Towards the development of high accuracy force fields: Understanding molecular interactions

This work focuses on some of the intra- and intermolecular interactions in current models (or force fields), and their effects on the phase behavior of pure organic compounds and of their mixtures. Various force fields are tested via Monte Carlo simulations in the grand canonical ensemble with the histogram reweighting technique, which is known for its high accuracy and precision. In a first part, the effects of the sampling method of the dihedral angle is investigated for linear alkanes. Results show that the magnitude of a Fourier series based torsional potential does not affect the phase behavior. However, changing the sampling method to a normal distribution causes deviations in the coexistence curves: the predicted liquid densities are lower than the experimental data, and the critical point is underestimated. These typical results are also observed in branched alkanes, perfluorocarbons, and functionalized molecules such as alcohols, ketones and ethers. Studies of the internal structure of normal alkanes shows that the average excluded volume is significantly reduced when a Gaussian sampling is used, compared to a potential using Fourier series.;Next, the repulsive exponent of the Lennard-Jones potential is changed into a variable and the effect of its magnitude on phase equilibrium of simple spheres and diatomic particles is investigated. The results show that the lower the repulsion the higher the critical point, and the steeper the slope of the saturation pressure. The critical point is shown to be varying linearly with the attractive domain of the intermolecular potential, which depends on the repulsion. Results show that by fixing the separation, the tuning of the repulsion and the usual Lennard-Jones parameters provides values of the acentric factor in very good agreement with experimental data for common diatomic fluids.;When looking at the performance of current force fields for perfluorocarbons in the reproduction of thermodynamic properties, no model using fixed bond length can accurately predict both the vapor liquid coexistence and the saturated vapor pressure simultaneously. Besides some models show inconsistencies as the predictions can be accurate for some molecules but incorrect for others within the same class of chemicals. The deviations in vapor pressure cause drastic changes in the phase behavior of mixtures of these compounds with regular alkanes.;Based on the results above, a new force field using tunable repulsion is developed to enable accurate predictions of both the vapor liquid coexistence and the saturation pressure of organic compounds. The molecules studied are linear alkanes, their perfluorinated homologues and alcohols. Results also show that the use or omission of long range corrections has some importance: the saturated densities, the critical point and the vapor pressure are affected. In terms of force field development, it is observed that a slightly stronger repulsion has to be used in order to correctly reproduce the thermodynamics of alkanes, and that for perfluorinated compounds, the repulsion has to be very high. For alcohols, a soft repulsion is required to compensate the strong Coulombic repulsions due to the negative charge on the oxygen atom. The phase behavior of an alkane+perfluorocarbon mixture can be predicted with better accuracy and precision using the newly developed force field instead of the ones studied previously.