Enrichment of Components at Vapour - Liquid Interfaces: A Study by Molecular Simulation and Density Gradient Theory
- 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.