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. Inclassical theory, a vapour-liquid interface is a two-dimensional object. In reality it is a region in whichproperties change over a few nanometres and the density changes continuously from its liquid bulk to its gasbulk value. Many mixtures show unexpected effects in that transition region. While the total density changesmonotonously 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 maximumwould be an insurmountable obstacle to mass transfer according to Fickian theory. Even if that argument isnot 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 noexperimental methods that can be used for direct studies of density profiles in such interfacial regions. Butsuch data can be obtained with theoretical methods, namely with molecular dynamics simulations (MD) aswell as with density gradient theory (DGT) or with density functional theory (DFT) combined with an equationof state (EOS).Studies from our group on the vapour-liquid interface of several real mixtures and a model fluid using thesemethods yield consistent results and reveal an important enrichment in some cases. Strong enrichment isfound at vapour-liquid interfaces in the systems in which one of the components is supercritical. These resultsindicate 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 ofthis finding for the modelling of these separation processes are discussed.