Theoretical studies of solid-fluid phase equilibria. Final report, September 1, 1990--December 31, 1993

摘要

Work focused in two directions: understanding the role of anisotropic molecular interactions such as those arising from molecular shape on solid-fluid equilibria and in developing a theoretical approach to solid mixtures. A study of the phase diagrams for hard dumbbells (hard sphere dimers), focused especially on the solid-fluid equilibria, was completed; it produced insights into how molecular shape determines the stable crystal structure and solid-fluid equilibria. Results were used to test theoretical predictions from density functional theories and from the cell theory, which were successfully extended to such systems. Results for hard dumbbells provided reference system data for a generalized van der Waals theory of solid-fluid equilibria of nonspherical molecules. This theory provides a basis for understanding trends in experimental solid-fluid equilibrium data, such as reduced triple point temperatures for different substances. A simple extension of this approach to systems with quadrupolar interactions provides a plausible explanation for some features of the phase diagram of CO(sub 2) such as its high triple point temperature and large density increase on freezing. The cell theory was extended to mixtures in the solid state. Key development was the incorporation of cell composition fluctuations into the theory. Comparisons with Monte Carlo simulation results for solid-fluid equilibria in hard sphere mixtures show that this theory is the most quantitatively accurate one available and is readily extended to attractive interactions without using a mean field approximation for composition dependence of such interactions.