以第一原理計算探討分子構型對流體相行為的影響

Abstract

流體的相行為在化學工業程序設計是不可或缺的資訊。預測純物質與混合物流體的相行為對於初期程序設計是相當重要的。在以往的研究方法中常使用官能基貢獻法或是定量結構與關係法等方式來進行項行為的預測。已有許多研究指出，分子構型也是影響流體相行為的一個重要原因，但是在常用的預測方法中往往選擇單一構型來進行預測，將分子構型對於性質的影響併入模型的參數中，但是這樣的計算方法往往需要大量的實驗數值來進行參數的迴歸，而且也會因為構型選擇限制了模型的準確度。在本研究中，我們提出以結合量子化學計算、分子動態模擬計算，藉由計算純物質分子的溶劑化自由能，獲取PR+COSMOSAC狀態方程式所需之參數a、b以及ω，當有了這些資訊便可以在數秒之內快速完成相行為預測，以及非剛性分子在不同相中構型的分布狀況，因此可以用來了解分子構型對於相行為的影響。我們選擇兩個簡單的分子1,2-dichloroethane和1,2-ethanediol來進行研究，前者僅有兩種主要的構型，在相行為以及構型分布比例的解釋上都能簡單的利用極性來說明。然而後者較為複雜有10種構型，極性大小和氫鍵面積同時影響到相行為以及構型分布比例。研究中我們我們主要探討純物質蒸氣壓、二成分系統的氣液相平衡以及液液相平衡，過程中我們將專注在不同環境對分子構型的影響，分析構型的比例及吉布絲自由能，最後得知藉由分子構型的考慮能使得該分子做到更準確的相行為預測及更大的應用範圍，此外還能從微觀角度得知環境對構型的影響。

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.