以第一原理計算探討分子構型對流體相行為的影響

張智為

doi:10.6342/NTU201700215

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以第一原理計算探討分子構型對流體相行為的影響

How Does Conformation Flexibility Influence the Fluid Phase Behavior: An Investigation from First Principles

張智為 , Masters　　Advisor：林祥泰　　

英文 DOI： 10.6342/NTU201700215

多尺度方法；狀態方程式；分子動態模擬；溶解；分子構型；相行為； multiscale approach ； PR+COSMOSAC equation of state ； molecular dynamics simulation ； solvation ； molecular conformation ； phase behavior

Abstract │ Reference (82) │ Altmetrics

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The modeling and prediction of fluid phase behavior have long been a focus of research. It is important information for process design. However, conventional predictive methods, such as the group contribution methods, usually ignore the structure details, e.g. the conformation of molecules. This limits such methods in providing properties of a chemical species in different chemical environments. Conformational distribution of flexible molecules may have a significant influence on physical properties.

In this work we use the predictive PR+COMOSAC EOS to determine the sensitivity of thermophysical properties to the detailed molecular conformations. The PR+COSMOSAC EOS provides thermodynamic properties of a fluid from the results of quantum mechanical solvation calculations, where the molecular structure is the only input. The PR+COSMOSAC EOS is used to provide the fugacity of chemical species in a mixture. The electrostatic and dispersion interactions in the EOS are determined from quantum-mechanical solvation calculations and molecular dynamics simulations at a few state points.

We examine the prediction of vapor pressure and partition coefficients for two typical systems (1,2-dichloroethane and 1,2-ethandiol) in order to better understand the effect of molecular conformation in phase behavior. Two main conformations (trans and gauche) are considered for 1,2-dichloroethane, and 10 conformations are analyzed for 1,2-ethandiol. In these two cases, different conformation will affect polarity and hydrogen bonding surface area. This causes the variation of intermolecular forces and reflects on the vapor pressure and different affinity to solvent. Therefore, contribution of each conformation will not be same in each phase and will be affected by different solvent. In this project we take the conformational transformations as chemical reactions with the equilibrium constant determined from ab initio G4 calculations. With all the EOS parameters obtained from molecular simulations at different scales, subsequent phase equilibrium predictions can be achieved within milliseconds. In this research, we find out that considering conformation of molecule can improve the phase behavior and make the molecule behave better in more kinds of systems. In addition, we can also know how chemical environment affect conformation in microscopic world.

Reference ( 82 ) 〈TOP〉

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[6] H. Renon, J.M. Prausnitz, Local Compositions in Thermodynamic Excess Functions for Liquid Mixtures, AIChE Journal 14 (1968) 135-144.
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