Jadran Vrabec

Vapour liquid equilibria of the Lennard-Jones fluid from the NpT plus test particle method

Using an improved version of the recently suggested NpT + test particle method, phase equilibria are determined for the Lennard-Jones fluid in the temperature range T* = 0·70 to T* = 1·30 in steps of ΔT* = 0·05. For the final results, gas simulations are necessary only at the two highest temperatures; for T* = 1·20 and downwards the gas phase can be described with sufficient accuracy by the Haar-Shenker-Kohler equation. The resulting vapour pressures, bubble and dew densities can be correlated by the equations , , , with the critical temperature and density being T*c = 1·310 and ρ*c = 0·314, respectively. Then a detailed comparison with other simulation results and perturbation theory results is given. Outstanding findings are that the present data are rather smooth, that they match quite well the early data of Hansen and Verlet (1969, Phys. Rev., 184, 151) and are much closer to perturbation theory than most previous results. Finally, using the vapour pressure equation and directly calculated volume chan...

Comprehensive study of the vapour–liquid coexistence of the truncated and shifted Lennard–Jones fluid including planar and spherical interface properties

Vapour–liquid equilibria of the Lennard–Jones potential, truncated and shifted at 2.5σ, are studied using molecular dynamics simulations, an attractive option for studying inhomogeneous systems. Comprehensive simulation data are reported for three cases: no interface, a planar interface, and a spherical interface between the coexisting phases, covering a wide range of temperatures. Spherical droplets are also studied for a range of radii between 5 and 16σ. The size dependence of the surface tension, based on the Irving–Kirkwood pressure tensor, and other properties is quantified for spherical interfaces. All simulation results are correlated with a consistent set of empirical equations. A comparison with the results of other authors as well as with experimental data for noble gases and methane is also presented.

Grand Equilibrium: vapour-liquid equilibria by a new molecular simulation method

A new molecular simulation method for the calculation of vapour-liquid equilibria of mixtures is presented. In this method, the independent thermodynamic variables are temperature and liquid composition. In the first step, one isobaric isothermal simulation for the liquid phase is performed, in which the chemical potentials of all components and their derivatives with respect to the pressure, i.e. the partial molar volumes, are calculated. From these results, first-order Taylor series expansions for the chemical potentials as functions of the pressure μ i (p) at constant liquid composition are determined. This information is needed, as the specified pressure in the liquid will generally not be equal to the equilibrium pressure, which has to be found in the course of a vapour simulation. In the second step, one pseudo grand canonical simulation for the vapour phase is performed, where the chemical potentials are set according to the instantaneous pressure p v using the previously determined function μ i (p v). In this way, results for the vapour pressure and vapour composition are achieved which are consistent for the given temperature and liquid composition. The new method is applied to the pure Lennard-Jones fluid, a binary and a ternary mixture of Lennard-Jones spheres, and shows very good agreement with corresponding data from the literature.

Prediction of self-diffusion coefficient and shear viscosity of water and its binary mixtures with methanol and ethanol by molecular simulation

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.

Molecular dynamics and experimental study of conformation change of poly(N-isopropylacrylamide) hydrogels in mixtures of water and methanol.

The conformation transition of poly(N-isopropylacrylamide) hydrogel as a function of the methanol mole fraction in water/methanol mixtures is studied both experimentally and by atomistic molecular dynamics simulation with explicit solvents. The composition range in which the conformation transition of the hydrogel occurs is determined experimentally at 268.15, 298.15, and 313.15 K. In these experiments, cononsolvency, i.e., collapse at intermediate methanol concentrations while the hydrogel is swollen in both pure solvents, is observed at 268.15 and 298.15 K. The composition range in which cononsolvency is present does not significantly depend on the amount of cross-linker. The conformation transition of the hydrogel is caused by the conformation transition of the polymer chains of its backbone. Therefore, conformation changes of single backbone polymer chains are studied by massively parallel molecular dynamics simulations. The hydrogel backbone polymer is described with the force field OPLS-AA, water with the SPC/E model, and methanol with the model of the GROMOS-96 force field. During simulation, the mean radius of gyration of the polymer chains is monitored. The conformation of the polymer chains is studied at 268, 298, and 330 K as a function of the methanol mole fraction. Cononsolvency is observed at 268 and 298 K, which is in agreement with the present experiments. The structure of the solvent around the hydrogel backbone polymer is analyzed using H-bond statistics and visualization. It is found that cononsolvency is caused by the fact that the methanol molecules strongly attach to the hydrogels backbone polymer, mainly with their hydroxyl group. This leads to the effect that the hydrophobic methyl groups of methanol are oriented toward the bulk solvent. The hydrogel+solvent shell hence appears hydrophobic and collapses in water-rich solvents. As more methanol is present in the solvent, the effect disappears again.

An accurate Van der Waals-type equation of state for the Lennard-Jones fluid

A new equation of state (EOS) is proposed for the Helmholtz energyF of the Lennard Jones fluid which represents the thermodynamic properties over a wide range of temperatures and densities. The EOS is written in the form of a generalized van der Waals equation.F =Fu +Fv. WhereFu is a hard body contribution andFA an anttractive dispersion force contribution. The expression forFH is closely related to the hybrid Barker Henderson pertubation theory. The construction ofFA is accomplished with the Setzmann Wagner optimization procedure on the basis of virial coefficients and critically assessed computer simulation data. A comparison with the EOS of Johnson et al. shows improvement in the description of the vapor liquid coexistence properties, thepvT data. and in peculiar, of the calorie properties. A comparison with the EOS of Kolafa and Nezbeda which appeared after the bulk of this work was finished shows still by about 30%.

A set of molecular models for carbon monoxide and halogenated hydrocarbons

Molecular models are presented for carbon monoxide and 53 halogenated methane, ethane, and ethene derivatives, among which are important alternative refrigerants. The models are based on the two-center Lennard-Jones plus point dipole or plus point quadrupole pair potentials. The model parameters were adjusted to experimental vapor–liquid equilibria of the pure fluids using a highly efficient procedure. The application of these models to the calculation of vapor–liquid equilibria and homogeneous fluid state points by molecular simulation shows good to excellent agreement with experimental results. The present molecular models describe the vapor pressures in most cases significantly better than models available in the literature. Typical mean relative deviations between simulation results and experiments are 0.5% for the saturated liquid density, 4% for the vapor pressure, and 3% for the enthalpy of vaporization. Due to the compatibility of the presented models, the prediction of vapor–liquid equilibria of ...

Vapour liquid equilibria of mixtures from the NpT plus test particle method

The NpT + test particle method for the calculation of vapour-liquid phase equilibria by molecular simulation is extended to binary mixtures. The independent thermodynamic variables are the temperature T and the liquid concentration x. On the liquid side, one NpT simulation is performed at a prescribed pressure which determines the chemical potentials and their derivatives with respect to the pressure, i.e., the partial molar volumes. On the vapour side, two NpT simulations are performed at two different concentrations and at a prescribed pressure yielding again the chemical potentials and their derivatives with respect to the pressure. Using first-order Taylor expansions for the chemical potentials, the vapour-liquid phase equilibria are obtained. As an example, the vapour-liquid phase equilibria for the mixture argon + methane are predicted at four temperatures and compared with experimental results. For that purpose, an optimized redetermination of the molecular parameters was made using only excess vol...

Comprehensive study of the vapour-liquid equilibria of the pure two-centre Lennard-Jones plus pointquadrupole fluid

Abstract Results of a systematic investigation of the vapour–liquid equilibria of 38 individual two-centre Lennard–Jones plus axial pointdipole model fluids (2CLJD) are reported over a range of reduced dipolar momentum 0≤μ ∗2 ≤20 and of reduced elongation 0≤L ∗ ≤1.0 . Temperatures investigated are from about 55 to 95% of the critical temperature of each fluid. The NpT + test particle method is used for the generation of vapour pressures, saturated densities, and saturated enthalpies. For the lowest temperatures, these data are calculated with highly accurate chemical potentials obtained from the gradual insertion method. Critical temperatures T ∗ c and densities ρ ∗ c are obtained from Guggenheim’s equations. Empirical correlations for critical data T ∗ c and ρ ∗ c as well as for saturated densities ρ′ ∗ and ρ″ ∗ , and vapour pressures p ∗ σ are developed as global functions of the model parameters. They describe the simulation data generally within their statistical uncertainties. Critical pressures and acentric factors of the 2CLJD fluid can be calculated from these correlations. The present results are a sound basis for adjustments of the model parameters μ ∗2 , L ∗ , σ and ϵ to experimental VLE data of real fluids.

Molecular model for carbon dioxide optimized to vapor-liquid equilibria

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%.