The liquid-vapour interface of pure fluids and mixtures: application of computer simulation and density functional theory

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.