Molecular dynamics simulation of the thermodynamic properties of mercury at pressures below 2.5 GPa and temperatures below 10000 K

Models of mercury were constructed by molecular dynamics using the interparticle potential of the embedded atom model (EAM) at temperatures below 10 000 K and pressures below 2.5 GPa. The thermodynamic properties of the models were presented on the isobars of 0.5, 1.0, 1.5, 2.0, and 2.5 GPa. The compressibility factors Z = pV/(RT) were calculated; the coordinates of the inversion points of the Joule–Thomson coefficient below 5600 K were found from the positions of minima on the Z(p, T) isobars. At densities above 8–9 g/cm³, the results of simulation agreed well with experiment; at lower densities there were discrepancies associated with a loss of metal properties by real mercury. The behavior of the models was analyzed in the region of the van der Waals loop. The calculated critical temperature of mercury was found to be significantly overestimated relative to the experiment. Modeling the “meta-mercury” with the EAM potential with excluded embedded potential contribution gave better agreement with the equation of state of mercury at lower densities. The states with Z = 1 can be observed below 1.0 GPa. The calculated temperature of the inversion of the Joule–Thomson coefficient increased monotonically to 5600 K as the pressure increased to 2.5 GPa.     

Molecular dynamics simulation of the homogeneous crystallization of liquid rubidium

The homogeneous crystallization of liquid rubidium models containing 500, 998, and 1968 particles in the basic cube was studied by the molecular dynamics method. The liquid crystallized over the temperature range 70–182.5 K predominantly with the formation of a body centered cubic (BCC) structure. The mechanism of crystallization was different from that accepted in classic nucleation theory. Crystallization developed as an increase in the number of atoms with Voronoi polyhedra of the 0-6-0-8 and 0-4-4-6 types, the formation of bound groups (clusters) from these atoms, and growth of these groups as in the coagulation of an impurity from a supersaturated solution. At the initial stage, bound groups had a very loose structure and included a fairly large number of atoms with polyhedra of other types. The linear dimension of the largest group rapidly approached the basic cube size. The atoms with the 0-6-0-8 and 0-4-4-6 Voronoi polyhedra played a leading role in crystallization and activated the transition of bound group atoms with other coordination types into a BCC coordination. The probability of formation of a bound group of a given size was found to be independent of the volume of the liquid model. Cluster size fluctuations especially strong over the temperature range 180–185 K played an important role in the formation of 0608 clusters of a threshold (“critical”) size.     

The Gibbs energies of monomeric and polymeric fullerene C60 phases at pressures up to 2.0 GPa and temperatures up to 800 K

The Gibbs energies of monomeric and polymeric orthorhombic, tetragonal, and rhombohedral fullerene C₆₀ phases were calculated. The results were used to determine the relative stabilities of the phases and construct the phase diagram of C₆₀ over the pressure and temperature ranges of up to 2.0 GPa and 800 K, respectively. The data obtained are compared with experimental data and calculation results reported by other authors.     

The speed of sound and derived thermodynamic properties of pure water at temperatures between (253 and 473) K and at pressures up to 400 MPa

The speed of sound in high-purity water has been measured in the temperature range (253 to 473) K at pressures up to 400 MPa. The experimental technique used was based on a double-path pulse-echo method with a single 5-MHz ultrasound transducer placed between two unequally spaced reflectors. The cell was calibrated in water at T = 298.15 K and p = 1 MPa against the speed of sound given by the 1995 equation-of-state formulation of the International Association for the Properties of Water and Steam (IAPWS-95) which, for that state point, has an uncertainty of 0.005%. Corrections for the effects of temperature and pressure on the path length difference are considered in detail. The estimated expanded relative uncertainty of the speed of sound determined in this work is shown to be between 0.03% and 0.04% at a confidence level of 95%. The density and isobaric specific heat capacity of water have been obtained in the temperature range (253.15 to 473.15) K at pressure up to 400 MPa by thermodynamic integration of the sound-speed data subject to initial values computed from IAPWS-95 on the isobar at p = 0.1 MPa. The speed of sound, density, and isobaric specific heat capacity were compared with IAPWS-95 with corresponding absolute relative deviations within 0.3%, 0.03%, and 1%, respectively at T ≥ 273.15 K and p ≤ 400 MPa; larger deviations, especially for heat capacity, were found at lower temperatures. The results imply that the uncertainties of properties computed from IAPWS-95 may be significantly reduced over the major part of the region investigated in this work.     

Density measurements of liquid 2-propanol at temperatures between (280 and 393) K and at pressures up to 10 MPa

Liquid densities for 2-propanol have been measured at T = (280, 300, 325, 350, 375, and 393) K from about atmospheric pressure up to 10 MPa using a vibrating tube densimeter. The period of vibration has been converted into density using the Forced Path Mechanical Calibration method. The R134a has been used as reference fluid for T ? 350 K and water for T > 350 K. The uncertainty of the measurements is lower than ±0.05%. The measured liquid densities have been correlated with a Starling BWR equation with an overall AAD of 0.025%. The same BWR equation agrees within an AAD lower than 0.2% with the experimental values available in the literature over the same temperature and pressure range.     

Densities and derived thermodynamic properties of 1-heptanol and 2-heptanol at temperatures from 313 K to 363 K and pressures up to 22 MPa

Experimental densities were determined in liquid phase for 1-heptanol and 2-heptanol at temperatures from 313 K to 363 K and pressures up to 22 MPa using a vibrating tube densimeter. Water and nitrogen were used as reference fluids for the calibration of the vibrating tube densimeter. The uncertainties of the experimental measurements in the whole range of reported data are estimated to be ±0.03 K for temperature, ±0.008 MPa for pressure, and ±0.20 kg · m^?3 for density. The experimental data are correlated using a short empirical equation of six parameters and the 11-parameter Benedict–Webb–Rubin–Starling equation of state (BWRS EoS) using a least square optimization. Statistical values to evaluate the different correlations are reported. Published density data of 1-heptanol are compared with values calculated with the 6-parameter equation using the parameters obtained in this work. The experimental data determined here are also compared with an available correlation for 1-heptanol. Densities of 2-heptanol at high pressure were not found in the literature and the data reported here represent the first set of data reported in the literature. Isothermal compressibilities and isobaric thermal expansivity are calculated using the 6-parameter equation for both alcohols within uncertainties estimated to be ±0.025 Gpa^?1 and ±4 × 10^?7 K^?1, respectively.     

Solubility of methane in octane + ethanol at temperatures from 303.15 K to 333.15 K and pressures up to 12.01 MPa

Solubility of methane in octane + ethanol was measured at temperatures ranging from 303.15 K to 333.15 K and pressures ranging from 2.60 MPa to 12.01 MPa. Experimental data were analyzed using the Soave-Redlich-Kwong equation of state with three types of mixing rules, and the estimated average deviation from the experimental solubility data was less than 3.5 %.     

Compressed liquid densities of 1-butanol and 2-butanol at temperatures from 313 K to 363 K and pressures up to 25 MPa

(p, ρ, T) properties were determined in liquid phase for 1-butanol and 2-butanol at temperatures from 313 K to 363 K and pressures up to 25 MPa using a vibrating tube densimeter. The uncertainty is estimated to be lower than ±0.2 kg · m⁻³ for the experimental densities. Nitrogen and water were used as reference fluids for the calibration of the vibrating tube densimeter. Experimental densities of 1-butanol and 2-butanol were correlated with a short empirical equation and the 11-parameter Benedict–Webb–Rubin–Starling equation of state (BWRS EoS) using a least square optimization. Statistical values to evaluate the different correlations were reported. Published densities of 1-butanol and 2-butanol are compared with values calculated with the BWRS EoS using the parameters obtained in this work. The experimental data determined here are also compared with available correlations for 1-butanol and 2-butanol.     

Two-phase simulation of the crystalline silicon melting line at pressures from -1 to 3 GPa

Results of a numerical investigation of crystalline silicon melting line within the range of pressures from -1 to 3 GPa are presented. A two-phase molecular dynamics method is applied to obtain temperature, pressure, and densities of solid and liquid phases on the melting line. Using a special procedure we ensure the strict control of the two-phase equilibrium in the simulation cell. To describe the interaction between the atoms four classic potentials have been chosen: the Stillinger-Weber one and three modified variants of the Tersoff potential. For the Stillinger-Weber and Tersoff potentials in the modification by Kumagai-Izumi-Hara-Sakai a good coincidence with experimental data on crystalline Si melting temperature is obtained within the range of pressure from 0 to 3 GPa. Calculations of the solid and liquid phase densities on the silicon melting line for the Stillinger-Weber potential are also in close agreement with experiments.     

Investigating the thermodynamic properties of lactones at various temperatures

The 11-undecanolide enthalpy of vaporization is obtained from the temperature dependence of the vapor pressure measured by transpiration. A conformational analysis of 8-octanolide (I), 11-undecanolide (II), and 13-tridecanolide (III) is performed. Quantum-chemical calculations are performed to study the moments of inertia, vibrational spectra, and most favorable conformations. The ideal gas thermodynamic properties of these compounds in the temperature range of 0-1500 K are determined. A procedure for the additive determination of polynomial coefficients for temperature-dependent properties is developed to assess the thermochemical data for lactones, lactides, and carboxylic hydroxyacids at 298.15–1000 K.     

Molecular level structure of the liquid/liquid interface. Molecular dynamics simulation and ITIM analysis of the water-CCl4 system

The molecular level properties of the liquid/liquid interface between water and CCl(4) are analysed in detail on the basis of molecular dynamics computer simulation. This analysis requires a full list of the molecules that are right at the interface in both phases. Such a list can be provided by the novel method for identifying truly interfacial molecules (ITIM). The full list of the truly interfacial molecules various properties (e.g., width, molecular level roughness) of the interface can be meaningfully analysed. The residence time of the molecules at the interface, the percolation of the water molecules at the interfacial layer as well as in the second layer beneath the surface, the preferred orientations of the interfacial water molecules and the dependence of these orientational preferences on the local curvature of the interface are also analysed and discussed in detail.     

Compressed liquid densities of ethane-propane mixtures between 10 and 49°C at pressures up to 9.6 MPa

Densities of compressed liquid ethane-propane mixtures are reported at five temperatures between 10.00 ad 48.90°C. For each isotherm the data cover eight compositions ranging between 30 and 95 mol% ethane at pressures up to 9.65 MPa. The excess volumes derived from the data are negative, extremely dependent on temperature and pressure, and can represent up to a 50% correction to the ideal mixture volume.     

Thermodynamic properties of mixtures of N-methyl-2-pyrrolidinone and methanol at temperatures between 298.15 K and 343.15 K and pressures up to 60 MPa

We report measurements of the speed of sound in mixtures of N-methyl-2-pyrrolidinone and methanol at temperatures between 298.15 K and 343.15 K and at pressures up to 60 MPa. The measurements were made using a dual path pulse-echo apparatus operating at a frequency of 5 MHz. We have also measured the isobaric specific heat capacity of each mixture as a function of temperature at ambient pressure, by means of a Setaram DSC III microcalorimeter. The experimental results have been combined with literature data for the density of the same mixtures as a functions of temperature at ambient pressure to obtain the density, isobaric specific heat capacity, and other thermodynamic properties at temperatures between 298.15 K and 343.15 K and at pressures up to 60 MPa. Detailed comparisons with the literature data are presented.     

Hydrogen adsorption on microporous materials at ambient temperatures and pressures up to 50 MPa

High surface area microporous adsorbents are often proposed as potential hydrogen storage materials, although typically at 77?K and less than 5?MPa. In this study, we focus on conditions more suitable for automotive applications by investigating the storage capacities of microporous materials at 298?K and at pressures up to 50?MPa. In an effort to derive trends within and across material classes, we examined a wide range of materials with varying microstructures including the activated carbons AX-21, KUA-5, and MSC-30; a zeolite templated carbon; a hypercrosslinked polymer; and the Metal Organic Frameworks MOF-177, IRMOF-20, MIL-53, ZIF-8, and Cu₃(btc)₂. The peak excess adsorption of these materials ranged from 0.8–1.8?wt.%, although many did not reach their maximum capacity even at high pressures. However, the total volumetric storage gains over compressed hydrogen gas were quite low and, in many cases, negative. In addressing ambient temperature adsorption at significantly higher pressures than previously reported, our data confirms and extends the range of validity of several existing DFT calculations. Furthermore, our data suggest that, for both activated carbons and MOFs, factors other than specific surface area govern ambient temperature adsorption capacity. Contrary to some reports, the high fractions of sub-nanometer pores in some of the investigated MOFs did not appear to enhance the excess adsorption even at high pressures. For on-board applications with ambient temperature storage, significant enhancements to the attractive force at the materials’ surface are required, beyond merely increasing specific surface area, or for MOFs, tuning of pore sizes.     

Speed of sound in liquid cyclic alkanes at temperatures between (283 and 343) K and pressures up to 20 MPa

The speed of sound in liquid cyclopentane, cyclohexane, methylcyclopentane and methylcyclohexane were measured, with a sing-around technique operated at a frequency of 2 MHz, at temperatures from (283 to 343) K and pressures up to 20 MPa with an estimated error of less than ± 0.2 per cent in the high-density region. From these measurements the densities and isobaric and isentropic compressibilities of each compound were estimated. The behaviour on the temperature and pressure in these quantities is discussed based on the difference in molecular shape between cyclic and normal alkanes with those for n-alkanes.     

Structural and optical properties of liquid CO2 for pressures up to 1 TPa

We report on the use of first-principles molecular dynamics calculations to examine properties of liquid carbon dioxide in the pressure-temperature range of 0-1 TPa and 200-100 000 K. The computed equations of state points are used to predict a series of shock Hugoniots with initial starting conditions that are relevant to existing and ongoing shock-wave experiments. A comparison with published measurements up to 70 GPa shows excellent agreement. We find that the liquid undergoes a gradual phase transition along the Hugoniot and have characterized this transition based on changes in bonding and structural properties as well as the conductivity and reflectivity of the fluid.