Properties of fluid interfaces are interesting for many applications. In the classical theory, the interface is a two-dimensional object, in reality it is a region in which properties change over a few nanometres. There are... [ view full abstract ]

Properties of fluid interfaces are interesting for many applications. In the classical theory, the interface is a two-dimensional object, in reality it is a region in which properties change over a few nanometres. There are presently no experimental methods which would yield information like density profiles in that interfacial region. The enrichment of components at the interface of mixtures is of particular interest as it might affect mass transfer. Such data can, however, be obtained with theoretical methods, namely with molecular simulations on the one side and equations of state (EOS) combined with Density Gradient Theory (DGT) or Density Functional Theory (DFT) on the other.

In the present work, we carry out a systematic study of interfacial properties of fluid mixtures with a focus on the enrichment of components both with molecular dynamics simulations and with an EOS+DGT. The studied fluid is described by the Lennard-Jones truncated and shifted (LJTS) potential. For that potential, based on available and new molecular simulation data, a new EOS is developed using perturbation theory: the PeTS-EOS (Perturbed Lennard-Jones truncated and shifted).

The development of the PETS-EOS for pure components is briefly described and it is shown that the new equation fits the simulation data well. It is then shown that the PeTS EOS can be extended to mixtures of LJTS fluids on the basis of van der Waals one fluid theory using the same mixing rules as in the molecular simulation. A modified Lorentz- Berthelot mixing rule is used, in which the mixed attractive interaction can be modified by a state independent parameter.

This is the basis for the comparative study of interfacial properties of mixtures, in which six binary mixtures of LJTS fluids were investigated. Keeping the size parameter of both fluids constant, the ratio of the pure fluid energy interaction parameter was varied as well as the mixture cross-interaction parameter. Based on corresponding states principles the study is designed so that a variety of technically important types of phase behaviour is covered (narrow/wide boiling, ideal/non-ideal, subcritical/supercritical second component). The agreement between the results from the molecular simulations and from EOS+DGT is found to be good for all six investigated mixtures not only regarding the phase diagrams but also regarding the studied interfacial properties, which include density profiles, enrichment of components, surface excess, and surface tension. Some systematic trends which are observed in comparing the results from both methods are discussed on the basis of the extensive new data. They are in line with recent findings form studies using models of real fluids [1, 2]. A special focus of the presentation is on the enrichment, which is defined here as the ratio of a component’s density maximum in the interfacial region to the larger value of the density of that component in either of the bulk phases. Conditions which favour the enrichment are identified.

[1] S. Becker et al., Fluid Phase Equilibr., 427 (2016)

[2] S. Werth, M. et al., Fluid Phase Equilibr. 427 (2016)