Molecular dynamics simulation results on hydrogen bonding in mixtures of methanol with CO2 at supercritical, liquid-like conditions are compared to 1H NMR spectroscopic data that have recently become available. The molecular models are parametrized using vapor-liquid equilibrium data only, which they reliably describe. A new molecular model for methanol of Lennard-Jones plus point charge type is presented. This molecular methanol model is investigated in terms of its capability to yield hydrogen-bonding statistics. Simple assumptions are made regarding the assignment of NMR chemical shifts to the different types of hydrogen-bonded species. Only two state-independent parameters are fitted to the large NMR data set on the basis of hydrogen-bonding statistics from molecular simulations. Excellent agreement between the molecular simulation results and the NMR data is found. This shows that the molecular models of the simple type studied here cannot only describe thermodynamic properties but also structural effects of hydrogen bonding in solutions.

The influence of the unlike Lennard-Jones (LJ) parameters on vapor-liquid equilibria of mixtures is investigated and the performance of eleven combining rules is assessed. In the first part of the work, the influence of the unlike LJ size and energy parameter on vapor pressure, bubble density and dew point composition is systematically studied for the mixtures CO+C 2 H 6 and N 2 +C 3 H 6 , respectively. It is found that mixture vapor pressure depends strongly both on the size and the energy parameter whereas the bubble density depends mostly on the size parameter and the dew point composition is rather insensitive to both parameters. In preceding work, unlike LJ parameters were adjusted to experimental binary vapor-liquid equilibria for 44 real mixtures. On the basis of these results, in the second part of the work eleven combining rules are assessed regarding their predictive power. A comparison with the adjusted unlike LJ parameters determined from the fit shows that none of the eleven combining rules yields appropriate parameters in general. To obtain an accurate mixture model, the unlike dispersive interaction should therefore be adjusted to experimental binary data. The results from the present work indicate that it is sufficient to use the Lorenz rule for the unlike LJ size parameter and to fit the unlike LJ energy parameter to the vapor pressure.In molecular simulations of a binary mixture A+B with pairwise additive potentials, three different interactions occur: two like interactions between molecules of the same type A-A and B-B, which are fully defined by the pure component models, and the unlike interaction between molecules of different type A-B. In mixtures consisting of polar molecules, the electrostatic part of the unlike interaction is fully determined by the laws of electrostatics. However, there is no rigorous physical framework that yields reliable unlike repulsion and dispersion parameters like the Lennard-Jones (LJ) parameters studied in the present work. For finding these parameters, combining rules were developed in the past based on physical and mathematical intuition or on empirical approaches. Eleven of these combining rules were investigated in the present work.These combining rules rely soley on pure component data, namely the LJ parameters and, in some cases, additionally the polarizablility α or the ionization potential I. Other combining rules, that are not discussed in this work, also employ dispersion force coefficients [1, 2, 3], diamagnetic susceptibility [4,5] or effective transition energies [6,7].Another approach for obtaining unlike LJ parameters is to adjust them directly to experimental binary data, for which a single data point may in principle be sufficient. Kohler et al. [8] fitted both unlike LJ parameters to experimental second virial coefficients of binary mixtures.Following this approach, Möller et al.[9] developed a method for adjusting the unlike LJ size and energy parameters to experimental excess volumes and enthalpies. Their investigations showed that the un...

Henry's law constants of the solutes methane, nitrogen, oxygen and carbon dioxide in the solvent ethanol are predicted by molecular simulation. The molecular models for the solutes are taken from previous work. For the solvent ethanol, a new rigid anisotropic united atom molecular model based on Lennard-Jones and Coulombic interactions is developed.It is adjusted to experimental pure component saturated liquid density and vapor pressure data. Henry's law constants are calculated by evaluating the infinite dilution residual chemical potentials of the solutes from 273 to 498 K with Widom's test particle insertion. The prediction of Henry's Law constants without the use of binary experimental data on the basis of the Lorentz-Berthelot combining rule agree well with experimental data, deviations are 20 %, except for carbon dioxide for which deviations of 70 % are reached. Quantitative agreement is achieved by using the modified Lorentz-Berthelot combining rule which is adjusted to one experimental mixture data point.

No abstract

No abstract