High frequency dynamics and structural relaxation process in liquid ammonia

The dynamic structure factor S(Q,omega) of liquid ammonia has been measured by inelastic x-ray scattering in the terahertz frequency region as a function of the temperature in the range of 220-298 K at a pressure P=85 bars. The data have been analyzed using the generalized hydrodynamic formalism with a three term memory function to take into account the thermal, the structural, (alpha) and the microscopic (mu) relaxation processes affecting the dynamics of the liquid. This allows to extract the temperature dependence of the structural relaxation time (tau(alpha)) and strength (Delta(alpha)). The former quantity follows an Arrhenius behavior with an activation energy E(a)=2.6+/-0.2 kcal/mol, while the latter is temperature independent suggesting that there are no changes in the interparticle potential and arrangement with T. The obtained results, compared with those already existing in liquid water and liquid hydrogen fluoride, suggest the strong influence of the connectivity of the molecular network on the structural relaxation.     

Totally inclusive cubic equation of state with five parameters for pure fluids

A totally inclusive cubic equation of state (cubic EOS) is proposed. Although, its form is fairly simple as compared with the present cubic equations, it can include all of them as special cases. The EOS has five parameters. By fitting the experimental critical isothermal for six typical substances combining the critical conditions, the generalized expressions for the five parameters at critical temperature are established. The temperature coefficients of the five parameters for 43 substances are determined by fitting the experimental data of vapor pressure and saturated liquid density. These coefficients are correlated with the critical compressibility factor and acentric factor to obtain the generalized expressions. The predicted saturated vapor pressure, saturated liquid density, critical isothermal and coexistence curve near the critical point show that the equation gives the best results when compared with the Redlich–Kwong–Soave (RKS) and Peng–Robinson (PR) EOS.     

Solvation pressure in chloroform

Molecular dynamics (MD) simulations of chloroform vapor and liquid at normal temperature and pressure and liquid under hydrostatic pressure are presented, giving bond lengths and vibrational frequencies as functions of pressure. The change in bond lengths between vapor and liquid at normal temperature and pressure is consistent with a pressure equivalent to the cohesive energy density (CED) of the liquid, supporting the solvation pressure model which predicts that solvated molecules or nanoparticles experience a pressure equal to the CED of the liquid. Experimental data for certain Raman frequencies of chloroform in the vapor phase, in the liquid, and in the liquid under pressure are presented and compared to MD. Results for C-Cl vibrational modes are in general agreement with the solvation pressure model whereas frequencies associated with the C-H bond are not. The results demonstrate that masking interactions exist in the real liquid that can be reduced or eliminated in simplified simulations.     

Local structure of a phase-separating binary mixture in a mesoporous glass matrix studied by small-angle neutron scattering

The mesoscopic structure of the binary system isobutyric acid + heavy water (D(2)O) confined in a porous glass (controlled-pore silica glass, mean pore width ca. 10 nm) was studied by small-angle neutron scattering at off-critical compositions in a temperature range above and below the upper critical solution point. The scattering data were analyzed in terms of a structure factor model similar to that proposed by Formisano and Teixeira [Eur. Phys. J. E 1, 1 (2000)], but allowing for both Ornstein-Zernike-type composition fluctuations and domainlike structures in the microphase-separated state of the pore liquid. The results indicate that the phase separation in the pores is shifted by ca. 10 K and spread out in temperature. Microphase separation is pictured as a transition from partial segregation at high temperature, due to the strong preferential adsorption of water at the pore wall, to a tube or capsule configuration of the two phases at low temperatures, depending on the overall composition of the pore liquid. Results for samples in which the composition of the pore liquid can vary with temperature due to equilibration with extra-pore liquid are consistent with this picture.     

Theoretical calculation of electrical resistivity in liquid sodium and potassium

As a direct numerical test of Ziman formula for the static resistivity of monovalent liquid metals, constant pressure resistivities of liquid potassium at 338 K and 408 K and liquid sodium at 373 K and 473 K have been calculated using a recently proposed form factor and a liquid structure factor obtained from Monte Carlo calculations. The procedure is fundamental in that no parameters have been adjusted to fit experimental values at any stage. The resistivity results are in good agreement with experiment at the above mentioned temperatures, but it appears that the formula may become less accurate at higher temperatures.     

Upward extrapolation of saturated liquid densities

The design of new energy conversion processes requires equations of state for the working fluids. For their construction saturated liquid densities are needed which are not available for some potential working fluids at higher temperatures. Hence, we investigate how Rackett-type equations behave in extrapolations from saturated liquid densities in the temperature range 0.5 ≤ T/T_c ≤ 0.75 up to the critical temperature T_c. Extrapolation methods with different inputs from the critical point are used: (a) no critical point data, (b) critical temperature, (c) critical temperature and pressure, and (d) critical temperature and compression factor. It is found that upward extrapolations of the saturated liquid densities without using critical point data can be done with some care and that the additional use of the critical temperature improves the quality of the predictions substantially.     

Vapor pressures and PVT properties of 1,1,1,3,3-pentafluoropropane (HFC-245fa)

The vapor pressures and PVT properties of superheated vapor and compressed liquid of 1,1,1,3,3-pentafluoropropane (HFC-245fa) were measured at wide range of temperature and pressure. The simple correlation for vapor pressures, compressibility factors of superheated vapor and specific volumes of liquid were developed on the basis of the present measurements. The critical pressure was calculated by extrapolating the developed vapor pressure equation to the critical temperature. Isothermal compressibility of liquid was calculated from the developed Tait equation. Specific volume data obtained show the good linearity in the Hudleston plots. Overall uncertainty in the vapor pressure, compressibility factor and specific volume measurements is estimated less than ±5 kPa, ±1.2% and ±0.09%, respectively.     

Molecular dynamics simulation of liquid trimethylphosphine

Structural and dynamical properties of liquid trimethylphosphine (TMP), (CH(3))(3)P, as a function of temperature is investigated by molecular dynamics (MD) simulations. The force field used in the MD simulations, which has been proposed from molecular mechanics and quantum chemistry calculations, is able to reproduce the experimental density of liquid TMP at room temperature. Equilibrium structure is investigated by the usual radial distribution function, g(r), and also in the reciprocal space by the static structure factor, S(k). On the basis of center of mass distances, liquid TMP behaves like a simple liquid of almost spherical particles, but orientational correlation due to dipole-dipole interactions is revealed at short-range distances. Single particle and collective dynamics are investigated by several time correlation functions. At high temperatures, diffusion and reorientation occur at the same time range as relaxation of the liquid structure. Decoupling of these dynamic properties starts below ca. 220 K, when rattling dynamics of a given TMP molecules due to the cage effect of neighbouring molecules becomes important.     

A New Technique for Modelling the Structure of Expanded Liquid Metals Along the Liquid-Vapour Coexistence Curve

Abstract

We have developed a simple technique for modelling the structure of expanded liquid metals along the liquid vapour coexistence curve which may be characterised as a ‘correlated percolation’ method. Starting from a model for the liquid at high density, e.g. near the triple point, obtained either by molecular dynamics simulation or reverse Monte Carlo modelling, we keep the size of the model and the atomic positions fixed and remove atoms, according to criteria which depend on the coordination number distribution, until the required lower density, corresponding to a higher temperature, is reached. Small random Gaussian displacements are then added to the position of each atom to account for the increased temperature. The structure factor of the resulting model is quite close to that measured experimentally. Changes in the structure factor as the liquid expands can thus be separated into the effects of density fluctuations and temperature (or entropy).     

Molecular dynamics simulations on the local order of liquid and amorphous ZnTe

Molecular dynamics studies of structural and dynamical correlations of molten and vitreous states under several conditions of density and temperature were performed. We use an effective recently proposed interatomic potential, consisting of two- and three-body covalent interactions which has successfully described the structural, dynamical, and structural phase transformation induced by pressure in ZnTe [D. S. Borges and J. P. Rino, Phys. Rev. B 72, 014107 (2005)]. The two-body term of the interaction potential consists of Coulomb interaction resulting from charge transfer, steric repulsion due to atomic sizes, charge-dipole interaction to include the effect of electronic polarizability of anions, and dipole-dipole (van der Waals) interactions. The three-body covalent term is a modification of the Stillinger-Weber potential. Molecular dynamics simulations in isobaric-isenthalpic ensemble have been performed for systems amounting to 4096 and 64 000 particles. Starting from a crystalline zinc-blende (ZB) structure, the system is initially heated until a very homogeneous liquid is obtained. The vitreous zinc telluride phase is attained by cooling the liquid at sufficiently fast cooling rates, while slower cooling rates lead to a disordered ZB crystalline structure. Two- and three-body correlations for the liquid and vitreous phases are analyzed through pair distribution functions, static structure factors, and bond angle distributions. In particular, the neutron static structure factor for the liquid phase is in very good agreement with both the reported experimental data and first-principles simulations.     

A new cubic equation of state for reservoir fluids

A new three-parameter cubic equation of state is developed with special attention to the application for reservoir fluids. One parameter is taken temperature dependent and others are held constant. The EOS parameters were evaluated by minimizing saturated liquid density deviation from experimental values and satisfying the equilibrium condition of equality of fugacities simultaneously. Then, these parameters were fitted against reduced temperature and Pitzer acentric factor. For calculating the thermodynamic properties of a pure component, this equation of state requires the critical temperature, the critical pressure, the acentric factor and the experimental critical compressibility of the substance. Using this equation of state, saturated liquid density, saturated vapor density and vapor pressure of pure components, especially near the critical point, are calculated accurately. The average absolute deviations of the predicted saturated liquid density, saturated vapor density and vapor pressure of pure components are 1.4%, 1.19% and 2.11%, respectively. Some thermodynamic properties of substances have also been predicted in this work.     

Structure and speciation in hydrous silica melts. 2. Pressure effects

The effect of pressure on structure and water speciation in hydrated liquid silica is examined over a range of temperatures and compositions. The Feuston-Garofalini (FG) potential is used in isobaric-isothermal Monte Carlo simulations carried out at four pressures (0.25, 1.0, 2.5, and 10 GPa) for seven temperatures (2000 < or = T < or= 9000 K) and five compositions (0.0 < or = x_w < or = 0.4). The FG potential yields a stable melt phase for p > or = 1.0 GPa and/or x_w < or = 0.1 for all temperatures. The volume minimum seen in previous simulations of pure and hydrated liquid silica using the FG potential persists up to 2.5 GPa but is no longer evident at 10 GPa. This is correlated with gradual structural changes of the liquid up to 2.5 GPa and with more significant changes at 10 GPa. Even at high overall concentrations of water (x_w = 0.4), only about 2% of oxygen atoms are present as molecular water species at the lowest temperature. This percentage decreases with increasing pressure and temperature.     

Cu-12％Al合金熔体内中程有序原子团簇

通过高温X射线衍射仪研究了Cu 12％Al（质量分数,下同）合金熔体结构,并用纯铜作对比实验.在1250 ℃时,发现Cu 12％Al合金熔体结构因子曲线上18.5 nm－1位置有预峰出现.随着温度的下降,预峰变得更加明锐.预峰的出现是液体中存在中程有序的标志.通过熔态旋淬法获得该合金的快速凝固条带,对条带进行固态X射线衍射分析,其结构是具有有序体心立方晶格的Cu3Al.熔体内的中程有序结构单元尺寸与快凝Cu3Al（111）晶面面间距d111数值一致.由双体分布函数得到的最近邻原子距离、配位数.结合原子团簇结构单元的几何模型,计算得出该体心立方的棱边长(a=3.00 10－10m)与文献中所提供的固态晶格常数（a=2.95 10－10 m）基本吻合.可证明该合金熔体中存在以DO3结构为基本单元的中程有序原子团簇,在液相线以上200 ℃温度范围内这种中程有序都能稳定存在,并随着温度的下降,中程有序的相关尺寸逐渐增大.     

Effect of temperature and pressure on the structure, dynamics, and hydrogen bond properties of liquid N-methylacetamide: a molecular dynamics study

The structure and dynamical properties of liquid N-methylacetamides (NMA) are calculated at five different temperatures and at four different pressures using classical molecular dynamics simulations. Our results are analyzed in terms of pressure-induced changes in structural properties by investigating the radial distribution functions of different atoms in NMA molecule. It is found that the first peak and also the second peak of C-O and N-H are well defined even at higher temperature and pressure. It is also observed that the number of hydrogen bonds increase with application of pressure at a given temperature. On the other hand, the calculated hydrogen bond energy (E(HB)) shows that the stability of hydrogen bond decreases with increasing of pressure and temperature. Various dynamical properties associated with translational and rotational motion of neat NMA are calculated and the self-diffusion coefficient of NMA is found to be in excellent agreement with the experiment and the behavior is non-Arrhenius at low temperatures with application of pressures. The single particle orientational relaxation time for dipole vector and N-C vector are also calculated and it is found that the orientational relaxation time follows Arrhenius behavior with a variation of temperature and pressure.     

Temperature, pressure, and isotope effects on the structure and properties of liquid water: a lattice approach

We present a lattice model to describe the effect of isotopic replacement, temperature, and pressure changes on the formation of hydrogen bonds in liquid water. The approach builds upon a previously established generalized lattice theory for hydrogen bonded liquids [B. A. Veytsman, J. Phys. Chem. 94, 8499 (1990)], accounts for the binding order of 1/2 in water-water association complexes, and introduces the pressure dependence of the degree of hydrogen bonding (that arises due to differences between the molar volumes of bonded and free water) by considering the number of effective binding sites to be a function of pressure. The predictions are validated using experimental data on the temperature and pressure dependence of the static dielectric constant of liquid water. The model is found to correctly reproduce the experimentally observed decrease of the dielectric constant with increasing temperature without any adjustable parameters and by assuming values for the enthalpy and entropy of hydrogen bond formation as they are determined from the respective experiments. The pressure dependence of the dielectric constant of water is quantitatively predicted up to pressures of 2 kbars and exhibits qualitative agreement at higher pressures. Furthermore, the model suggests a--temperature dependent--decrease of hydrogen bond formation at high pressures. The sensitive dependence of the structure of water on temperature and pressure that is described by the model rationalizes the different solubilization characteristics that have been observed in aqueous systems upon change of temperature and pressure conditions. The simplicity of the presented lattice model might render the approach attractive for designing optimized processing conditions in water-based solutions or the simulation of more complex multicomponent systems.     

Interpretation of boundaries in the pressure–temperature diagram for liquid tellurium and their relationship to the melting-curve maximum

Research on liquid tellurium is reviewed and interpreted in an effort to obtain a consistent picture of the effect of pressure and temperature on the structure and properties of tellurium in the liquid state. Electrical resistance experiments on liquid tellurium support the existence of reaction boundaries in the p–T diagram. The boundaries are believed to identify stages in the reaction whereby the 2-coordinate covalent chain structure transforms to a 3-coordinate covalent network structure which eventually dissociates with pressure and temperature into a metallic system. The increasing liquid density associated with the molecular reaction is given as the cause of the maximum on the P–T melting curve. The melting curve maximum in tellurium is shown to be related to maxima in selenium and sulfur. Quenching tellurium from different liquid fields, delineated by reaction boundaries, resulted in materials with different x-ray spectra.     

Surface phase behavior in Gibbs monolayers of bis(ethylene glycol) mono-n-tetradecyl ether at the air-water interface

We present the adsorption kinetics and surface morphology of the adsorbed monolayers of bis(ethylene glycol) mono-n-tetradecyl ether (C14E2) by film balance and Brewster angle microscopy. A cusp point followed by a plateau region in the pressure (pi)-time (t) adsorption isotherm indicates a first-order phase transition in the coexistence region between a lower density liquid expanded (LE) phase and a higher density liquid condensed (LC) phase. A variety of condensed phase domains surrounded by the homogeneous LE phase are observed just after the appearance of the phase transition. The domains are of a spiral or striplike structure at lower temperatures. This characteristic shape of the domains is because of strong dipole-dipole repulsion between the molecules. At 18 degrees C, the domains are found to be quadrant structures. A slight increase in subphase temperature (around 1 degrees C) brings about a quadrant-to-circular shape transition in the domains. The circular domains return to quadrant structures as the subphase temperature is lowered. The domains completely disappear when the temperature is increased beyond 19 degrees C, suggesting that the critical temperature for the condensed domain formation is 19 degrees C. Above this temperature, the hypothetical surface pressure necessary for the phase transition exceeds the actual surface pressure attainable from a solution of concentration greater than or equal to the critical micelle concentration. An increase in molecular motion with increasing temperature results in a higher degree of chain flexibility. As a result, the molecules cannot accumulate in the condensed phase form when the subphase temperature is above 19 degrees C.     

Effect of temperature on the interfacial behavior of a polystyrene-b-poly(methyl methacrylate) diblock copolymer at the air/water interface

Monolayers of a polystyrene-poly(methyl methacrylate) (PS-PMMA) diblock copolymer at the air-water interface were studied by measuring the surface pressure-area isotherms at several temperatures. Langmuir film balance experiments and atomic force microscopy showed that the diblock copolymer molecules formed surface micelles. In the plot of the surface pressure versus surface area per repeating unit, the monolayer changed from the gas phase to the liquid expanded phase at lower surface pressure for systems at low temperature compared to those at high temperature. In addition, a plateau, corresponding to the transition from the liquid expanded to liquid condensed phase, appeared in that plot at lower surface pressure for systems with a higher subphase (water) temperature. Hysteresis was observed in the compression-expansion cycle process. Increasing the subphase temperature alleviated this hyteresis gap, especially at low surface pressures. The minimum in the plot of the surface pressure versus surface area per repeating unit in the expansion process (which arises from the transition) and the transition plateau appeared more vividly at higher water temperature. These dynamic experimental results show that PS-PMMA diblock copolymers, in which both blocks are insoluble in water, do not form complicated entanglements in two-dimensional space. Although higher water temperature provided more entropy to the chains, and thus more conformational freedom, it did not change the surface morphology of the condensed film because both blocks of PS-PMMA are insoluble in water.     

Fe68Si32合金液态结构与固态组织的相关性

利用液态X射线衍射仪研究了Fe68Si32合金的液态结构，获得了结构因子、径向分布函数、原子间的最近邻距离和配位数.结果表明，在1250～1450 ℃范围内液态合金的最近邻距离为0.259～0.260 nm，配位数为10.3(±0.2)；液态合金的结构因子在Q=15.5 nm－1处有一明显的预峰存在.根据预峰的特性，建立了Fe68Si32熔体的结构模型，即体心立方结构的有序Fe3Si原子团以共面的方式形成Fe3Si面心立方超结构(DO3)；合金在1250 ℃的径向分布函数的Gauss分解结果与合金的面心立方模型吻合较好.预峰的产生是面心立方超结构原子团中Si原子之间相互关联的外在表现. Fe68Si32合金的固态X射线衍射显示合金中含有Fe3Si相，而且其特征峰与合金的结构因子的峰位基本一致，表明Fe68Si32合金的液固态结构之间联系紧密.     

Solid + liquid equilibria of (n-alkane + cyclohexane) mixtures at high pressures

Solid+liquid equilibria (SLE) of [n-alkanes (tridecane, hexadecane, octadecane, or eicosane) + cyclohexane] at very high pressures up to about 1.0 GPa have been investigated in the temperature range from 293 to 363 K using a thermostated apparatus for the measurements of transition pressures from the liquid to the solid state in two component isothermal solutions. The freezing temperature of each mixture increases monotonously with increasing pressure. The eutectic point of the binary systems shifts to a higher temperature and to a higher n-alkane concentration with increasing pressure. The pressure–temperature–composition relation of the high-pressure solid–liquid equilibria, a polynomial based on the general solubility equation at atmospheric pressure, was satisfactorily used. Additionally, the SLE of the binary system (tridecane+cyclohexane) at normal pressure was measured by the dynamic method. The results at high pressure for all systems were compared to that at normal pressure.