Molecular-Based Study of Reversed Micelles in Supercritical in Carbon Dioxide for Solvent Substitution in the U.S. Chemical Industry

点击翻译按钮获取中文摘要

9613555 Cummings It is proposed to use small angle x-ray (SAXS) and neutron (SANS) scattering plus complementary molecular dynamics (MD) and Monte Carlo (MC) simulations to develop a molecular-level understanding of the formation and stability of reverse micelles (RMs) in supercritical carbon dioxide (SC-CO2). The results of this project will contribute to the fundamental knowledge base for the U.S. chemical industry to substitute SC-CO2 for many of the toxic or hazardous solvents presently used in most industrial chemical reaction and separation processes. Despite its attractive features as a solvent substitute, SC-CO2 is not a good solvent for certain classes of substances, e.g. 1) water, water-soluble, and polar substances or 2) most high molecular weight substances such as synthetic polymers or proteins. Techniques have been proposed, however, for processing these insoluble substances in SC-CO2 through the use of RMs. Recently, after a decade-long search, appropriate surfactants have been discovered to disperse both aqueous systems and polymers in RMs andSC-CO2. The aim of this work is to understand how the detailed molecular characteristics of the newly discovered surfactants contribute to the formation of RMs to understand what makes a successful CO2 -philic surfactant tail, to characterize the interfacial layer between the continuous SC-CO2 phase and the microscopic RM core, to establish the basis for predicting and measuring the size and shape of RMs, in short, to provide the molecular foundations for the needed knowledge base for widespread industrial utilization RMs in SC-CO2 as substitute for liquid solvents. It is proposed to apply four molecular-based techniques to study RMs in SC-CO2: 1) SANS for its ability to vary contrast between specific molecular components through selective deuteration, 2) SAXS for its high productivity and different contrast compared with SANS, 3) MC for its efficient exploration of the configuration space, and 4) MD for its capability to explore dynamical processes at a molecular level. Scattering can tell much about the microstructure (spheres? rods? lamella? biocontinuous structures?) and macrostructure (clustering?) of RMs which is unavailable by other means. Both the 30-m SANS instrument at ORNL with the existing high pressure SAXS cell, will be used to characterize RMs with both polymer and water cores. And the massively parallel Intel Paragon supercomputers at ORNL, will be used to simulate RMs from the newly discovered surfactants in SC-CO2 using accurate models developed from the OPLS parameter set. The combination of the power of the Paragon supercomputers and the efficiency of our parallel techniques will permit determination of the stability of preconstructed RMs. A primary goal will be to make quantitative comparisons between the stable RM structures as determined by SANS and SAXS experiments and those determined by simulation. Then, it is proposed to use simulation experiments, with available thermodynamic data, to understand the balance of interactions that is required to form stable RMs in SC-CO2, and thus to understand the molecular structure required for a successful surfactant. With anticipated success in the proposed SANS, SAXS, MC, and MD studies, a full repertoire of molecular-based tools for study of RMs in SC-CO2 would be available. With these tools it is proposed to test and adapt existing thermodynamic models of aqueous RMs, e.g., those Nagarajan, Blankschtein, and Holland, to RMs in SC-CO2 . With both molecular and thermodynamic models, it is proposed to suggest novel surfactants for synthesis and testing. Achieving even a small fraction of this degree of success, will substantially enhance the knowledge base for widespread substitution of SC-CO2 to replace conventional liquid solvents in the U.S. chemical industry. A more competitive industry with reduced emissions will be the ultimate result. ***