Temperature and pressure study of Brillouin transverse modes in the organic glass-forming liquid orthoterphenyl

Role of anisotropy in understanding the molecular grounds for density scaling in dynamics of glass-forming liquids

Molecular Dynamics (MD) simulations of glass-forming liquids play a pivotal role in uncovering the molecular nature of the liquid vitrification process. In particular, much focus was given to elucidating the interplay between the character of intermolecular potential and molecular dynamics behaviour. This has been tried to achieve by simulating the spherical particles interacting via isotropic potential. However, when simulation and experimental data are analysed in the same way by using the density scaling approaches, serious inconsistency is revealed between them. Similar scaling exponent values are determined by analysing the relaxation times and pVT data obtained from computer simulations. In contrast, these values differ significantly when the same analysis is carried out in the case of experimental data. As discussed thoroughly herein, the coherence between results of simulation and experiment can be achieved if anisotropy of intermolecular interactions is introduced to MD simulations. In practice, it has been realized in two different ways: (1) by using the anisotropic potential of the Gay-Berne type or (2) by replacing the spherical particles with quasi-real polyatomic anisotropic molecules interacting through isotropic Lenard-Jones potential. In particular, the last strategy has the potential to be used to explore the relationship between molecular architecture and molecular dynamics behaviour. Finally, we hope that the results presented in this review will also encourage others to explore how 'anisotropy' affects remaining aspects related to liquid-glass transition, like heterogeneity, glass transition temperature, glass forming ability, etc.

Viscosity of Ionic Liquids: Theories and Models

Ionic liquids (ILs) offer a wide range of promising applications due to their unique and designable properties compared to conventional solvents. Further development and application of ILs require correlating/predicting their pressure-viscosity-temperature behavior. In this review, we firstly introduce methods for calculation of thermodynamic inputs of viscosity models. Next, we introduce theories, theoretical and semi-empirical models coupling various theories with EoSs or activity coefficient models, and empirical and phenomenological models for viscosity of pure ILs and IL-related mixtures. Our modelling description is followed immediately by model application and performance. Then, we propose simple predictive equations for viscosity of IL-related mixtures and systematically compare performances of the above-mentioned theories and models. In concluding remarks, we recommend robust predictive models for viscosity at atmospheric pressure as well as proper and consistent theories and models for P-η-T behavior. The work that still remains to be done to obtain the desired theories and models for viscosity of ILs and IL-related mixtures is also presented. The present review is structured from pure ILs to IL-related mixtures and aims to summarize and quantitatively discuss the recent advances in theoretical and empirical modelling of viscosity of ILs and IL-related mixtures.

The origin of the density scaling exponent for polyatomic molecules and the estimation of its value from the liquid structure

In this article, we unravel the problem of interpreting the density scaling exponent for the polyatomic molecules representing the real van der Waals liquids. Our studies show that the density scaling exponent is a weighted average of the exponents of the repulsive terms of all interatomic interactions that occur between molecules, where the potential energy of a given interaction represents its weight. It implies that potential energy is a key quantity required to calculate the density scaling exponent value for real molecules. Finally, we use the well-known method for potential energy estimation and show that the density scaling exponent could be successfully predicted from the liquid structure for fair representatives of the real systems.

Isomorphs in nanoconfined liquids

We study in this paper the possible existence of Roskilde-simple liquids and their isomorphs in a rough-wall nanoconfinement. Isomorphs are curves in the thermodynamic phase diagram along which structure and dynamics are invariant in suitable nondimensionalized units. Two model liquids using molecular dynamics computer simulations are considered: the single-component Lennard-Jones (LJ) liquid and the Kob-Andersen binary LJ mixture, both of which in the bulk phases are known to have good isomorphs. Nanoconfinement is implemented by adopting a slit-pore geometry with fcc crystalline walls; this implies inhomogenous density profiles both parallel and perpendicular to the confining walls. Despite this fact and consistent with an earlier study [Ingebrigtsen et al., Phys. Rev. Lett., 2013, 111, 235901] we find that these two nanoconfined liquids have isomorphs to a good approximation. More specifically, we show good invariance along the isomorphs of inhomogenous density profiles, mean-square displacements, and higher-order structures probed using the topological cluster classification algorithm. Our study thus provides an alternative framework for understanding nanoconfined liquids.

On the Segmental Dynamics and the Glass Transition Behavior of Poly(2-vinylpyridine) in One- and Two-Dimensional Nanometric Confinement

Geometric nanoconfinement, in one and two dimensions, has a fundamental influence on the segmental dynamics of polymer glass-formers and can be markedly different from that observed in the bulk state. In this work, with the use of dielectric spectroscopy, we have investigated the glass transition behavior of poly(2-vinylpyridine) (P2VP) confined within alumina nanopores and prepared as a thin film supported on a silicon substrate. P2VP is known to exhibit strong, attractive interactions with confining surfaces due to the ability to form hydrogen bonds. Obtained results show no changes in the temperature evolution of the α-relaxation time in nanopores down to 20 nm size and 24 nm thin film. There is also no evidence of an out-of-equilibrium behavior observed for other glass-forming systems confined at the nanoscale. Nevertheless, in both cases, the confinement effect is seen as a substantial broadening of the α-relaxation time distribution. We discussed the results in terms of the importance of the interfacial energy between the polymer and various substrates, the sensitivity of the glass-transition temperature to density fluctuations, and the density scaling concept.

The behavior of conductivity dynamic modulus and its connection to thermodynamic bulk modulus in ionic liquids

In this paper, we investigate the interplay between the dynamics and thermodynamics of aprotic ionic liquids in the supercooled and normal liquid states. For this purpose, the conductivity dynamic modulus Mσp-T, being defined as the ratio of activation energy (Ep) and activation volume (VT), and its relation to bulk modulus BT under isobaric and isothermal conditions is examined. We found that both isobaric cooling and isothermal compression lead to an increase in Mσp-T. Specifically, Mσp-T(P)T rises linearly similar to BT. Consequently, a direct linear relationship between Mσp-T(P)T and BT is established under isothermal conditions.

High-pressure experiments as a novel perspective to study the molecular dynamics of glass-forming materials confined at the nanoscale

Herein, we report the pioneering high-pressure dielectric studies on the dynamics of a model van der Waals glass-forming liquid bisphenol-A diglycidyl ether (DGEBA) infiltrated into anodic aluminum oxide (AAO) templates of the mean pore sizes, d = 150 and d = 18 nm. It was found that although the shape of the structural relaxation process varies with the confinement, it remains constant under varying thermodynamic conditions for a given pore diameter. Consequently, the time-temperature-pressure (TTP) rule satisfied for the majority of bulk substances is also obeyed for the spatially restricted liquid. We have also shown for the first time that there is a decoupling between the core and interfacial mobility at elevated pressure. Moreover, it was noted that the structural dynamics of the former fraction of molecules becomes systematically shorter with respect to the bulk DGEBA during the compression. The enhanced structural dynamics of the core material, as well as the varying pressure coefficients of the glass transition temperature of the interfacial and core molecules, have been discussed in the context of a distinct evolution in their free volume/density packing with respect to the bulk DGEBA, and a change in the interfacial tension, which may lead to the enhanced wettability of the liquid adsorbed onto the pore walls under different thermodynamic conditions. The performed high-pressure measurements offer novel perspectives to explore the combination of two different effects, compression and confinement, which might be a breakthrough in the study of the glass transition phenomenon and the behavior of soft materials confined at the nanoscale.

Exploring the connection between the density-scaling exponent and the intermolecular potential for liquids on the basis of computer simulations of quasireal model systems

In this paper, based on the molecular dynamics simulations of quasireal model systems, we propose a method for determination of the effective intermolecular potential for real materials. We show that in contrast to the simple liquids, the effective intermolecular potential for the studied systems depends on the thermodynamic conditions. Nevertheless, the previously established relationship for simple liquids between the exponent of the inverse power law approximation of intermolecular potential and the density-scaling exponent is still preserved when small enough intermolecular distances are considered. However, our studies show that molecules approach each other at these very short distances relatively rarely. Consequently, only sparse interactions between extremely close molecules determine the value of the scaling exponent and then strongly influence the connection between dynamics and thermodynamics of the whole system.

A model-free analysis of configurational properties to reduce the temperature- and pressure-dependent segmental relaxation times of polymers

The segmental relaxation time data for poly(vinyl acetate), poly(vinyl chloride), and linear and star polystyrene are analyzed using a model-free method to determine how the temperature- and pressure-dependent relaxation times, τ, scale with the relative configurational thermodynamic properties. The model-free method assumes no specific mathematical form, such as reciprocal linearity, and the configurational properties are referred to an isochronal state to eliminate the bias associated with the definition of the ideal glassy state. The scaling ability of a given configurational property is strongly material-dependent with the logarithm of τ scaling better with TS_c and H_c for poly(vinyl acetate), with TS_c, H_c, and U_c for poly(vinyl chloride), and with TS_c, H_c, and V_c for linear and star polystyrene. The choice of the isochronal reference state does not qualitatively affect the results.

Testing density scaling in nanopore-confinement for hydrogen-bonded liquid dipropylene glycol

Recently, it has been demonstrated that the glassy dynamics of the molecular liquids and polymers confined at the nanoscale level might satisfy the density scaling law (ρ γ /T) with the same value of the scaling exponent, γ, as that determined from the high-pressure studies of the bulk material. In this work, we have tested the validity of this interesting experimental finding for strongly hydrogen-bonded molecular liquid, dipropylene glycol (DPG), which is known to violate the ρ γ /T scaling rule in the supercooled liquid bulk state. The results of the independent dielectric relaxation studies carried out on increased pressure and in nanopores, have led to an important finding that when the density change induced by geometrical confinement is not very large, DPG can still obey the density scaling law with the same value of the scaling exponent as that found for the bulk sample. In this way, we confirm that the information obtained from the universal density scaling approach applied to nanoscale confined systems is somehow consistent with the macroscopic ones and that in both cases the same fundamental rules governs the glass-transition dynamics.

Connecting 1D and 2D Confined Polymer Dynamics to Its Bulk Behavior via Density Scaling

Under confinement, the properties of polymers can be much different from the bulk. Because of the potential applications in technology and hope to reveal fundamental problems related to the glass-transition, it is important to realize whether the nanoscale and macroscopic behavior of polymer glass-formers are related to each other in any simple way. In this work, we have addressed this issue by studying the segmental dynamics of poly(4-chlorostyrene) (P4ClS) in the bulk and upon geometrical confinement at the nanoscale level, in either one- (thin films on Al substrate) or two- (within alumina nanopores) dimensions. The results demonstrate that the segmental relaxation time, irrespective of the confinement size or its dimensionality, can be scaled onto a single curve when plotted versus ρ^γ/T with the same single scaling exponent, γ = 3.1, obtained via measurements at high pressures in bulk. The implication is that the macro- and nanoscale confined polymer dynamics are intrinsically connected and governed by the same underlying rules.

The effect of molecular architecture on the physical properties of supercooled liquids studied by MD simulations: Density scaling and its relation to the equation of state

Theoretical concepts in condensed matter physics are typically verified and also developed by exploiting computer simulations mostly in simple models. Predictions based on these usually isotropic models are often at odds with measurement results obtained for real materials. One of the examples is an intriguing problem within the density scaling idea that has attracted attention in recent decades due to its hallmarks of universality, i.e., the fact that the difference between the density scaling exponent and the exponent of the equation of state is observed for real materials, whereas it has not been reported for the model system. In this paper, we use new model molecules of simple but anisotropic architecture to study the effect of molecular anisotropy on the dynamic and thermodynamic properties of the system. We identify the applicable range of intermolecular interactions for a given physical process, and then we explain the reason for observed differences between the behavior of the model and real systems. It demonstrates that the new model systems open broad perspectives for simulation and theoretical research, for example, into unifying concepts in the glass transition physics.

Breakdown of the Simple Arrhenius Law in the Normal Liquid State

It is common practice to discuss the temperature effect on molecular dynamics of glass formers above the melting temperature in terms of the Arrhenius law. Using dielectric spectroscopy measurements of dc conductivity and structural relaxation time on the example of the typical glass former propylene carbonate, we provide experimental evidence that this practice is not justified. Our conclusions are supported by employing thermodynamic density scaling and the occurrence of inflection points in isothermal dynamic data measured at elevated pressure. Additionally, we propose a more suitable approach to describe the dynamics both above and below the inflection point based on a modified MYEGA model.

Thermodynamic Scaling of the Dynamics of a Strongly Hydrogen-Bonded Glass-Former

We probe the temperature- and pressure-dependent specific volume (v) and dipolar dynamics of the amorphous phase (in both the supercooled liquid and glass states) of the ternidazole drug (TDZ). Three molecular dynamic processes are identified by means of dielectric spectroscopy, namely the α relaxation, which vitrifies at the glass transition, a Johari-Goldstein β JG relaxation, and an intramolecular process associated with the relaxation motion of the propanol chain of the TDZ molecule. The lineshapes of dielectric spectra characterized by the same relaxation time (isochronal spectra) are virtually identical, within the studied temperature and pressure ranges, so that the time-temperature-pressure superposition principle holds for TDZ. The α and β JG relaxation times fulfil the density-dependent thermodynamic scaling: master curves result when they are plotted against the thermodynamic quantity Tv γ , with thermodynamic exponent γ approximately equal to 2. These results show that the dynamics of TDZ, a system characterized by strong hydrogen bonding, is characterized by an isomorphism similar to that of van-der-Waals systems. The low value of γ can be rationalized in terms of the relatively weak density-dependence of the dynamics of hydrogen-bonded systems.

Scaling of the dynamics of flexible Lennard-Jones chains: Effects of harmonic bonds

The previous paper [A. A. Veldhorst et al., J. Chem. Phys. 141, 054904 (2014)] demonstrated that the isomorph theory explains the scaling properties of a liquid of flexible chains consisting of ten Lennard-Jones particles connected by rigid bonds. We here investigate the same model with harmonic bonds. The introduction of harmonic bonds almost completely destroys the correlations in the equilibrium fluctuations of the potential energy and the virial. According to the isomorph theory, if these correlations are strong a system has isomorphs, curves in the phase diagram along which structure, dynamics, and the excess entropy are invariant. The Lennard-Jones chain liquid with harmonic bonds does have curves in the phase diagram along which the structure and dynamics are invariant. The excess entropy is not invariant on these curves, which we refer to as "pseudoisomorphs." In particular, this means that Rosenfeld's excess-entropy scaling (the dynamics being a function of excess entropy only) does not apply for the Lennard-Jones chain with harmonic bonds.

Communication: High pressure specific heat spectroscopy reveals simple relaxation behavior of glass forming molecular liquid

The frequency dependent specific heat has been measured under pressure for the molecular glass forming liquid 5-polyphenyl-4-ether in the viscous regime close to the glass transition. The temperature and pressure dependences of the characteristic time scale associated with the specific heat is compared to the equivalent time scale from dielectric spectroscopy performed under identical conditions. It is shown that the ratio between the two time scales is independent of both temperature and pressure. This observation is non-trivial and demonstrates the existence of specially simple molecular liquids in which different physical relaxation processes are both as function of temperature and pressure/density governed by the same underlying "inner clock." Furthermore, the results are discussed in terms of the recent conjecture that van der Waals liquids, like the measured liquid, comply to the isomorph theory.

Adam-Gibbs model in the density scaling regime and its implications for the configurational entropy scaling

To solve a long-standing problem of condensed matter physics with determining a proper description of the thermodynamic evolution of the time scale of molecular dynamics near the glass transition, we have extended the well-known Adam-Gibbs model to describe the temperature-volume dependence of structural relaxation times, τ_α(T, V). We also employ the thermodynamic scaling idea reflected in the density scaling power law, τ_α = f(T⁻¹V^−γ), recently acknowledged as a valid unifying concept in the glass transition physics, to differentiate between physically relevant and irrelevant attempts at formulating the temperature-volume representations of the Adam-Gibbs model. As a consequence, we determine a straightforward relation between the structural relaxation time τ_α and the configurational entropy S_C, giving evidence that also S_C(T, V) = g(T⁻¹V^−γ) with the exponent γ that enables to scale τ_α(T, V). This important findings have meaningful implications for the connection between thermodynamics and molecular dynamics near the glass transition, because it implies that τ_α can be scaled with S_C.

The Modified VFT law of glass former materials under pressure: Part II: Relation with the equation of state

The dynamical properties of glass formers (GFs) as a function of P, V, and T are reanalyzed in relation with the equations of state (EOS) proposed recently (Eur. Phys. J. E 37, 113 (2014)). The relaxation times τ of the cooperative non-Arrhenius α process and the individual Arrhenius β process are coupled via the Kohlrausch exponent n S(T, P). In the model n S is the sigmoidal logistic function depending on T (and P, and the α relaxation time τ α of GFs above T g verifies the pressure-modified VFT law: log τ α ∼ E β /nsRT, which can be put into a form with separated variables: log τ α ∼ f(T)g(P). From the variation of n S and τ α with T and P the Vogel temperature T 0 (τ α → ∝, n S = 0) and the crossover temperature (also called the merging or splitting temperature) T B (τ α ∼ τ β, n S ∼ 1) are determined. The proposed sm-VFT equation fits with excellent accuracy the experimental data of fragile and strong GFs under pressure. The properties generally observed in organic mineral and metallic GFs are explained: a) The Vogel temperature is independent of P (as suggested by the EOS properties), the crossover is pressure-dependent. b) In crystallizable GFs the T B (P) and Clapeyron curves T m(P) coincide. c) The α and β processes have the same ratio of the activation energies and volume, E*/V* (T- and P-independent), the compensation law is observed, this ratio depends on the anharmonicity Slater-Grüneisen parameter and on the critical pressure P* deduced from the EOS. d) The properties of the Fan Structure of the Tangents (FST) to the isotherms and isobars curves log τ versus P and T and to the isochrones curves P(T). e) The scaling law log τ = f(V (Λ) ) and the relation between Γ and γ. We conclude that these properties should be studied in detail in GFs submitted to negative pressures.

Thermodynamic scaling of molecular dynamics in supercooled liquid state of pharmaceuticals: Itraconazole and ketoconazole

Pressure-Volume-Temperature (PVT) measurements and broadband dielectric spectroscopy were carried out to investigate molecular dynamics and to test the validity of thermodynamic scaling of two homologous compounds of pharmaceutical activity: itraconazole and ketoconazole in the wide range of thermodynamic conditions. The pressure coefficients of the glass transition temperature (dT(g)/dp) for itraconazole and ketoconazole were determined to be equal to 183 and 228 K/GPa, respectively. However, for itraconazole, the additional transition to the nematic phase was observed and characterized by the pressure coefficient dT(n)/dp = 258 K/GPa. From PVT and dielectric data, we obtained that the liquid-nematic phase transition is governed by the relaxation time since it occurred at constant τ(α) = 10(-5) s. Furthermore, we plotted the obtained relaxation times as a function of T(-1)v(-γ), which has revealed that the validity of thermodynamic scaling with the γ exponent equals to 3.69 ± 0.04 and 3.64 ± 0.03 for itraconazole and ketoconazole, respectively. Further analysis of the scaling parameter in itraconazole revealed that it unexpectedly decreases with increasing relaxation time, which resulted in dramatic change of the shape of the thermodynamic scaling master curve. While in the case of ketoconazole, it remained the same within entire range of data (within experimental uncertainty). We suppose that in case of itraconazole, this peculiar behavior is related to the liquid crystals' properties of itraconazole molecule.

Scaling of the dynamics of flexible Lennard-Jones chains

The isomorph theory provides an explanation for the so-called power law density scaling which has been observed in many molecular and polymeric glass formers, both experimentally and in simulations. Power law density scaling (relaxation times and transport coefficients being functions of ρ(γ(S)), where ρ is density, T is temperature, and γ(S) is a material specific scaling exponent) is an approximation to a more general scaling predicted by the isomorph theory. Furthermore, the isomorph theory provides an explanation for Rosenfeld scaling (relaxation times and transport coefficients being functions of excess entropy) which has been observed in simulations of both molecular and polymeric systems. Doing molecular dynamics simulations of flexible Lennard-Jones chains (LJC) with rigid bonds, we here provide the first detailed test of the isomorph theory applied to flexible chain molecules. We confirm the existence of isomorphs, which are curves in the phase diagram along which the dynamics is invariant in the appropriate reduced units. This holds not only for the relaxation times but also for the full time dependence of the dynamics, including chain specific dynamics such as the end-to-end vector autocorrelation function and the relaxation of the Rouse modes. As predicted by the isomorph theory, jumps between different state points on the same isomorph happen instantaneously without any slow relaxation. Since the LJC is a simple coarse-grained model for alkanes and polymers, our results provide a possible explanation for why power-law density scaling is observed experimentally in alkanes and many polymeric systems. The theory provides an independent method of determining the scaling exponent, which is usually treated as an empirical scaling parameter.

Variation of the dynamic susceptibility along an isochrone

Koperwas et al. showed in a recent paper [Phys. Rev. Lett. 111, 125701 (2013)] that the dynamic susceptibility χ4 as estimated by dielectric measurements for certain glass-forming liquids decreases substantially with increasing pressure along a curve of constant relaxation time. This observation is at odds with other measures of dynamics being invariant and seems to pose a problem for theories of glass formation. We show that this variation is in fact consistent with predictions for liquids with hidden scale invariance: Measures of dynamics at constant volume are invariant along isochrones, called isomorphs in such liquids, but contributions to fluctuations from long-wavelength fluctuations can vary. This is related to the known noninvariance of the isothermal bulk modulus. Considering the version of χ4 defined for the NVT ensemble, data from simulations of a binary Lennard-Jones liquid show in fact a slight increase with increasing density. This is a true departure from the formal invariance expected for this quantity.

Density-temperature scaling of the fragility in a model glass-former

Dynamical quantities e.g. diffusivity and relaxation time for some glass-formers may depend on density and temperature through a specific combination, rather than independently, allowing the representation of data over ranges of density and temperature as a function of a single scaling variable. Such a scaling, referred to as density-temperature (DT) scaling, is exact for liquids with inverse power law (IPL) interactions but has also been found to be approximately valid in many non-IPL liquids. We have analyzed the consequences of DT scaling on the density dependence of the fragility in a model glass-former. We find the density dependence of kinetic fragility to be weak, and show that it can be understood in terms of DT scaling and deviations of DT scaling at low densities. We also show that the Adam-Gibbs relation exhibits DT scaling and the scaling exponent computed from the density dependence of the activation free energy in the Adam-Gibbs relation, is consistent with the exponent values obtained by other means.

Microscopically based calculations of the free energy barrier and dynamic length scale in supercooled liquids: the comparative role of configurational entropy and elasticity

We compute the temperature-dependent barrier for α-relaxations in several liquids, without adjustable parameters, using experimentally determined elastic, structural, and calorimetric data. We employ the random first order transition (RFOT) theory, in which relaxation occurs via activated reconfigurations between distinct, aperiodic minima of the free energy. Two different approximations for the mismatch penalty between the distinct aperiodic states are compared, one due to Xia and Wolynes (Proc. Natl. Acad. Sci. U. S. A. 2000, 97, 2990), which scales universally with temperature as for hard spheres, and one due to Rabochiy and Lubchenko (J. Chem. Phys. 2013, 138, 12A534), which employs measured elastic and structural data for individual substances. The agreement between the predictions and experiment is satisfactory, given the uncertainty in the measured experimental inputs. The explicitly computed barriers are used to calculate the glass transition temperature for each substance--a kinetic quantity--from the static input data alone. The temperature dependence of both the elastic and structural constants enters the temperature dependence of the barrier over an extended range to a degree that varies from substance to substance. The lowering of the configurational entropy, however, seems to be the dominant contributor to the barrier increase near the laboratory glass transition, consistent with previous experimental tests of the RFOT theory using the XW approximation. In addition, we compute the temperature dependence of the dynamical correlation length, also without using adjustable parameters. These agree well with experimental estimates obtained using the Berthier et al. (Science 2005, 310, 1797) procedure. Finally, we find the temperature dependence of the complexity of a rearranging region is consistent with the picture based on the RFOT theory but is in conflict with the assumptions of the Adam-Gibbs and "shoving" scenarios for the viscous slowing down in supercooled liquids.

NVU perspective on simple liquids' quasiuniversality

The last half-century of research into the structure, dynamics, and thermodynamics of simple liquids has revealed a number of approximate universalities. This paper argues that simple liquids' reduced-coordinate constant-potential-energy hypersurfaces constitute a quasiuniversal family of compact Riemannian manifolds parametrized by a single number; from this follows the quasiuniversalities.

Pressure coefficient of the glass transition temperature in the thermodynamic scaling regime

We report that the pressure coefficient of the glass transition temperature, dT(g)/dp, which is commonly used to determine the pressure sensitivity of the glass transition temperature T(g), can be predicted in the thermodynamic scaling regime. We show that the equation derived from the isochronal condition combined with the well-known scaling, TV(γ) = const, predicts successfully values of dT(g)/dp for a variety of glass-forming systems, including van der Waals liquids, polymers, and ionic liquids.

Scaling of volumetric data in model systems based on the Lennard-Jones potential

The crucial problem for better understanding the nature of glass transition and related relaxation phenomena is to find proper interrelations between the molecular dynamics and thermodynamics of viscous systems. To make progress towards this goal the recently observed density scaling of viscous liquid dynamics has been very intensively and successfully studied in the past few years. However, previous attempts at related scaling of volumetric data yielded results inconsistent with those found from the density scaling of molecular dynamics. In this paper, we show that volumetric data obtained from simulations in simple molecular models based on the Lennard-Jones (LJ) potential, such as the Kob-Andersen binary LJ liquid, its repulsive inverse power-law version, and the Lewis-Wahnström o-terphenyl model, can be scaled by using the same value of the exponent, which scales dynamic quantities and is directly related to the exponent of the repulsive inverse power law that underlies short-range approximations of the LJ potential.

Communication: thermodynamics of condensed matter with strong pressure-energy correlations

We show that for any liquid or solid with strong correlation between its NVT virial and potential-energy equilibrium fluctuations, the temperature is a product of a function of excess entropy per particle and a function of density, T = f(s)h(ρ). This implies that (1) the system's isomorphs (curves in the phase diagram of invariant structure and dynamics) are described by h(ρ)/T = Const., (2) the density-scaling exponent is a function of density only, and (3) a Grüneisen-type equation of state applies for the configurational degrees of freedom. For strongly correlating atomic systems one has h(ρ) = ∑(n)C(n)ρ(n/3) in which the only non-zero terms are those appearing in the pair potential expanded as ν(r) = ∑(n)ν(n)r(-n). Molecular dynamics simulations of Lennard-Jones type systems confirm the theory.

Many-Body Nature of Relaxation Processes in Glass-Forming Systems

Most glass-forming systems are composed of basic units interacting with each other with a nontrivial anharmonic potential. Naturally, relaxation and diffusion in glass formers is a many-body problem. Results from recent experimental studies are presented to show the effects of many-body relaxation and diffusion manifested on the dynamic properties of glass formers. Considering that the effects are general and critical, the problem of glass transition will not be solved until the many-body nature of the relaxation process has been incorporated fundamentally into any theory.

Pressure-energy correlations in liquids. V. Isomorphs in generalized Lennard-Jones systems

This series of papers is devoted to identifying and explaining the properties of strongly correlating liquids, i.e., liquids with more than 90% correlation between their virial W and potential energy U fluctuations in the NVT ensemble. Paper IV [N. Gnan et al., J. Chem. Phys. 131, 234504 (2009)] showed that strongly correlating liquids have "isomorphs," which are curves in the phase diagram along which structure, dynamics, and some thermodynamic properties are invariant in reduced units. In the present paper, using the fact that reduced-unit radial distribution functions are isomorph invariant, we derive an expression for the shapes of isomorphs in the WU phase diagram of generalized Lennard-Jones systems of one or more types of particles. The isomorph shape depends only on the Lennard-Jones exponents; thus all isomorphs of standard Lennard-Jones systems (with exponents 12 and 6) can be scaled onto a single curve. Two applications are given. One tests the prediction that the solid-liquid coexistence curve follows an isomorph by comparing to recent simulations by Ahmed and Sadus [J. Chem. Phys. 131, 174504 (2009)]. Excellent agreement is found on the liquid side of the coexistence curve, whereas the agreement is less convincing on the solid side. A second application is the derivation of an approximate equation of state for generalized Lennard-Jones systems by combining the isomorph theory with the Rosenfeld-Tarazona expression for the temperature dependence of the potential energy on isochores. It is shown that the new equation of state agrees well with simulations.

Communication: Thermodynamic scaling of the Debye process in primary alcohols

The molecular dynamics of hydrogen-bonded liquids usually does not satisfy the thermodynamic scaling. However, very recently, two opposite conclusions about validity of thermodynamical scaling in monohydroxy alcohol, 2-ethyl-1-hexanol, were presented by Reiser et al. [J. Chem. Phys. 132, 181101 (2010)] and Fragiadakis et al. [J. Chem. Phys. 132, 144505 (2010)]. In this communication we present new experimental results that can explain this ostensible contradiction.

Density scaling in viscous systems near the glass transition

In this paper, a general equation of state (EOS) valid for fluids in the vicinity of the glass transition is derived on the basis of its isothermal precursor. This EOS is able to predict the density scaling of both isobaric and isothermal PVT data and it explicitly involves the scaling exponent γ(EOS), which is most likely straightforwardly related to the exponent of the inverse power law of some effective potential valid for viscous systems. This EOS and the density scaling are very successfully tested for representatives of several material classes (van der Waals liquids, polymer melts, ionic liquids, and even strongly hydrogen-bonded systems). Additionally, if the thermodynamic scaling of primary relaxation times can be achieved with the scaling exponent γ for a given material, then the value γ(EOS) found from fitting its PVT data to the EOS enables us to evaluate the value γ, which is always considerably smaller than γ(EOS).

Comment on "Density scaling of the diffusion coefficient at various pressures in viscous liquids"

The thermodynamic scaling idea enables an elegant description of relaxation dynamics near the glass transition and provides a linkage between thermodynamics and molecular dynamics of supercooled liquids. A lot of effort has been put into finding functions, f(T(-1)V(-γ)), which can scale dynamical quantities such as relaxation time, viscosity, and diffusion coefficient. Within the trend, Papathanassiou [A. N. Papathanassiou, Phys. Rev. E 79, 032501 (2009)] has recently derived a new scaling function for the reduced diffusion coefficient. In this Comment, we report that this scaling function is not sufficiently grounded, because it is based on some scaled equation of state which predicts significantly different values of the scaling exponent γ from volumetric data in comparison with those found from the scaling of relaxation or viscosity data. Moreover, we would like to point out that any scaling function f cannot be straightforwardly formulated on the basis of any equation of state which yields the scaling exponent γ different from that used to scale the dynamic quantity.

Pressure-energy correlations in liquids. IV. "Isomorphs" in liquid phase diagrams

This paper is the fourth in a series devoted to identifying and explaining the properties of strongly correlating liquids, i.e., liquids where virial and potential energy correlate better than 90% in their thermal equilibrium fluctuations in the NVT ensemble. For such liquids we here introduce the concept of "isomorphic" curves in the phase diagram. A number of thermodynamic, static, and dynamic isomorph invariants are identified. These include the excess entropy, the isochoric specific heat, reduced-unit static and dynamic correlation functions, as well as reduced-unit transport coefficients. The dynamic invariants apply for both Newtonian and Brownian dynamics. It is shown that after a jump between isomorphic state points the system is instantaneously in thermal equilibrium; consequences of this for generic aging experiments are discussed. Selected isomorph predictions are validated by computer simulations of the Kob-Andersen binary Lennard-Jones mixture, which is a strongly correlating liquid. The final section of the paper relates the isomorph concept to phenomenological melting rules, Rosenfeld's excess entropy scaling, Young and Andersen's approximate scaling principle, and the two-order parameter maps of Debenedetti and co-workers. This section also shows how the existence of isomorphs implies an "isomorph filter" for theories for the non-Arrhenius temperature dependence of viscous liquids' relaxation time, and it explains isochronal superposition for strongly correlating viscous liquids.