Fluid phase interface properties of acetone, oxygen, nitrogen and their binary mixtures by molecular simulation.

Vapor-liquid equilibria (VLE) of the pure substances acetone, oxygen and nitrogen as well as their binary mixtures are studied by molecular dynamics (MD) simulation with a direct approach. Thereby, particular attention is paid to the interface behavior on the molecular level, yielding total and partial density profiles as well as surface tension data. The classical approach by van der Waals is used to analyze the total density profiles. It is found that an extended function is needed to describe those profiles for the mixtures containing acetone, due to the strong adsorption of the volatile component at the vapor side of the interface. Based on these representations the interface thickness is studied. The surface tension results are compared to experimental data, correlations thereof and results from other molecular approaches. Due to the scarcity of experiments, the parachor method is employed to obtain predictive surface tension data for the mixtures. Following the same approach, the present surface tension results are correlated for the mixtures containing acetone.

Excess equimolar radius of liquid drops.

The curvature dependence of the surface tension is related to the excess equimolar radius of liquid drops, i.e., the deviation of the equimolar radius from the radius defined by the macroscopic capillarity approximation. Based on the Tolman [J. Chem. Phys. 17, 333 (1949)] approach and its interpretation by Nijmeijer et al. [J. Chem. Phys. 96, 565 (1991)], the surface tension of spherical interfaces is analyzed in terms of the pressure difference due to curvature. In the present study, the excess equimolar radius, which can be obtained directly from the density profile, is used instead of the Tolman length. Liquid drops of the truncated and shifted Lennard-Jones fluid are investigated by molecular dynamics simulation in the canonical ensemble, with equimolar radii ranging from 4 to 33 times the Lennard-Jones size parameter σ. In these simulations, the magnitude of the excess equimolar radius is shown to be smaller than σ/2. This suggests that the surface tension of liquid drops at the nanometer length scale is much closer to that of the planar vapor-liquid interface than reported in studies based on the mechanical route.