Research Summary: The objective of this project is the investigation of phase equilibrium properties of fluid mixtures by computer simulation in a new ensemble. The Gibbs ensemble Monte Carlo simulation technique enables the direct determination of the properties of two coexisting fluid phases (e.g., a liquid at equilibrium with its vapor) from a single computer experiment. The first part of this project will be the extension of the Gibbs technique to multicomponent systems. The method will then be applied for the determination of the effect of intermolecular potential parameters (size, shape and polarity of the components of a mixture) on the mixture critical curves and the presence of azeotropes, liquid-liquid and fluid-fluid immiscibility gaps. By selecting appropriate models for the intermolecular interactions, the behavior of systems that approximate real mixtures of engineering interest will then be studied. Finally, a comparison between direct computer simulation results and theoretical or semi-empirical models will be performed. Innovation/Novelty: The Gibbs method (recently proposed by the PI) is the first practical technique available for exact determination of phase equilibria in complex systems with given intermolecular interactions. Until now, the calculation of phase equilibria from simulation was extremely difficult because a large number of laborious free-energy determinations was required. This research will provide extensive phase equilibrium data on well-defined model systems. Such data are not currently available, despite the significant role they can play in the development of improved theoretical and semi-empirical models for the thermodynamic behavior of fluids. In addition, realistic mixtures of engineering importance will be studied for the first time by exact simulation methods. Technical Impact and Significance: Detailed knowledge of the phase behavior of fluid mixtures is needed for the efficient design and operation of many separation processes in chemical engineering (e.g., distillation, liquid-liquid extraction, enhanced oil recovery), but similar needs may arise in many other fields, including geophysics, biomedicine and the study of dense planetary atmospheres. The proposed research will significantly contribute to the development of molecular simulation techniques (to complement experiment and theory) for this purpose. The new simulation methodology can also find applications for the molecular-based study of a wide range of industrially important phenomena. Examples of such possibilities would be the study of adsorption by solids and physical equilibria in microporous catalysts (relevant for the oil and chemical process industries), micelle formation and stability in surfactant sytems (important for the cosmetics and pharmaceuticl industries, as well as for the study of living systems), and equilibria in the presence of semi-permeable membranes (significant for the development of new separation methods for the biotechnology industry).
