Molecular dynamics simulations of pure methane and carbon dioxide hydrates: lattice constants and derivative properties

ABSTRACT

We report extensive molecular dynamics simulation results of pure methane and carbon dioxide hydrates at pressure and temperature conditions that are of interest to various practical applications. We focus on the calculation of the lattice constants of the two pure hydrates and their dependence on pressure and temperature. The calculated lattice constants are correlated using second order polynomials which are functions of either temperature or pressure. Finally, the obtained correlations are used in order to calculate two derivative properties, namely the isothermal compressibility and the isobaric thermal expansion coefficient. The current simulation results are also compared against reported experimental measurements and other simulation studies and good agreement is found for the case of isothermal compressibility. On the other hand, for the case of isobaric thermal expansion coefficient good agreement is found only with other simulation studies, while the simulation studies are in disagreement with experiments, particularly at low temperatures.     

Validity of some regularities of dense fluids for ionic liquids

Five of the best known regularities have been investigated for different classes of ionic liquids. The regularities are near linearity of the isothermal bulk modulus as function of pressure, common intersection point of isotherms of the bulk modulus as a function of density, common intersection point of isotherms of isobaric expansion coefficient or isothermal compressibility versus pressure, and common intersection point for the isobars of internal pressure as a function of temperature. These regularities were evaluated by Tait and GMA equation of states.     

Equilibrium defect properties of sodium in the high temperature range

The defect structure of high purity sodium (99.99%) was examined by using precise lattice parameter data. It is shown that these data are not in contradiction to the existence of an appreciable amount of divacancies in addition to the single vacancy contribution. The consequences of this result are discussed with respect to the diffusion mechanism, the isobaric expansion coefficient, the isobaric specific heat, the isothermal compressibility and the Grüneisen-parameter.     

Elasticity of a magnetic lattice near the magnetic critical point

The isothermal elastic constants and the coefficient of anomalous thermal expansion of a magnetic lattice are discussed. The spin system is described by the Ising model with an exchange coupling depending on lattice spacing. A behavior of the elastic constants and the coefficient of thermal expansion is found which is in qualitative agreement with experiments. The isothermal compressibility remains positive nearT _c and no thermo-mechanical instability occurs (which would lead to a first-order phase transitions), in contrast to earlier theories.     

U_(1-x)Pu_xO_2热膨胀性质分子动力学模拟研究

基于部分离子势函数的分子动力学方法,研究了U_(1-x)Pu_xO_2(x=0.07,0.15,0.25和0.5)在300—3000 K温度范围和0—1.5 GPa下的热力学性质,研究发现,在等压和不同温度下,随着钚(Pu)比率的增加,U_(1-x)Pu_xO_2的晶格常数线性减小,线性膨胀系数有所增大,等温压缩系数减小,通过拟合得到了零压下U_(1-x)Pu_xO_2晶格常数和线膨胀系数随Pu比率变化的经验表达式。     

Molar volume, thermal expansivity and isothermal compressibility of trans-decahydronaphthalene up to 200MPa and 446K

The molar volume isotherms of trans-decahydronaphthalene （C10H18） between 293 and 446 K and at pressures from 10 to 200 MPa have been determined. A modified Tait equation of state is used to fit each experimental molar volume isotherm with a maximum average deviation of 0.029%. The thermal expansivity （cubic expansion coefficient） α and isothermal compressibility κ were determined by fitting the slopes of the isobaric curves and isotherms, respectively. The coefficients in the equation Vm=C1＋C2T＋C3T^2-C4p-C5pT have been fitted with an average deviation of 1.03%.     

The Debye temperature and Grüneisen parameter of the CaLa<Subscript>2</Subscript>S<Subscript>4</Subscript>-La<Subscript>2</Subscript>S<Subscript>3</Subscript> system

The thermal expansion coefficient of solid solutions in the CaLa₂S₄-La₂S₃ system at a temperature of 300 K is investigated experimentally. The Debye temperature, the Grüneisen parameter, and the isothermal compressibility coefficient of solid solutions in the system under investigation are determined from the experimental thermal expansion coefficient. It is demonstrated that, upon substitution of calcium ions for cation vacancies in La₂S₃, the Debye temperature decreases, the isothermal compressibility coefficient increases, and the Grüneisen parameter remains constant for all compositions in the CaLa₂S₄-La₂S₃ system. A correlation between the ionic radii of Ca²⁺ and La³⁺, the concentration of cation vacancies, and the rigidity of the lattice, on the one hand, and the Debye temperature, the Grüneisen parameter, and the isothermal compressibility coefficient, on the other, is revealed for the studied samples.     

Thermodynamic and magnetic properties of LaSn3

Thermal expansion coefficient between 77 and 900K, isothermal compressibility in the 0–80 Kbar pressure range, magnetic susceptibility between 77 and 1300 K and heat capacity at constant pressure in the 20–300 K temperature range were determined for the LaSn₃ compound. From the experimental data, the specific heat at constant volume was calculated and the thermal dependence of the Debye's parameter θ_D was obtained. The electron contribution to the heat capacity was also determined from the high temperature data. The magnetic properties confirm that there is no evidence of the existence of a magnetic moment localized on La atoms, in contrast with a previous report and in agreement with the general assumptions. A little anomaly found in the expansion coefficient, in the isothermal compressibility and in the specific heat is discussed in terms of a lattice order-disorder phenomenon.     

Calculation of thermodynamic properties of Ni nanoclusters via selected equations of state based on molecular dynamics simulations

We present an approach for constant-pressure molecular dynamics simulations. This approach is especially designed for finite systems, for which no periodic boundary condition applies. A molecular dynamics (MD) simulation for Ni nanoclusters is used to calculate their pressure–volume–temperature (p–v–T) data for the temperature range 200 K≤T≤400 K, and pressures up to 600 kbar. Isothermal sets of p–v–T data were generated by the simulation; each set was fitted by three equations of state (EoSs): Linear Isotherm Regularity-II (LIRII), Birch–Murnaghan (BM), and EOS III. It is found that the MD data are satisfactorily reproduced by the EoSs with reasonable precision. Some features of the EoSs criteria, such as the temperature dependences of the coefficients, the isothermal bulk modulus and its pressure derivative at the zero-pressure limit, and isobaric thermal expansion for Ni nanoclusters, are investigated. We have found that same EoSs are valid for both bulk Ni and Ni nanoclusters, but with different values of the parameters, which depend on the cluster size and temperature. An increase in bulk modulus with decrease of cluster size can be observed. Also, an increase in isobaric expansion coefficient with decrease of cluster size has been found.     

Theoretical Study of Compressibility and Thermoelasticity of LaNi5-xAlx （x= 0.3, 0.5 and 1.0）

The compressibility, the temperature dependence of bulk modulus, the pressure dependence of normalized volume V/V0, thermal expansion coefficient and Debye temperature of LaNi5-xAlx compounds are successfully obtained using the first-principles plane-wave pseudopotential （PW-PP） method, the EOSFIT6.0 software and the quasiharmonic Debye model. The rapid decrease of relative lattice constant a/a0 shows that the deformation is easier in directions normal to the c-axis than that along it. The relationships between bulk modulus B and pressure at different temperatures are also analysed. It is found that the bulk modulus B increases monotonically with increasing pressure. Moreover, the pressure dependences of thermal expansion and Debye temperature are also successfully obtained. The calculated results are in agreement with the experimental data.     

Infrared spectroscopy and ultrasonic velocimetry as a tool for probing conformational flexibility of proteins

Abstract

Because of the crucial importance of structural fluctuations for function and stability of proteins, there is a strong interest for the relationships between structural fluctuations, the parameters of protein denaturation and the kinetics of H/D-exchange. Structural fluctuations can be described by volume and entropy fluctuations and these quantities are accessible via the isothermal compressibility, the thermal expansion and the isobaric heat capacity.

Our aim is to present the principal problem concerning the experimental procedures to answer those questions using lysozyme and α-lactalbumin. Whereas the transition parameters and the kinetics of the H/D-exchange were obtained using FTIR spectroscopy, the adiabatic compressibility was obtained by a combination of ultrasonic velocimetry and densitometry. It could be shown that the stability of the investigated proteins is correlated with reduced volume fluctuations. The expected direct correlation between H/D exchange rates and structural fluctuations could not be seen and it is assumed that the interactions are more complex than from the intuitive point of view.     

关于二级相变中热力学物理量突变的分析与自旋链模拟

理论上对二级相变中热容、体膨胀系数、等温压缩系数三个热力学物理量的变化进行了分析,进而使用自旋链模型的基态能量和von Neumann熵随模型参数变化的突变行为对二级相变中这三个物理量的变化所引起的系统序变进行模拟,得到二级相变中热容、体膨胀系数、等温压缩系数的变化是由于系统内部微观上发生关联序变的结果。     

Analysis of thermal expansivity and isothermal compressibility of liquid hexane and chloroform

In the present paper, we have analysed the pressure dependences of thermal expansivity (α _p ) and isothermal compressibility (K _T ) for the liquid hexane and chloroform. We have used the formulae based on the pseudospinodal model and the Sanchez et al [1,2] equation of state obtained from the Pade approximation. The thermal expansion coefficient and isothermal compressibility follow a power law function of pressures. It is found that the quantities, α _p as well as K _T decreases with the increasing pressure. The isothermal compressibility decreases more rapidly in comparison to thermal expansivity. The pressure at which the spinodal condition is satisfied, i.e. α _p and K _T both tend to infinity, has been found to be a characteristic property of the liquid hexane and chloroform.     

Linear dependence of the phonon thermal resistance of nonmetallic crystals on the isobaric thermal strain

This paper reports on the results of investigations into the thermal resistance W and thermal expansion coefficient β for single-crystal samples of Si, SiO₂, Al₂O₃, and NaCl. The available experimental data on the thermal resistance and thermal expansion coefficient for materials with different types of interatomic bonding and different Landau criteria for convection are analyzed. It is demonstrated that the reduced phonon thermal resistance is equal to the isobaric thermal strain at any temperature.     

Molecular dynamics simulation of P-V-T relationship of ZnO with rock-salt structure using pair-wise interactions

Molecular dynamics (MD) method is used to investigate the behavior of the pressure-volume-temperature (P-V-T) relationship, lattice constant and thermal expansivity for ZnO with rock-salt structure at high pressures and temperatures. The interionic potential is taken to be the sum of pair-wise additive Coulomb, van der Waals attraction, and repulsive interactions. The isothermal and isobaric properties are discussed from the corresponding P-V-T relationship, and it is shown that the MD simulation is successful in reproducing the measured volumes of ZnO over a wide range of temperature and pressure. Meanwhile, the equations of state parameters including lattice constant, linear thermal expansion coefficient, and isothermal bulk modulus are calculated and compared with the available experimental data and the latest theoretical results. At an extended pressure and temperature range, P-V-T relationship, lattice constant, and linear thermal expansion coefficient have been predicted. The structural and thermodynamic properties of ZnO with rock-salt structure are summarized in the pressure 0-100 GPa ranges and the temperature up to 3100 K.     

Temperature dependent anomalous fluctuations in water: shift of ≈1 kbar between experiment and classical force field simulations

Here we report on the temperature dependence of the anomalous behaviour of water in terms of (i) its growth in tetrahedral structures, (ii) instantaneous spatial correlations from small angle x-ray scattering (SAXS) data, (iii) estimates of thermodynamic response functions of isothermal compressibility and (iv) thermal expansion coefficient. Water’s thermal expansion coefficient is estimated for the first time at supercooled conditions from liquid water’s structure factor. We used previously published data from classical force-fields of TIP4P/2005 and iAMOEBA to compare experimental data with molecular dynamics simulations and observe that these force-fields underestimate water’s anomalous behaviour but perform better upon increasing pressure. We demonstrate that the molecular dynamics simulations can describe better the temperature dependent anomalous behaviour of ambient pressure water if simulated at 1?kbar. The deviation in anomalous fluctuations in the simulations is not restricted to ≈228?K but extends all the way to ambient temperatures.     

General Considerations on the Finite-Size Corrections for Coulomb Systems in the Debye--Hückel Regime

We study the statistical mechanics of classical Coulomb systems in a low coupling regime (Debye--Hückel regime) in a confined geometry with Dirichlet boundary conditions for the electric potential. We use a method recently developed by the authors which relates the grand partition function of a Coulomb system in a confined geometry with a certain regularization of the determinant of the Laplacian on that geometry with Dirichlet boundary conditions. We study several examples of fully confining geometry in two and three dimensions and semi-confined geometries where the system is confined only in one or two directions of the space. We also generalize the method to study systems confined in arbitrary geometries with smooth boundary. We find a relation between the expansion for small argument of the heat kernel of the Laplacian and the large-size expansion of the grand potential of the Coulomb system. This allow us to find the finite-size expansion of the grand potential of the system in general. We recover known results for the bulk grand potential (in two and three dimensions) and the surface tension (for two-dimensional systems). We find the surface tension for three-dimensional systems. For two-dimensional systems our general calculation of the finite-size expansion gives a proof of the existence a universal logarithmic finite-size correction predicted some time ago, at least in the low coupling regime. For three-dimensional systems we obtain a prediction for the curvature correction to the grand potential of a confined system.     

超临界流体传热综述

超临界流体是温度和压力均高于临界点的流体,在临界点附近物体的粘性、密度、比热以及其它一些性质变化非常剧烈,处于超临界状态时物质的气液两相性质非常接近,这种特殊的物性使得超临界流体在诸多领域有着非常广泛的应用。总结了以前国内外学者对于超临界流体传热的研究,主要集中于CO2和水的实验研究,由众多实验表明换热恶化容易发生在赝临界点附近、高热流密度、低质量流量和向上流动的情况下,换热系数的峰值也出现在赝临界点附近,并随压力和温度的变化会有所改变。还对提出的一系列关联式进行比较和分析。     

常压下固态KNO2热力学性质的密度泛函理论研究

采用密度泛函理论结合准谐德拜模型研究常压下300~725 K间KNO₂立方结构的热力学性质,重点分析常压下定压热容、定容热容、熵、德拜温度、体膨胀系数、平衡体积和体弹模量随温度的变化.结果显示,常压下计算的定压热容随温度的变化与实验数据符合较好,而计算的熵与实验数据相差较大.计算得到KNO₂的平均体膨胀系数约为1.837 8×10^-5K^-1,常温下(300 K)KNO₂的德拜温度约为667.13K.     

The thermodynamics of elastic deformation—I: Equation of state for solids

The isobaric and isothermal volume derivatives of In B, In μ and In μ' are investigated, where B, μ and μ' are the isothermal bulk modulus and the two shear moduli, respectively, of a cubic crystal. In the case of the bulk modulus, the temperature independence of αB (where α is the volume thermal expansion) for a large number of materials, ensures that the derivatives are constant and approximately equal, while for the shear moduli, evidence is advanced that the isothermal derivatives are constant along an isotherm, but not along an isobar except at high temperatures near the melting point. The relationships satisfied by the bulk modulus enable the explicit temperature and pressure dependence of the molar volume, V, thermal expansion, and bulk modulus to be determined. The most adequate representation of the volume dependence of the Grüneisen parameter, γ, appears to be that γ/V is independent of volume between the Debye and melting temperatures.