁(T) vs an independently measured reference time τ_ref(T). Using single-frequency scans, (i) allows one to extract the relaxation stretching as well as the NMR coupling constant. Surveying 26 data sets, we find Kohlrausch functions with exponents 0.39 < β_K ≤ 0.67. Plots of the spin-spin relaxation rate R₂─rescaled by the NMR coupling constant─as a function of temperature allow one to test how well site-specific NMR relaxations couple to a given reference process. Upon cooling of flexible molecule liquids, the site-specific dynamics is found to merge, suggesting that near T_g the molecules reorient essentially as a rigid entity. This presents a possible resolution for the much lower stretching parameters reported here at high temperatures that contrast with the ones that were reported to be universal in a recent DLS study close to T_g. Our analysis underlines that deuteron relaxation is a uniquely powerful tool to probe single-particle reorientation.

Comparative analysis of dielectric, shear mechanical and light scattering response functions in polar supercooled liquids

The studies of molecular dynamics in the vicinity of liquid–glass transition are an essential part of condensed matter physics. Various experimental techniques are usually applied to understand different aspects of molecular motions, i.e., nuclear magnetic resonance (NMR), photon correlation spectroscopy (PCS), mechanical shear relaxation (MR), and dielectric spectroscopy (DS). Universal behavior of molecular dynamics, reflected in the invariant distribution of relaxation times for different polar and weekly polar glass-formers, has been recently found when probed by NMR, PCS, and MR techniques. On the other hand, the narrow dielectric permittivity function ε*(f) of polar materials has been rationalized by postulating that it is a superposition of a Debye-like peak and a broader structural relaxation found in NMR, PCS, and MR. Herein, we show that dielectric permittivity representation ε*(f) reveals details of molecular motions being undetectable in the other experimental methods. Herein we propose a way to resolve this problem. First, we point out an unresolved Johari–Goldstein (JG) β-relaxation is present nearby the α-relaxation in these polar glass-formers. The dielectric relaxation strength of the JG β-relaxation is sufficiently weak compared to the α-relaxation so that the narrow dielectric frequency dispersion faithfully represents the dynamic heterogeneity and cooperativity of the α-relaxation. However, when the other techniques are used to probe the same polar glass-former, there is reduction of relaxation strength of α-relaxation relative to that of the JG β relaxation as well as their separation. Consequently the α relaxation appears broader in frequency dispersion when observed by PCS, NMR and MR instead of DS. The explanation is supported by showing that the quasi-universal broadened α relaxation in PCS, NMR and MR is captured by the electric modulus M*(f) = 1/ε*(f) representation of the dielectric measurements of polar and weakly polar glass-formers, and also M*(f) compares favorably with the mechanical shear modulus data G*(f).

The relaxation behavior of supercooled and glassy imidacloprid

Employing dielectric spectroscopy, oscillatory shear rheology, and calorimetry, the present work explores the molecular dynamics of the widely used insecticide imidacloprid above and below its glass transition temperature. In its supercooled liquid regime, the applied techniques yield good agreement regarding the characteristic structural (alpha) relaxation times of this material. In addition, the generalized Gemant-DiMarzio-Bishop model provides a good conversion between the frequency-dependent dielectric and shear mechanical responses in its viscous state, allowing for an assessment of imidacloprid's molecular hydrodynamic radius. In order to characterize the molecular dynamics in its glassy regime, we employ several approaches. These include the application of frequency-temperature superposition (FTS) to its isostructural dielectric and rheological responses as well as use of dielectric and calorimetric physical aging and the Adam-Gibbs-Vogel model. While the latter approach and dielectric FTS provide relaxation times that are close to each other, the other methods predict notably longer times that are closer to those reflecting a complete recovery of ergodicity. This seemingly conflicting dissimilarity demonstrates that the molecular dynamics of glassy imidacloprid strongly depends on its thermal history, with high relevance for the use of this insecticide as an active ingredient in technological applications.

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.

The relationship between charge and molecular dynamics in viscous acid hydrates

Oscillatory shear rheology has been employed to access the structural rearrangements of deeply supercooled sulfuric acid tetrahydrate (SA4H) and phosphoric acid monohydrate, the latter in protonated (PA1H) and deuterated (PA1D) forms. Their viscoelastic responses are analyzed in relation to their previously investigated electric conductivity. The comparison of the also presently reported dielectric response of deuterated sulfuric acid tetrahydrate (SA4D) and that of its protonated analog SA4H reveals an absence of isotope effects for the charge transport in this hydrate. This finding clearly contrasts with the situation known for PA1H and PA1D. Our analyses also demonstrate that the conductivity relaxation profiles of acid hydrides closely resemble those exhibited by classical ionic electrolytes, even though the charge transport in phosphoric acid hydrates is dominated by proton transfer processes. At variance with this dielectric simplicity, the viscoelastic responses of these materials depend on their structural compositions. While SA4H displays a "simple liquid"-like viscoelastic behavior, the mechanical responses of PA1H and PA1D are more complex, revealing relaxation modes, which are faster than their ubiquitous structural rearrangements. Interestingly, the characteristic rates of these fast mechanical relaxations agree well with the characteristic frequencies of the charge rearrangements probed in the dielectric investigations, suggesting appearance of a proton transfer in mechanical relaxation of phosphoric acid hydrates. These findings open the exciting perspective of exploiting shear rheology to access not only the dynamics of the matrix but also that of the charge carriers in highly viscous decoupled conductors.

Molecular dynamics simulations vs field-cycling NMR relaxometry: Structural relaxation mechanisms in the glass-former glycerol revisited

We combine field-cycling (FC) relaxometry and molecular dynamics (MD) simulations to study the rotational and translational dynamics associated with the glassy slowdown of glycerol. The ¹H NMR spin-lattice relaxation rates R₁(ω) probed in the FC measurements for different isotope-labelled compounds are computed from the MD trajectories for broad frequency and temperature ranges. We find high correspondence between experiment and simulation. Concerning the rotational motion, we observe that the aliphatic and hydroxyl groups show similar correlation times but different stretching parameters, while the overall reorientation associated with the structural relaxation remains largely isotropic. Additional analysis of the simulation results reveals that transitions between different molecular configurations are slow on the time scale of the structural relaxation at least at sufficiently high temperatures, indicating that glycerol rotates at a rigid entity, but the reorientation is slower for elongated than for compact conformers. The translational contribution to R₁(ω) is well described by the force-free hard sphere model. At sufficiently low frequencies, universal square-root laws provide access to the molecular diffusion coefficients. In both experiment and simulation, the time scales of the rotational and translational motions show an unusually large separation, which is at variance with the Stokes-Einstein-Debye relation. To further explore this effect, we investigate the structure and dynamics on various length scales in the simulations. We observe that a prepeak in the static structure factor S(q), which is related to a local segregation of aliphatic and hydroxyl groups, is accompanied by a peak in the correlation times τ(q) from coherent scattering functions.

Molecular heterogeneities in the thermal expansivity of polyalcohols

Density is the key quantity for nearly all the numerous theories of the (dynamic) glass transition of supercooled liquids and melts. As mean field quantity, it is used to describe correlations and heterogeneities between regions consisting of several molecules. In contrast, the question how density is created by the interactions (i.e., bonds) within a molecule and to its nearest neighbors is almost unexplored. To investigate this for the example of a homologous series of polyalcohols (glycerol, threitol, xylitol, and sorbitol), Fourier-Transform InfraRed (FTIR) spectroscopy is carried out in a wide range of temperatures from far above to far below the calorimetric glass transition T_g. This enables us to determine the potentials and hence the bond lengths of specific intramolecular and intermolecular interactions. While the former has an expansion coefficient of (∼0.1 pm/100 K) with only smooth changes, the latter shows a 30-40 times stronger response with pronounced kinks at T_g. A comparison with the overall expansion based on mass density reveals that one has to separate between strong (OH⋅⋅⋅O) and weak (CH⋅⋅⋅O) intermolecular hydrogen (H)-bridges. Despite the fact that the latter dominates glassy dynamics, their expansivity is 5 times smaller than that of the weak H-bridges. It is to be expected that such heterogeneities on intramolecular and intermolecular scales are a general phenomenon in liquids and glassy systems demonstrating especially the necessity of atomistic simulations.

Reorientational dynamics of highly asymmetric binary non-polymeric mixtures – a dielectric spectroscopy study

Two separated relaxations α₁ and α₂ with different temperature dependences are identified in the mixtures. They are attributed to the dynamics associated with the high-T_g (α₁) and the low-T_g component (α₂) with distinct T_g concentration dependences.

Control of electron transfer by protein dynamics in photosynthetic reaction centers

Trehalose and glycerol are low molecular mass sugars/polyols that have found widespread use in the protection of native protein states, in both short- and long-term storage of biological materials, and as a means of understanding protein dynamics. These myriad uses are often attributed to their ability to form an amorphous glassy matrix. In glycerol, the glass is formed only at cryogenic temperatures, while in trehalose, the glass is formed at room temperature, but only upon dehydration of the sample. While much work has been carried out to elucidate a mechanistic view of how each of these matrices interact with proteins to provide stability, rarely have the effects of these two independent systems been directly compared to each other. This review aims to compile decades of research on how different glassy matrices affect two types of photosynthetic proteins: (i) the Type II bacterial reaction center from Rhodobacter sphaeroides and (ii) the Type I Photosystem I reaction center from cyanobacteria. By comparing aggregate data on electron transfer, protein structure, and protein dynamics, it appears that the effects of these two distinct matrices are remarkably similar. Both seem to cause a "tightening" of the solvation shell when in a glassy state, resulting in severely restricted conformational mobility of the protein and associated water molecules. Thus, trehalose appears to be able to mimic, at room temperature, nearly all of the effects on protein dynamics observed in low temperature glycerol glasses.

Low-Frequency (Gigahertz to Terahertz) Depolarized Raman Scattering Off n-Alkanes, Cycloalkanes, and Six-Membered Rings: A Physical Interpretation

Molecular liquids have long been known to undergo various distinct intermolecular motions, from fast librations and cage-rattling oscillations to slow orientational and translational diffusion. However, their resultant gigahertz to terahertz spectra are far from simple, appearing as broad shapeless bands that span many orders of magnitude of frequency, making meaningful interpretation troublesome. Ad hoc spectral line shape fitting has become a notoriously fine art in the field; a unified approach to handling such spectra is long overdue. Here we apply ultrafast optical Kerr-effect (OKE) spectroscopy to study the intermolecular dynamics of room-temperature n-alkanes, cycloalkanes, and six-carbon rings, as well as liquid methane and propane. This work provides stress tests and converges upon an experimentally robust model across simple molecular series and range of temperatures, providing a blueprint for the interpretation of the dynamics of van der Waals liquids. This will enable the interpretation of low-frequency spectra of more complex liquids.

Suppression of Orientational Correlations in the Viscous-Liquid State of Hyperquenched Pressure-Densified Glycerol

Glycerol pressurized to 2 kbar and hyperquenched from the bulk liquid at rates of about -10 000  K/s, has been frozen to an extreme out-of-equilibrium state. As compared to conventionally cooled melts, the resulting material exhibits lower orientational correlations, enabling the observation of a secondary relaxation peak in the ambient-pressure dielectric response. The hyperquenching rather than the pressurizing part of the preparation protocol induces the observed structural changes. These vanish entirely only well above the glass transition temperature of the equilibrium liquid and are evidence for strong similarities between hyperquenched and vapor-deposited glass formers.

Ionic conductivity and relaxation dynamics in plastic crystals with nearly globular molecules

We have performed a dielectric investigation of the ionic charge transport and the relaxation dynamics in plastic crystalline 1-cyano-adamantane (CNA) and in two mixtures of CNA with the related plastic crystals adamantane or 2-adamantanon. Ionic charge carriers were provided by adding 1% of Li salt. The molecules of these compounds have nearly globular shape and, thus, the so-called revolving-door mechanism assumed to promote ionic charge transport via molecular reorientations in other PC electrolytes should not be active here. Indeed, a comparison of the dc resistivity and the reorientational α-relaxation times in the investigated PCs reveals complete decoupling of both dynamics. Similar to other PCs, we find a significant mixing-induced enhancement of the ionic conductivity. Finally, these solid-state electrolytes reveal a second relaxation process, slower than the α-relaxation, which is related to ionic hopping. Due to the mentioned decoupling, it can be unequivocally detected and is not superimposed by the reorientational contributions as found for most other ionic conductors.

Relaxation and vibrational properties in metal alloys and other disordered systems

The relaxation dynamics and the vibrational spectra of amorphous solids, such as metal alloys, have been intensely investigated as well separated topics in the past. The aim of this review is to summarize recent results in both these areas in an attempt to establish, or unveil, deeper connections between the two phenomena of relaxation and vibration. Theoretical progress in the area of slow relaxation dynamics of liquid and glassy systems and in the area of vibrational spectra of glasses and liquids is reviewed. After laying down a generic modelling framework to connect vibration and relaxation, the physics of metal alloys is considered where the emergence of power-law exponents has been identified both in the vibrational density of states (VDOS) as well as in density correlations. Also, theoretical frameworks which connect the VDOS to the relaxation behaviour and mechanical viscoelastic response in metallic glasses are reviewed. The same generic interpretative framework is then applied to the case of molecular glass formers where the emergence of stretched-exponential relaxation in dielectric relaxation can be put in quantitative relation with the VDOS by means of memory-function approaches. Further connections between relaxation and vibration are provided by the study of phonon linewidths in liquids and glasses, where a natural starting point is given by hydrodynamic theories. Finally, an agenda of outstanding issues including the appearance of compressed exponential relaxation in the intermediate scattering function of experimental and simulated systems (metal alloys, colloidal gels, jammed packings) is presented in light of available (or yet to be developed) mathematical models, and compared to non-exponential behaviour measured with macroscopic means such as mechanical spectroscopy/rheology.

Dielectric relaxation and proton field-cycling NMR relaxometry study of dimethyl sulfoxide/glycerol mixtures down to glass-forming temperatures

Mixtures of glycerol and dimethyl sulfoxide (DMSO) are studied by dielectric spectroscopy (DS) and by 1H field-cycling (FC) NMR relaxometry in the entire concentration range and down to glass-forming temperatures (170-323 K). Molecular dynamics is accessed for 0 < xDMSO ≤ 0.64, at higher concentration phase separation occurs. The FC technique provides the frequency dependence of the spin-lattice relaxation rate which is transformed to the susceptibility representation and thus allows comparing NMR and DS results. The DS spectra virtually do not change with xDMSO and T, only the relaxation times become shorter. This is in contrast to the non-associated mixture toluene/quinaldine for which strong spectral changes occur. The FC relaxation spectra of glycerol in solution with DMSO or (deuterated) DMSO-d6 display a bimodal structure with a high-frequency part reflecting rotational and a low-frequency part reflecting translational dynamics. Regarding the rotational contribution in the glycerol/DMSO-d6 mixtures, no spectral change with xDMSO and T is observed. Yet, the non-deuterated mixture reveals a broader relaxation spectrum. Time constants τrot(T) probed by the two techniques complement each, a range 10-11 s < τ < 10 s is covered. The glass transition temperature Tg(xDMSO) is determined, yielding Tg = 149.5 ± 1 K of pure DMSO by extrapolation. Analysing the low-frequency FC NMR spectra allows to determine the diffusion coefficient Dtrans. Its logarithm shows a linear xDMSO-dependence as does lg τrot. The ratio Dtrans/Drot is independent of xDMSO and its low value indicates large separation of translation and rotation. The corresponding unphysically small hydrodynamic radius indicates strong failure of Stokes-Einstein-Debye relation. Such anomaly is taken as characteristics of a 3d hydrogen-bonded network. We conclude, although DMSO is an aprotic liquid the molecule is continuously incorporated in the hydrogen network of glycerol. Both molecules display common dynamics, i.e., no decoupling of the component dynamics is found in contrast to quinaldine/toluene with a similar Tg difference of its components.

Reorientational dynamics of trimethoxyboroxine: A molecular glass former studied by dielectric spectroscopy and 11B nuclear magnetic resonance

In this work, trimethoxyboroxine (TMB) is identified as a small-molecule glass former. In its viscous liquid as well as glassy states, static and dynamic properties of TMB are explored using various techniques. It is found that, on average, the structure of the condensed TMB molecules deviates from threefold symmetry so that TMB's electric dipole moment is nonzero, thus rendering broadband dielectric spectroscopy applicable. This method reveals the super-Arrhenius dynamics that characterizes TMB above its glass transition, which occurs at about 204 K. To extend the temperature range in which the molecular dynamics can be studied, ¹¹B nuclear magnetic resonance experiments are additionally carried out on rotating and stationary samples: Exploiting dynamic second-order shifts, spin-relaxation times, line shape effects, as well as stimulated-echo and two-dimensional exchange spectroscopy, a coherent picture regarding the dynamics of this glass former is gained.

Main and secondary relaxations of non-polymeric high-Tg glass formers as revealed by dielectric spectroscopy

A series of high-Tg glass formers with Tg values varying between 347 and 390 K and molar masses in the range of 341 and 504 g mol-1 are investigated by dielectric spectroscopy. They are compared to paradigmatic reference systems. Differently polar side groups are attached to a rigid non-polar core unit at different positions. Thereby, the dielectric relaxation strength varies over more than two decades. All the relaxation features typical of molecular glass formers are rediscovered, i.e. stretching of the main (α-) relaxation, a more or less pronounced secondary (β-) process, and a fragility index quite similar to that of other molecular systems. The position of the polar nitrile side group influences the manifestation of the β-relaxation. The α-relaxation stretching displays the trend to become less with higher relaxation strength Δεα, confirming recent reports. Typical for a generic β-process is the increase of its amplitude above Tg, which is found to follow a power-law behaviour as a function of the ratio τα/τβ with a universal exponent; yet, its relative amplitude to that of the α-relaxation varies as does the temporal separation of both processes. The mean activation energy of the β-process as well as the width of the energy distribution gβ(E) increases more or less systematically with Tg. The latter is determined from the dielectric spectra subjected to a scaling procedure assuming a thermally activated process. Plotting gβ(E) as a function of the reduced energy scale E/Tg, the distributions are centred between 19-35 and their widths differ by a factor 2-3.

Dynamically asymmetric binary glass formers studied by dielectric and NMR spectroscopy

We investigate the component dynamics in asymmetric binary glass formers. Focusing on the dielectric spectra of the high-T_g components m-tricresyl phosphate and quinaldine mixed with toluene as low-T_g component, the broadend spectra cannot be described by Kohlrausch or Cole-Davidson (CD) functions. Instead, we apply a generalized CD function which allows to control the width of the susceptibility independently of its high-frequency flank. The spectra show a common broadening and failure of the frequency-temperature superposition with increasing toluene concentration. This is confirmed by stimulated echo experiments showing an increased stretching of the probed orientational correlation function. In analogy to the definition of T_g, we consider "isodynamic points". For each component, a different but linear concentration dependence of 1/T_iso is revealed, indicating different time scales. Qualitativly, we do not find significant differences for the present mixtures with T_g-contrasts of 63-89K compared to those with larger T_g-contrast ( [Formula: see text] K): Whereas the high-T_g component shows relaxation features similar to those of neat glass formers, yet, with "atypical" weak relaxation broadening, the faster low-T_g component displays pronounced dynamic heterogeneities. This is supported by scrutinizing NMR relaxation data of several mixtures investigated previously as a function of concentration. A universal evolution of the dynamics of the high-T_g as well as the low-T_g component is suggested for mixtures with high [Formula: see text]T_g .

A microscopic look at the Johari-Goldstein relaxation in a hydrogen-bonded glass-former

Understanding the glass transition requires getting the picture of the dynamical processes that intervene in it. Glass-forming liquids show a characteristic decoupling of relaxation processes when they are cooled down towards the glassy state. The faster (βJG) process is still under scrutiny, and its full explanation necessitates information at the microscopic scale. To this aim, nuclear γ-resonance time-domain interferometry (TDI) has been utilized to investigate 5-methyl-2-hexanol, a hydrogen-bonded liquid with a pronounced βJG process as measured by dielectric spectroscopy. TDI probes in fact the center-of-mass, molecular dynamics at scattering-vectors corresponding to both inter- and intra-molecular distances. Our measurements demonstrate that, in the undercooled liquid phase, the βJG relaxation can be visualized as a spatially-restricted rearrangement of molecules within the cage of their closest neighbours accompanied by larger excursions which reach out at least the inter-molecular scale and are related to cage-breaking events. In-cage rattling and cage-breaking processes therefore coexist in the βJG relaxation.

Plastic-crystalline solid-state electrolytes: Ionic conductivity and orientational dynamics in nitrile mixtures

Many plastic crystals, molecular solids with long-range, center-of-mass crystalline order but dynamic disorder of the molecular orientations, are known to exhibit exceptionally high ionic conductivity. This makes them promising candidates for applications as solid-state electrolytes, e.g., in batteries. Interestingly, it was found that the mixing of two different plastic-crystalline materials can considerably enhance the ionic dc conductivity, an important benchmark quantity for electrochemical applications. An example is the admixture of different nitriles to succinonitrile, the latter being one of the most prominent plastic-crystalline ionic conductors. However, until now, only few such mixtures were studied. In the present work, we investigate succinonitrile mixed with malononitrile, adiponitrile, and pimelonitrile to which 1 mol. % of Li ions was added. Using differential scanning calorimetry and dielectric spectroscopy, we examine the phase behavior and the dipolar and ionic dynamics of these systems. We especially address the mixing-induced enhancement of the ionic conductivity and the coupling of the translational ionic mobility to the molecular reorientational dynamics, probably arising via a "revolving-door" mechanism.

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.