In separation processes not only thermodynamic bulk but also interfacial properties play a crucial role. In classical theory, a vapour-liquid interface is a two-dimensional object. In reality it is a region in which properties change over a few nanometres and the density changes continuously from its liquid bulk to its gas bulk value. Many mixtures show unexpected effects in that transition region. While the total density changes monotonously from the bulk vapour to the bulk liquid, this does not hold for the molarities of the components. The molarities of the light boiling component can have a distinct maximum at the interface. That maximum would be an insurmountable obstacle to mass transfer according to Fickian theory. Even if that argument is not adopted, it shows that there is good reason to believe that the maximum may affect mass transfer and, hence, fluid separation processes like absorption or distillation. Unfortunately, there are currently no experimental methods that can be used for direct studies of density profiles in such interfacial regions. But such data can be obtained with theoretical methods, namely with molecular dynamics simulations (MD) as well as with density gradient theory (DGT) or with density functional theory (DFT) combined with an equation of state (EOS).
Studies from our group on the vapour-liquid interface of several real mixtures and a model fluid using these methods yield consistent results and reveal an important enrichment in some cases. Strong enrichment is found at vapour-liquid interfaces in the systems in which one of the components is supercritical. These results indicate that mixtures, which are typical for absorption processes usually show an important enrichment, whereas this is not the case for mixtures that are typically separated by distillation. Possible consequences of this finding for the modelling of these separation processes are discussed.