It is demonstrated that thermal and mutual diffusivities of binary mixtures of n-octacosane (n-C28H58) with carbon monoxide (CO), hydrogen (H2), and water (H2O) are simultaneously accessible by dynamic light scattering (DLS). As the light-scattering signals originating from thermal and concentration fluctuations appear in similar time scales, different data evaluation strategies were tested to achieve minimum uncertainties in the resulting transport properties. To test the agreement of the respective theoretical model with the DLS signals in the regression, an improved multifit procedure is introduced. With the selected data evaluation strategy, uncertainties of 4 to 15% and 4 to 30% in the thermal and mutual diffusivities, respectively, could be obtained for the binary mixtures. The mutual diffusivities for the mixtures measured at temperatures ranging from 398 to 523 K and pressures of 5 to 30 bar at saturation conditions are in good agreement with molecular dynamics simulations and data from the literature.

The self-diffusion and mutual diffusion coefficients of hydrogen (H(2)), carbon monoxide (CO), and water (H(2)O) in n-alkanes were studied by molecular dynamics simulation. n-Alkane molecules were modeled based on the TraPPE united atom force field. NPT molecular dynamics (MD) simulations were performed for n-C(12) to n-C(96) at different temperature and pressure values to validate the accuracy of the force field. In all cases, good agreement was obtained between literature experimental data and model predictions for the density and structure properties of the n-alkanes. Subsequently, the self-diffusion coefficient of the three light components in the various n-alkanes was calculated at different temperatures. Model predictions were in very good agreement with limited experimental data. Furthermore, the Maxwell-Stefan diffusion coefficients of H(2) and CO in two n-alkanes, namely n-C(12) and n-C(28), were calculated based on long MD NVT simulations for different solute concentrations in the n-alkanes. Finally, the Fick diffusion coefficient of the components was calculated as a product of the Maxwell-Stefan diffusion coefficient and a thermodynamic factor. The latter was estimated from the statistical associating fluid theory (SAFT). The Fick diffusion coefficient was found to be higher than the Maxwell-Stefan diffusion coefficient for H(2) and CO in n-C(28). The empirical Darken equation was used to estimate the Maxwell-Stefan diffusion coefficient, and calculations were found to be in good agreement with simulation results.

Theoretical approaches suggest that dynamic light scattering (DLS) signals from low-molecular-weight ternary mixtures are governed by fluctuations in temperature as well as two individual contributions from fluctuations in concentration that are related to the eigenvalues of the Fick diffusion matrix. Until now, this could not be proven experimentally in a conclusive way. In the present study, a detailed analysis of DLS signals in ternary mixtures consisting of n-dodecane (n-CH) and n-octacosane (n-CH) with dissolved hydrogen (H), carbon monoxide (CO), or water (HO) as well as of n-CH or n-CH with dissolved H and CO is given for temperatures up to 523 K and pressures up to 4.1 MPa. Thermal diffusivities of pure n-CH and n-CH as well as thermal and mutual diffusivities of their binary mixtures being the basis for the ternary mixtures with dissolved gas were studied for comparison purposes. For the investigated ternary mixtures, three individual signals could be distinguished in the time-resolved analysis of scattered light intensity by using photon correlation spectroscopy (PCS). For the first time, it could be evidenced that these signals are clearly associated with hydrodynamic modes. In most cases, the fastest mode observable for ternary mixtures is associated with the thermal diffusivity. The two further modes obviously related to the molecular mass transport are observable on different time scales and comparable to the modes associated with the concentration fluctuations in the respective binary mixtures. Comparison of the experimental data with results from molecular dynamics simulations revealed very good agreement.

Poly(dimethylsiloxane) (PDMS) is a widely used polymer for a number of industrial applications. In order that PDMS is selected for a specific application, accurate knowledge of its physical properties is necessary. Physical properties can be either measured or calculated based on reliable suitable methods. Molecular simulation using realistic models is a powerful tool for the elucidation of microscopic structure of polymers and the subsequent estimation of macroscopic physical properties. In this work, a force field is developed for the prediction of thermodynamic and structure properties of PDMS melts. Force field development was based on existing force fields for PDMS together with fitting to experimental thermodynamic data at ambient pressure. Extensive NPT molecular dynamics (MD) simulations were performed at different temperature and pressure values. In all cases, good agreement was obtained between literature experimental data and model predictions for the melt density. Calculations are reported also for the solubility parameter of the polymer melt at different temperatures. Furthermore, radial distribution functions for the intra-and intermolecular interactions are presented and shown to be in good agreement with previous literature work. The new force field is used subsequently for the calculation of solubility of 17 different compounds in PDMS using the Widom test particle insertion method. The solubility of n-alkanes from methane to n-hexane at 300 and 450 K and different pressures was calculated. In addition, solubility calculations for n-perfluoroalkanes at 300 and 450 K and for noble and light gases at 300, 375, and 450 K and ambient pressure were performed. Model predictions are in very good agreement with experimental data, in all cases. The infinite dilution solubility coefficient is shown to increase with temperature for very light gases and decrease for the heavier ones.