Volume 14 Number 3	July 1998

In Vivo Deployment of Palmaz-Schatz Stent Sparse Matrix Methods Fluid Phase Equilibria Modeling the Dynamics of RNA Seminar Synopsis Visitors Research Reports ver the next decade, it will become possible to gain microscopic-level insight into the behavior of complex chemical systems by making use of theoretical advances and by employing high-speed computational resources. Only with this molecular-based understanding will the research enterprise be able to develop chemicals and materials that meet the increasing needs of society. The computer experiment is unique because it allows researchers to study well-defined systems under well-controlled physical conditions with a non-invasive approach. The researcher can specify input parameters such as molecular structure of the constituents, their concentrations, and pressure and temperature. Once specified, they can follow the phase space trajectory of the system. Analysis of the trajectory allows researchers to determine mechanical as well as thermal properties and ultimately, to learn how molecular architecture and composition influence function. Presently, due to limitations in theoretical techniques, force fields, and computational resources, the range of systems to which this kind of molecular modeling can be successfully applied is restricted. It is the hallmark of Professor Ilja Siepmann and his research group, graduate students Marcus Martin, Bin Chen, and Nikolaj Zhuravlev, of the Chemistry Department at the University of Minnesota, to not merely apply the present level of theory, but to develop new computational tools expanding the range of chemistry that can be studied. FIG. 1: Vapor-liquid coexistence curves for ethane, n-pentane, and n-octane. Experimental data and critical points are shown as dashed line and crosses. Simulation results are shown for thre difference force fields: OPLS (blue circles), SKS (red squares), and TraPPE (green diamonds). Since hydrocarbons are the most important source of energy and the basic feedstock for most chemical industries, it is not surprising that the experimental determination and modeling of their behavior has been a subject widely studied. Although the thermophysical properties of pure low-molecular-weight hydrocarbons have been determined experimentally, many important areas remain difficult and costly to access. Experimental difficulties are caused by the necessity to study systems at high temperature and high pressure (or high shear rate), the lack of pure samples, and the immense variety of technologically important fluid mixtures. Molecular modeling and other theoretical prediction methods are needed to complement the experimental data as well as to improve the rational design of efficient chemical processes. Phase equilibria are a challenging task for simulation, but great strides have been made over the last few years. John Valleau and Athanassios Panagiotopoulos pioneered novel techniques (thermodynamic-scaling and Gibbs-ensemble Monte Carlo) for the efficient calculation of phase equilibria of simple fluids. Professor Siepmann and coworkers developed the configurational-bias Monte Carlo method that opened the door to calculations for complex fluids with articulated structures. The nature of the research--requiring the prediction of coexistence properties at many state points--is ideally suited to an embarrassingly parallel computational strategy by employing one processor per state point. However, in some cases, individual simulations can take days or weeks of central processing unit (CPU) time on an SGI R10000 processor. FIG. 2: Pressure--composition phase diagram for the binary system of supercritical ethane and n-heptane. Dashed lines and symbols represent the experimental and simulation data (green: T = 366 K; red: T = 450 K). Due to their sensitivity to the choice of interaction parameters, comparison between experimentally known coexistence curves and those evaluated from simulations constitutes an outstanding tool to improve parametrizations of force fields. A simple and accurate force field (TraPPE) for linear and branched alkanes has already been developed in this research (see FIG. 1). Plans are being made to extend efforts to the whole range of important organic solvents including cyclic alkanes, perfluorinated alkanes, alkenes, aromatics, ethers, ketones, and fatty acids. Multicomponent phase equilibria are calculated to ensure that the new force fields are directly transferable from pure substances to mixtures. Simulation data are most valuable for high-molecular-weight hydrocarbons that are thermally unstable at temperatures well below their critical points and difficult to purify. Simulations of long linear alkanes were instrumental in resolving important questions about the influence of chain length on the critical properties. Evidence was provided for large deviations from the principle of corresponding states for three triacontane isomers. Currently, focus is being placed on predictions of phase equilibria at high temperatures and pressures that are important for supercritical extractions and enhanced oil recovery. An example of the prediction of supercritical phase equilibria is shown in FIG. 2. Investigations will not be limited to binary systems because the addition of small amounts of further components can often markedly change thermophysical properties. This is the case with entrainers used to enhance the solubility power of supercritical solvents.   HOME | BULLETINS | CONTACT US | PREVIOUS ARTICLE | NEXT ARTICLE | THIS ISSUE   This information is available in alternative formats upon request by individuals with disabilities. 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