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Molecular dynamics simulations are reported for the evaporation of a liquid into vacuum, where a Lennard-Jones type fluid with truncated and shifted potential at 2.5σ is considered. Vacuum is enforced locally by particle deletion and the liquid is thermostated in its bulk so that heat flows to the planar interface driving stationary evaporation. The length of the non-thermostated transition region between the bulk liquid and the interface Ln is under study. First, it is found for the reduced bulk liquid temperature Tl/Tc = 0.74 (Tc is the critical temperature) that by increasing Ln from 5.2σ to 208σ the interface temperature Ti drops by 17% and the evaporation flux decreases by a factor of 4.4. From a series of simulations for increasing values of Ln, an asymptotic value Ti∞ of the interface temperature for Ln → ∞ can be estimated which is 21% lower than the bulk liquid temperature Tl. Second, it is found that the evaporation flux is solely determined by the interface temperature Ti, independent on Tl or Ln. Combining these two findings, the evaporation coefficient α of a liquid thermostated on a macroscopic scale is estimated to be α ≈ 0.14 for Tl/Tc = 0.74. [ABSTRACT FROM AUTHOR]

The Grüneisen parameter γG is widely used for studying thermal properties of solids at high pressure and also has received increasing interest in different applications of non-ideal fluid dynamics. Because there is a lack of systematic studies of the Grüneisen parameter in the entire fluid region, this study aims to fill this gap. Grüneisen parameter data from molecular modelling and simulation are reported for 28 pure fluids and are compared with results calculated from fundamental equations of state that are based on extensive experimental data sets. We show that the Grüneisen parameter follows a general density-temperature trend and characterize the fluid systems by specifying a span of minimum and maximum values of γG. Exceptions to this trend can be found for water. [ABSTRACT FROM AUTHOR]

Long range corrections for molecular simulations of inhomogeneous fluids with a spherical interface are presented. Correction terms for potential energy, force and virial are derived for the monatomic Lennard–Jones fluid. The method is generalised to the Mie potential and arbitrary molecular structures, employing a numerically efficient centre of mass cut-off scheme. The results are validated by a series of droplet simulations for one-centre and two-centre Lennard–Jones fluids with different cut-off radii rc. Systems with r c = 8 σ provide a check of self-consistence. Further, a system containing a bubble is investigated for the one-centre Lennard–Jones fluid. The equilibrium properties are almost completely independent on the cut-off radius. In comparison with vapour–liquid equilibrium data for systems without a curved interface, all properties show the expected behaviour. Simulation data are used to approximate the surface tension, which is in good agreement with the findings for planar interfaces, thus verifying the present corrections. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are conducted to investigate the evaporation of the truncated ( 2.5 σ) and shifted Lennard–Jones fluid into vacuum. Evaporation is maintained under stationary conditions, while the bulk liquid temperature and the thermal driving force gradient are varied over wide ranges. It is found that the particle flux and the energy flux solely depend on the interface temperature. Both of these quantities are correlated to estimate their values for macroscopically large systems. The latter is analysed by a hydrodynamic energy balance, considering conductive heat transport by Fourier's law. Following the Hertz–Knudsen approach, the evaporation coefficient is determined and found to be in good agreement with literature data based on the kinetic equation for fluids and molecular dynamics. [ABSTRACT FROM AUTHOR]

Intercalation phenomena of kaolinite in aqueous potassium acetate and in hexyl-amine solutions are studied by large scale molecular dynamics simulations. The simulated kaolinite particle is constructed from ~6.5 x 106 atoms, producing a particle size of ~100 nm x 100 nm x 10 nm. The simulation with potassium acetate results in a stable kaolinite-potassium acetate complex, with a basal spacing that is in close agreement with experimental data. The simulation with hexyl-amine shows signs of the experimentally observed delamination of kaolinite (the initial phase of the formation of nanoscrolls from the external layers). [ABSTRACT FROM AUTHOR]

Expressions for the thermodynamic factor matrix Γ of quaternary mixtures are derived in terms of Kirkwood–Buff integrals and implemented into the massively parallel simulation tool ms2. To assess these expressions, a liquid-like supercritical quaternary Lennard–Jones mixture is sampled throughout its entire composition range, employing molecular dynamics in the canonical ensemble. Good agreement is found between numerical chemical potential derivatives and the results from the present Kirkwood–Buff integral expressions. Moreover, the limits of the thermodynamic factor matrix for pure, binary and ternary subsystems are discussed. [ABSTRACT FROM AUTHOR]

This work presents new molecular models for alkali and halide ions in aqueous solution. The force fields were parameterized with respect to the reduced liquid solution density at 293.15 K and 1 bar, considering all possible ion combinations simultaneously. The experimental target data are reproduced with a high accuracy over a wide range of salinity. The ion models predict structural properties of electrolyte solutions well, such as pair correlation functions and hydration numbers. The force fields provide good predictions of the properties studied here in combination with different models for water. [ABSTRACT FROM AUTHOR]

Density, self-diffusion coefficient, and shear viscosity of pure liquid water are predicted for temperatures between 280 and 373 K by molecular dynamics simulation and the Green-Kubo method. Four different rigid nonpolarizable water models are assessed: SPC, SPC/E, TIP4P, and TIP4P/2005. The pressure dependence of the self-diffusion coefficient and the shear viscosity for pure liquid water is also calculated and the anomalous behavior of these properties is qualitatively well predicted. Furthermore, transport properties as well as excess volume and excess enthalpy of aqueous binary mixtures containing methanol or ethanol, based on the SPC/E and TIP4P/2005 water models, are calculated. Under the tested conditions, the TIP4P/2005 model gives the best quantitative and qualitative agreement with experiments for the regarded transport properties. The deviations from experimental data are of 5% to 15% for pure liquid water and 5% to 20% for the water + alcohol mixtures. Moreover, the center of mass power spectrum of water as well as the investigated mixtures are analyzed and the hydrogen-bonding structure is discussed for different states. [ABSTRACT FROM AUTHOR]

A molecular model for carbon dioxide is presented, and the parameters of the Lennard-Jones sites, the bond length, and the quadrupole moment are optimized to experimental vapor-liquid equilibrium data. The resulting molecular model shows mean unsigned deviations to the experiment over the whole temperature range from triple point to critical point of 0.4% in saturated liquid density, 1.8% in vapor pressure, and 8.1% in enthalpy of vaporization. The molecular model is assessed by comparing predicted thermophysical properties with experimental data and a reference equation of state for a large part of the fluid region. The average deviations for density and residual enthalpy are 4.5% and 1.7%, respectively. The model is also capable to predict the radial distribution function, the second virial coefficient, and transport properties, the average deviations of the latter are 12%. [ABSTRACT FROM AUTHOR]

Grand canonical molecular dynamics (GCMD) is applied to the nucleation process in a metastable phase near the spinodal, where nucleation occurs almost instantaneously and is limited to a very short time interval. With a variant of Maxwell’s demon, proposed by McDonald [Am. J. Phys. 31, 31 (1963)], all nuclei exceeding a specified size are removed. In such a steady-state simulation, the nucleation process is sampled over an arbitrary time span and all properties of the metastable state, including the nucleation rate, can be obtained with an increased precision. As an example, a series of GCMD simulations with McDonald’s demon is carried out for homogeneous vapor to liquid nucleation of the truncated-shifted Lennard-Jones (tsLJ) fluid, covering the entire relevant temperature range. The results are in agreement with direct nonequilibrium MD simulation in the canonical ensemble. It is confirmed for supersaturated vapors of the tsLJ fluid that the classical nucleation theory underpredicts the nucleation rate by two orders of magnitude. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulation is applied to the condensation process of supersaturated vapors of methane, ethane, and carbon dioxide. Simulations of systems with up to a 106 particles were conducted with a massively parallel MD program. This leads to reliable statistics and makes nucleation rates down to the order of 1030 m-3 s-1 accessible to the direct simulation approach. Simulation results are compared to the classical nucleation theory (CNT) as well as the modification of Laaksonen, Ford, and Kulmala (LFK) which introduces a size dependence of the specific surface energy. CNT describes the nucleation of ethane and carbon dioxide excellently over the entire studied temperature range, whereas LFK provides a better approach to methane at low temperatures. [ABSTRACT FROM AUTHOR]

The speed of sound of liquid squalane is measured with the double-path pulse-echo technique, utilizing a piezoelectric quartz (8 MHz) positioned between two reflectors with distinct path lengths. Calibration of the apparatus is carried out with water, for which highly accurate reference data exist. The present experiments with squalane cover the temperature range from 298.15 K to 493.15 K and a pressure from 0.1 MPa to 20 MPa. The total relative expanded ( k = 2 ) uncertainty U r (w) for the speed of sound is estimated to be ±0.1 %. The present data are consistent with literature values, which are, however, only available at ambient pressure. To validate the present speed of sound measurements, the density and isobaric heat capacity are integrated numerically from the sampled data over most of the measurement range, employing rigorous thermodynamic identities. With deviations of about 0.1 %, the resulting density data are favorably compared with literature values. The resulting isobaric heat capacity, for which just a single reference with data at pressures above the ambient exists, deviates by up to 6 %. [ABSTRACT FROM AUTHOR]

p, ρ, T data of the supersaturated vapor of the Lennard-Jones fluid are obtained by molecular dynamics simulations. The metastable state points are identified before a phase separation takes place. An estimation of the location of the spinodal is given. The results are compared to two theoretically based equations of state and one empirical equation of state which was parametrized also taking into account metastable state points. The pressure obtained by simulation is found to be lower than that from both theoretically based equations of state, which do not account for the inhomogeneous density distribution of the supersaturated vapor. [ABSTRACT FROM AUTHOR]