Phase equilibria of diatomic Lennard-Jones molecules using Monte Carlo simulation

The overall aim of this research is to gain an understanding of the nature of phase equilibria. We use Monte Carlo computer simulation to explore the vapor-liquid and solid-liquid phase behavior of simple nonspherical molecules.; We first calculate vapor-liquid phase diagrams for binary mixtures of diatomic Lennard-Jones molecules using Monte Carlo simulations and the Gibbs-Duhem integration method. We plot pressure versus composition vapor-liquid phase diagrams for the binary mixtures O2-N 2, CO2-C 2H6 and N2- C2H6 at different temperatures. Our results are in good agreement with experimental data, especially away from the critical point. We then add a quadrupole term to the two-center Lennard-Jones potential model and we observe that this further improves the agreement with experimental data. We also investigate the dependence of Henry's constant on the temperature, pressure and binary interaction parameter.; We explore the effect of varying the molecular size ratio from sigma 11/sigma22 = 1.0 to 1.40, intermolecular attraction ratio from epsilon11/epsilon22 = 0.70 to 1.20 and binary interaction parameter from delta12 = 0.70 to 1.10 on the dumbbell mixture's phase behavior. At low sigma11/sigma22 ratios, the pressure at the azeotrope decreases and the azeotrope shifts towards pure component 1. At high sigma11/sigma22 ratios, the phase envelope widens and the azeotrope of the mixture shifts towards pure component 2. The changes in the epsilon11/epsilon22 ratio affect the azeotrope in a similar fashion. At the high epsilon 11/epsilon22 ratios, the azeotrope disappears and the phase diagram terminates at the critical point. At low delta12 values, the mixture forms a heteroazeotrope, and at high delta12 values, the azeotrope changes from a positive azeotrope to a negative azeotrope.; We then examine the solid-liquid phase equilibria for systems containing pure Lennard-Jones dumbbell molecules and their mixtures. We begin by calculating the equations of state for systems containing C2 H6, CO2 and F 2, all of which are modeled by a two-center Lennard-Jones potential for linear diatomic molecules. We then use the Frenkel-Ladd thermodynamic integration method to calculate the free energies. The equations of state and the free energies are used to obtain solid-liquid coexistence points which are needed to start the Gibbs-Duhem integration. The solid-liquid phase equilibria for pure C2H6, CO2 and F2 and binary mixtures of Lennard-Jones dumbbells are predicted using the Gibbs-Duhem integration method. (Abstract shortened by UMI.)...