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.

Vapor-liquid interfacial properties of the system cyclohexane + CO2: Experiments, molecular simulation and density gradient theory

Properties of the vapor-liquid interface of the binary mixture cyclohexane + CO2 as well as for the two pure substances are reported. The data were obtained from pendant drop experiments (Exp), molecular dynamics (MD) simulation, and density gradient theory (DGT) in combination with the PCP-SAFT equation of state. The following interfacial properties were studied: surface tension (Exp, MD, DGT), relative adsorption (Exp, MD, DGT), enrichment (MD, DGT), and interfacial thickness (MD, DGT). The measurements were carried out at temperatures between 303.15 K and 373.15 K and pressures up to 6 MPa. Furthermore, bulk VLE properties were computed by MD and PCP-SAFT and compared to experimental data from the literature. Data from experiment, MD, and DGT were found to be in good agreement throughout.

Vapor−Liquid Interface of the Lennard-Jones Truncated and Shifted Fluid: Comparison of Molecular Simulation, Density Gradient Theory, and Density Functional Theory

The vapor-liquid interface of the Lennard-Jones truncated and shifted (LTJS) fluid with a cut-off radius of 2.5 σ is investigated for temperatures covering the range between the triple point and the critical point. Three different approaches to model the vapor-liquid interface are used: molecular dynamics (MD) simulations, density gradient theory (DGT) and density functional theory (DFT). The surface tension, pressure and density profiles, including the oscillatory layering structure of the fluid at the interface, are investigated. The PeTS (Perturbed truncated and shifted) equation of state and PeTS-i functional, based on perturbation theory, are used to calculate the Helmholtz free energy in the DGT and DFT approach. They are consistent with the LJTS force field model. Overall, both DGT and DFT describe the results from computer experiments well. An oscillatory layering structure is found in MD and DFT.

Interfacial Properties of Binary Lennard-Jones Mixtures by Molecular Simulation and Density Gradient Theory

A systematic study of interfacial properties of binary mixtures of simpleuids wascarried out by molecular dynamics (MD) simulation and density gradient theory(DGT). Theuids are described by the Lennard-Jones truncated and shifted potentialwith truncation radius of 2.5 diameters (LJTSuid). The following interfacialproperties were studied: surface tension, relative adsorption, enrichment, and interfacialthickness. A recently developed equation of state for the LJTSuid (PeTS EOS)was used as basis for the DGT. Six binary mixtures (components 1 + 2) were studiedat a constant temperature, which was chosen such that the high-boiling component1 is subcritical while the low-boiling component 2 is either subcritical or supercritical.Furthermore, a parameter ? in the combination rule for the unlike dispersiveinteraction was varied such that the resulting mixtures showed three types of behavior:high-boiling azeotrope, ideal, and low-boiling azeotrope. The parametersof the LJTS potential, including ?, were also used in the PeTS EOS without anyadjustment. Despite this simple approach, excellent agreement between the resultsof the PeTS EOS and the MD results for the phase equilibrium and the interfacialproperties is observed. Enrichment at the interface is only found for the low-boilingcomponent 2. The enrichment increases with decreasing concentration of component2 and is favored by high boiling point di?erences of the pure components 1 and 2 andpositive deviations from Raoult's law for the mixture 1 + 2.

Molecular modelling techniques for predicting liquid–liquid interfacial properties of methanol plus alkane (n-hexane, n-heptane, n-octane) mixtures

Liquid–liquid interfacial properties of methanol plus n-alkane mixtures are investigated by two complementary molecular modelling techniques: molecular dynamic simulations (MD) and density gradient theory (DGT) coupled with the PC-SAFT equation of state.