The conditions for which the interfacial enrichment at vapor-liquid interfaces occurs were elucidated. Methods for predicting the enrichment, which cannot be measured directly, were developed and successfully tested. The predictions from molecular simulations and density gradient theory agree very well. This holds both for model systems, which were studied systematically, and real systems. Experimental studies of the surface tension and the relative adsorption were carried out to validate the models. A comprehensive data base was established that contains all available data on the enrichment. Based on the results from the present work, the enrichment can now be estimated easily for basically any real system. Overall, the work in ENRICO has led to a significantly improved understanding of the nanoscopic equilibrium properties of vapor-liquid interfaces.
Furthermore, the nanoscopic mass transfer through vapor-liquid interfaces was studied using molecular simulations. Two new non-equilibrium molecular dynamics (NEMD) simulation methods (steady state and instationary) were developed for this purpose. Using these methods, the influence of the enrichment on the mass transfer on the nanoscopic level was confirmed. The molecular simulations yielded, furthermore, a wealth of insights into nanoscopic non-equilibrium processes during mass transfer across vapor-liquid interfaces, such as rebound of particles from the interface. The nanoscopic mass transfer through interfaces was also investigated using a continuum model, which was based on a new formulation of the Cahn-Hilliard equations.
Different experiments were carried out to study whether these nanoscopic findings translate into a significant mass transfer resistance on the macroscale. A novel type of laminar jet apparatus, which can be operated at pressures up to 15 bar, as well as a new method to study gas-liquid mass transfer based on magnetic resonance imaging (MRI) were developed and applied for the investigations. For comparison, diffusion in bulk liquid phases was studied by pulse-field gradient nuclear magnetic resonance spectroscopy (PFG-NMR), accompanied by molecular simulation studies. While these experimental studies have yielded an important amount of useful information on diffusion in liquid mixtures, we were not able to give a proof of the influence of the enrichment of components at the vapor-liquid interface on the macroscopic mass transfer. This is no contradiction to the findings on the nanoscale, it basically indicates that the nanoscopic resistance is small compared to macroscopic effects. While this holds for cases with simple fluid dynamics (laminar or stagnant), we cannot exclude that the enrichment influences the mass transport in turbulent situations in which the enrichment may influence the surface renewal.
The scientific results from ENRICO were disseminated in a large number of presentations at scientific conferences and papers, as well as in workshops. Awareness has been created in the chemical industry for the enrichment at vapor-liquid interfaces as well as for the insights in separation processes that can be obtained with molecular simulations.
