Liquid polyamorphism: Possible relation to the anomalous behaviour of water

We present evidence from experiments and computer simulations supporting the hypothesis that water displays polyamorphism, i.e., water separates into two distinct liquid phases. This concept of a new liquid-liquid phase transition is finding application to other liquids as well as water, such as silicon and silica. Specifically, we investigate, the relation between changes in dynamic and thermodynamic anomalies arising from the presence of the liquid-liquid critical point in (i) Two models of water, TIP5P and ST2, which display a first order liquid-liquid phase transition at low temperatures; (ii) the Jagla model, a spherically symmetric two-scale potential known to possess a liquid-liquid critical point, in which the competition between two liquid structures is generated by repulsive and attractive ramp interactions; and (iii) A Hamiltonian model of water where the idea of two length/energy scales is built in. This model also displays a first order liquid-liquid phase transition at low temperatures besides the first order liquid-gas phase transition at high temperatures. We find a correlation between the dynamic fragility crossover and the locus of specific heat maxima C_P^max (“Widom line”) emanating from the critical point. Our findings are consistent with a possible relation between the previously hypothesised liquid-liquid phase transition and the transition in the dynamics recently observed in neutron scattering experiments on confined water. More generally, we argue that this connection between C_P^max and the dynamic crossover is not limited to the case of water, a hydrogen bonded network liquid, but is a more general feature of crossing the Widom line, an extension of the first-order coexistence line in the supercritical region. Dedicated to Armin Bunde on the occasion of his 60th birthday.     

Mixturelike behavior near a liquid-liquid phase transition in simulations of supercooled water

In simulations of a waterlike model (ST2) that exhibits a liquid-liquid phase transition, we test for the occurrence of a thermodynamic region in which the liquid can be modeled as a two-component mixture. We assign each molecule to one of two species based on the distance to its fifth-nearest neighbor, and evaluate the concentration of each species over a wide range of temperature and density. Our concentration data compare well with mixture-model predictions in a region between the liquid-liquid critical temperature and the temperature of maximum density. Fits of the model to the data in this region yield accurate estimates for the location of the critical point. We also show that the liquid outside the region of density anomalies is poorly modeled as a simple mixture.     

液体-液体相变与反常特性

绝大多数物质的液态密度随温度降低而增大,即常见的热胀冷缩现象.但存在一类物质,如水及第四主族的硅、锗等,其液态密度在一定温度范围内随温度的升高而增大,即密度反常现象.此外,该类物质还存在动力学反常(密度越大粒子运动越快)、热力学反常(热力学量的涨落随温度降低而升高)等其他反常特性.这类材料的化学性质千差万别,但却具有相似的物理反常特性.进一步的理论研究发现部分材料具有两种液态,即高密度液态和低密度液态,两者之间存在一级相变.因此,反常特性与液体-液体相变是否有直接关联是一个值得深入研究的课题.本文主要介绍了具有液体-液体相变的一类材料及其反常特性,包括高温高压下氢的液体-液体相变及其超临界现象,镓的反常特性及其与液体-液体相变的关联等.     

Structure,thermodynamic and transport properties of imidazolium-based bis(trifluoromethylsulfonyl)imide ionic liquids from molecular dynamics simulations

Molecular dynamics (MD) simulations have been performed in order to investigate the properties of [C _n mim⁺][Tf₂N^?] (n?=?4,?8,?12) ionic liquids (ILs) in a wide temperature range (298.15?498.15?K) and at atmospheric pressure (1 bar). A previously developed methodology for the calculation of the charge distribution that incorporates ab initio quantum mechanical calculations based on density functional theory (DFT) was used to calculate the partial charges for the classical molecular simulations. The wide range of time scales that characterize the segmental dynamics of these ILs, especially at low temperatures, required very long MD simulations, on the order of several tens of nanoseconds, to calculate the thermodynamic (density, thermal expansion, isothermal compressibility), structural (radial distribution functions between the centers of mass of ions and between individual sites, radial-angular distribution functions) and dynamic (relaxation times of the reorientation of the bonds and the torsion angles, self-diffusion coefficients, shear viscosity) properties. The influence of the temperature and the cation's alkyl chain length on the above-mentioned properties was thoroughly investigated. The calculated thermodynamic (primary and derivative) and structural properties are in good agreement with the experimental data, while the extremely sluggish dynamics of the ILs under study renders the calculation of their transport properties a very complicated and challenging task, especially at low temperatures.     

Using clathrate hydrates for gas storage and gas-mixture separations: experimental and computational studies at multiple length scales

ABSTRACT

Clathrate hydrates have characteristic properties that render them attractive for a number of industrial applications. Of particular interest are the following two cases: (i) the incorporation of large amounts of gas molecules into the solid structure has resulted in considering hydrates as possible material for the storage/transportation of energy or environmental gases, and (ii) the selective incorporation of guest molecules into the solid structure has resulted in considering hydrates for gas-mixture separations. For the proper design of such industrial applications, it is essential to know accurately a number of thermodynamic, structural and transport properties. Such properties can either be measured experimentally or calculated at different scales that span the molecular scale-up to the continuum scale. By using clathrate hydrates as a particular case study, we demonstrate that performing studies at multiple length scales can be utilised in order to obtain properties that are essential to process design.     

Ginzburg-Landau theory of phase transitions in pseudo-one-dimensional systems

The static and dynamic properties of weakly coupled chains undergoing a phase transition are reviewed. The discussion is based on the functional generalization of the Ginzburg-Landau theory, including systems with real and complex order parameter. Various predictions of the theory, such as static correlations, renormalized phonon frequencies and a central resonance in the dynamic form factor near structural instabilities are discussed and compared with recent experiments on linear conductors that undergo a Peierls transition. New results are obtained for the thermodynamic anomalies near the onset of 3-d ordering and for the dynamic form factor of systems with an incommensurate Peierls distortion.     

Extension of the effective solid-fluid Steele potential for Mie force fields

Molecular simulation of fluid systems in the presence of surfaces require computationally expensive calculations due to the large number of solid–fluid pair interactions involved. Representing the explicit solid as a continuous wall with an effective potential can significantly reduce the computational time and allows exploring larger temporal and spatial scales. The well-known (10-4-3) Steele potential is one such analytic expression that faithfully represents the effective solid–fluid interactions for homonuclear crystalline solids with hexagonal lattice symmetry. However, this and most of the effective potentials found in the literature have been developed for fluids and solids interacting exclusively through Lennard-Jones potentials. In this work, we extend the Steele model to obtain the effective wall–fluid potentials for Mie force fields. We perform molecular dynamics simulations of coarse-grained fluids modelled via the SAFT force field approach in the presence of explicit and implicit surfaces to compare structural and dynamic properties in both representations. Also, we study the adsorption of ethane into slit-like pores with explicit and implicit surfaces via grand canonical Monte Carlo simulations. We explore the validity and the improvement in the simulation performance as well as the limitations of the proposed expression.     

Alchemical molecular dynamics for inverse design

We present a molecular dynamics (MD) implementation of an extended statistical mechanical ensemble that includes ‘alchemical’ degrees of freedom describing particle attributes as thermodynamic variables. We demonstrate the use of this alchemical MD method in inverse design simulations of particles interacting via the Oscillating Pair Potential (OPP) and the Lennard–Jones–Gauss potential (LJG) – two general, previously studied models for which phase diagrams are known. We show that alchemical MD can quickly and efficiently optimise pair potentials for target structures within a specified design space in the low-temperature regime, where internal energy adequately represents the features of the alchemical free energy landscape. We show that alchemical MD can be also used to inversely design pair potentials to achieve target materials properties (here, bulk modulus) directly, without explicit knowledge of the structure–property relationship. Alchemical MD can easily be generalised and applied to any target materials properties or structures and used with any differentiable interaction potential.     

Dynamics and correlation length scales of a glass-forming liquid in quiescent and sheared conditions

We numerically study dynamics and correlation length scales of a colloidal liquid in both quiescent and sheared conditions to further understand the origin of slow dynamics and dynamic heterogeneity in glass-forming systems. The simulation is performed in a weakly frustrated two-dimensional liquid, where locally preferred order is allowed to develop with increasing density. The four-point density correlations and bond-orientation correlations, which have been frequently used to capture dynamic and static length scales ξ in a quiescent condition, can be readily extended to a system under steady shear in this case. In the absence of shear, we confirmed the previous findings that the dynamic slowing down accompanies the development of dynamic heterogeneity. The dynamic and static length scales increase with α-relaxation time τ(α) as a power law [Formula: see text], with μ?>?0. In the presence of shear, both viscosity and τ(α) have power-law dependences on shear rate in the marked shear-thinning regime. However, the dependence of correlation lengths cannot be described by power laws in the same regime. Furthermore, the relation [Formula: see text] between length scales and dynamics holds for not too strong shear where thermal fluctuations and external forces are both important in determining the properties of dense liquids. Thus, our results demonstrate a link between slow dynamics and structure in glass-forming liquids even under nonequilibrium conditions.     

Atoms in a radio-frequency-dressed optical lattice

We load cold atoms into an optical lattice dramatically reshaped by radio-frequency coupling of state-dependent lattice potentials. This radio-frequency dressing changes the unit cell of the lattice at a subwavelength scale, such that its curvature and topology departs strongly from that of a simple sinusoidal lattice potential. Radio-frequency dressing has previously been performed at length scales from mm to tens of mum, but not at the single-optical-wavelength scale. At this length scale significant coupling between adiabatic potentials leads to nonadiabatic transitions, which we measure as a function of lattice depth and dressing amplitude. We also investigate the dressing by measuring changes in the momentum distribution of the dressed states.     

Molecular dynamic studies on materials under laser shocks

Molecular dynamic (MD) simulations offer a powerful means of understanding the microscopic characteristics of shock-propagation through solids and fluids, especially for the short spatial and temporal scales relevant to laser-driven shocks. First-principles molecular dynamics can be directly compared with time-resolved experimental measurements, and methods based on empirical (embedded-atom) potentials fitted to first-principles quantum-mechanical calculations are effective for MD simulations of shock propagation through many millions of atoms. In comparison, thermodynamic approaches based on free-energy considerations do not provide detailed information about mechanical-relaxation or phase-transformation processes within the shock front. We illustrate these ideas by way of embedded-atom simulations of shock-wave propagation through copper crystals of different orientation.     

Effect of magnetoelastic interaction on the thermodynamics of ferromagnets: Simulation by the method of spin-lattice dynamics

Thermodynamic properties of systems with coupled magnetic and lattice degrees of freedom are analyzed by the numerical spin-lattice dynamics (SLD) method. A scheme of numerical integration is developed for SLD equations in a thermostat, that follows the earlier formulated approaches and is modified to describe systems with realistic interatomic interactions. The method proposed allows one to calculate the spectral density of oscillations, heat capacity, magnetization, and thermal expansion coefficient within a single scheme. It is established that, due to short-range magnetic order, the interplay between magnetic and lattice degrees of freedom contributes to the thermodynamic properties of the system even in the paramagnetic state. It is shown that there exist two mechanisms how the spin-lattice interaction influences the thermodynamic properties: static and dynamic mechanisms; the first is determined by its contribution to the thermal expansion of the lattice, and the second, by the dynamic interaction between magnetic moments and crystal lattice vibrations.     

Inversion of sequence of anomalies in core-softened systems with attraction

In this paper we present a simulation study of water-like anomalies in a purely repulsive core-softened system and in a system with attraction described in our previous publications. We investigate the anomalous regions for systems with the same functional form of the potentials but with different parameters and show that the order of the region of anomalous diffusion and the region of density anomaly is inverted with increasing the width of the repulsive shoulder and the depth of the attractive well. It is shown that while the density anomaly is always inside the region of the structural anomaly, the diffusion anomaly can change its location depending on the parameters of the potential. In the presence of the attraction in the potential, the system demonstrates the silica-like behavior.     

Glass-forming liquids

When liquids are cooled below their melting temperature T_m, sometimes they do not solidify immediately but remain in supercooled state up to temperatures far below the melting point. Under certain conditions, they can solidify into the form of amorphous solids without crystallizing. A supercooled liquid and a amorphous solid are metastable phases, which are not completely understood in terms of structural arrangement and thermodynamic behaviour. But by far the most interesting feature is the glass transition which is the manifestation of a true thermodynamic transition and a dynamic event since a dramatic dynamic arrest intervenes. A unified theory of supercooled liquids and glass transition does not yet exist and, more specifically, the link between the sharp increase of the relaxation time and the correlation length is a question still largely open. This article presents in the most elementary manner a brief overview of this delicate issue.     

The Concavity of Entropy and Extremum Principles in Thermodynamics

We revisit the concavity property of the thermodynamic entropy in order to formulate a general proof of the minimum energy principle as well as of other equivalent extremum principles that are valid for thermodynamic potentials and corresponding Massieu functions under different constraints. The current derivation aims at providing a coherent formal framework for such principles which may be also pedagogically useful as it fully exploits and highlights the equivalence between different schemes. We also elucidate the consequences of the extremum principles for the general shape of thermodynamic potentials in relation to first-order phase transitions.