Studies on Fluids and Fluid Mixtures: Connecting Theory with Experiment

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This grant is supported jointly by the Division of Materials Research and the Chemistry Division. The research is targeted at correlating microscopic structure and macroscopic behavior for fluids and their mixtures, with a particular emphasis on complex fluids. While the research tools are theoretical , the focus is on making connections with experiment and, where appropriate, simulation. An unusual opportunity afforded by this work is the ability to compare the results for the lattice and continuum models using the same theoretical approach. The behavior of complex fluids has been of high interest recently, stimulated by the increasingly sophisticated kinds of measurements accessible to scientists as well as the greatly expanded ability to simulate mixtures of dense fluids. The research proposed here builds upon the work of the PI and her group in using the Born-Green-Yvon (BGY) integral equation technique to model lattice and continuum fluids and mixtures. The lattice theory yields simple closed-form expressions for thermodynamic quantities. Advantages include its accessibility to non-theorists, and the ability to test it using lattice simulation results on complex systems. The continuum theory is capable of tackling more subtle issues involving the interplay between local structure and bulk properties. However, numerical methods are required and simulation data on mixtures are limited. The development of analogous lattice and continuum theories creates opportunities for determining which properties are sensitive to the imposition of a lattice constraint. The lattice studies proposed here focus on three projects: re-deriving the BGY theory to describe films and interfaces and studying the transition to the bulk; increasing the complexity of systems studied to include ternary mixtures; and mapping the thermodynamic results of the BGY theory onto the formalism of the Flory-Huggins chi parameter, the most widely-used characteristic parameter in the polymer community. This work will build on the demonstrated ability of the lattice BGY theory to describe simple and polymeric fluids and mixtures, and will exploit recently developed strategies for extracting much information from a minimal amount of experimental data. On the continuum side, the BGY theory has been used recently to study square-well fluids of up to 16-mers, and n-alkanes. These results will inform the proposed study of small branched alkanes, a project which will lead to a greater understanding of packing effects on fluid properties, and consequently of what may be lost when the lattice BGY theory is used. The second project focuses on square-well mixtures of short chain fluids; the components will range between monomers and 8-mers, thereby stretching the limit of current simulation results. This research will require consideration of the concentration dependence of the inter- and intramolecular distributions: how to treat it, and under what conditions it will become important. In this work connections between experimental data/analysis and the theoretical tools developed by the PI to study complex fluids will be expanded and strengthened. The outcome is that a more sophisticated combination of strategies, accessible to the materials community, will be available in solving problems relating to understanding connections between microscopic structure and macroscopic behavior. Local presentations by undergraduates and graduates will help them develop teaching skills. Results will also be disseminated through conference presentations by the PI and her research group at national meetings, and publications. The PI's efforts in research and teaching mentorship has resulted in an increase in the number of women in the ranks of graduate and postgraduate students and in faculty; it is expected that such effects will continue.