In this work we present new molecular dynamics simulation results for the liquid-vapor interface of the pure Lennard-Jones fluid. Our aims were further investigations on the simulation setup and the simulation parameters to obtain reliable data for the coexisting densities as well as for the surface tension. The influence of the cutoff distance to the interfacial properties is investigated and long-range corrections to both the dynamics and the surface tension are applied. It is found that the saturated liquid densities from the surface simulations agree with those from the NpTϩtest particle method within 1% for sufficiently large simulation boxes; the saturated vapor densities agree within 4%. In order to obtain reliable values for the surface tension, cutoff radii of at least 5 molecular diameters supplemented by a tail correction are required.

A new equation of state (EOS) is proposed for the Helmholtz energy F 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 = FH + F^, where FH is a hard body contribution and F^ an attractive dispersion force contribution. The expression for F H is closely related to the hybrid Barker-Henderson pertubation theory. The construction of F^ 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, the pvT data, and in peculiar, of the caloric properties. A comparison with the EOS of Kolafa and Nezbeda which appeared after the bulk of this work was finished shows still an improvement in the standard deviations of the pressure and internal energy by about 30%.

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In this work we present and discuss new molecular dynamics simulation procedures and the application of density functional theory to the liquid-vapour interface of pure fluids and their liquid mixtures. Our aim was to further investigate the simulation set-up and parameters to obtain reliable simulation data for the phase behaviour, interfacial structure and surface tension. The influence of box geometries and summation techniques is discussed. In the application of the density functional theory we analysed the influence of different approximations within the theory on the calculation of interfacial behaviour and optical properties. In particular, the attractive free-energy term of a local density functional approach is modified by introducing an analytical representation of the radial distribution function of the uniform reference fluid. The calculated liquid and gas densities and surface tensions are in good agreement with recent molecular dynamics simulations. But the results clearly show that capillary-wave contributions, acting on different scales of length and time, have to be taken into consideration in predicting both surface tensions and optical properties such as ellipticity and specular reflectivity.

The pendant drop method, combined with efficient temperature control of the measuring cell, allows high precision in surface tension measurements. The surface tensions of heptane, toluene, N,N-dimethylformamide, cyclohexane, N-methyl-2-pyrrolidone, and propanone were measured as a function of temperature using the pendant drop method. The results were compared with literature data. The surface tension and density of toluene + heptane and N,N-dimethylformamide + toluene at atmospheric pressure were measured over a temperature range. Gibbs excess surface concentrations are derived from the experimental surface tensions, and the influence of activity coefficients is discussed.