Molecular modeling of solid -fluid equilibrium for complex molecules
This work is concerned with providing the fundamental understanding of solid-fluid phase equilibrium of complex molecules by using Monte Carlo simulation, with a special emphasis on long chain n-alkanes and simple chiral molecule systems. The goal of the work is to look into the solid-fluid equilibrium thermodynamics from perspective view of intermolecular interactions, especially the repulsive interactions. The solid-fluid phase behavior of longer chain n-alkanes can be complicated by a substantial loss of orientational order about the chain axis in the solid phase near the melting point. The solid state formed under these circumstances is called a rotator phase. We have investigated the properties of the solid and fluid phases of a molecular model of n-alkanes with chain lengths in the range C17-- C21, focusing especially on the occurrence of rotator phases. The models we have been using are hard-core united atom models. The bond lengths and bond angles are fixed. Internal rotations are modeled using simple torsional potentials that allow for small fluctuations around the trans and gauche conformations. We have made extensive calculations of the free energy and equation of state of the model for longer chains and have developed order parameters for tracking the degree of orientational order around the chain axis. The model exhibits both a rotator phase and a more orientationally ordered solid phase. Our results indicate that the transition between these two solid phases is first order with a small density change. We also use the models to investigate the solution thermodynamics and solid-fluid phase equilibrium in mixtures of n-alkanes. The phase diagrams show that solid-solid and solid-fluid phase coexistence can occur in those systems. Solid-solid microphase separation was observed directly in the simulations of some binary mixtures. In addition, for some states modulated superstructures were also observed, similar to those seen in experiments on solid phase alkane mixtures.
Techniques developed to study chain systems have been extended to chiral molecules with emphasis on the stability of solid phase racemic compounds. The solid-fluid and solid-solid phase equilibrium has been determined for molecular models representative of chiral molecules and enantiomeric mixtures. The models consist of four hard sphere interaction sites of different diameters in a tetrahedral arrangement with the fifth hard sphere interaction site at the center of the tetrahedron. A simulated annealing method has been applied to generate the close packing structure for both the pure components and racemic compounds of the model. The volumetric properties and free energies of the pure enantiomers and binary mixtures were calculated in both fluid and solid phases using isobaric Monte Carlo simulations. The models exhibit ideal solution behavior in the fluid phase with little chiral discrimination. In the solid phase the effects of chirality are much greater. Solid-fluid phase behavior involving the pure enantiomer solids and also racemic compounds was calculated. The calculations indicate that, depending on the relative sizes of the hard sphere interaction sites, packing effects alone can be sufficient to stabilize a racemic compound with respect to the pure enantiomer solids.