【作者】 张阳；

【导师】 杨基础；

【作者基本信息】 清华大学 ， 化学工程与技术， 2005， 硕士

【摘要】 本论文采用Monte Carlo方法和分子动力学(molecular dynamics)方法研究了纯超临界二氧化碳体系、纯乙醇体系以及经典的超临界二氧化碳+携带剂体系。检验了诸多半经验型势能函数在超临界领域的计算精度,同时也得到了这些流体的许多宏观和微观物理参数。1.采用正则系综的Monte Carlo方法和新提出的EPM-M系列势能模型对ρ=100-900 kg/m3,T=313,333,353 K范围内不同状态下的超临界二氧化碳进行了模拟计算。模拟量包括体系的PVT关系和微观结构等物理量,结果与实验值符合良好。此外,由于EPM2-M模型中加入了偶极矩,因此在对介电常数的模拟方面表现优异。2.利用NPT系综的Monte Carlo方法,在较大温度、压力范围内(473K–623 K;10 MPa–70 MPa)比较了OPLS-UA和TraPPE-UA两种模型对超临界乙醇PVT性质的模拟结果。应用TraPPE-UA模型模拟了超临界乙醇的微观结构和氢键特性,结果显示虽然高温破坏了超临界乙醇体系的氢键网络结构,但乙醇分子间仍部分保留氢键作用,这一结果与NMR的实验值符合。研究了水、乙醇、甲醇三种强缔合纯流体在超临界状态下的氢键强弱。3.采用分子动力学方法研究了组成、温度、密度等因素对超临界二氧化碳+携带剂体系的介电常数、扩散系数和微观结构的影响。所用的携带剂模型为TraPPE-UA,二氧化碳模型为EPM2。携带剂含量较低时介电常数的模拟结果与实验值相符,而携带剂含量高时则计算结果偏大;动力学数据说明携带剂分子在体系中的扩散系数小于二氧化碳分子;微观结构数据表明醇类在超临界二氧化碳中以聚集体的形式出现且氢键作用与纯流体体系相比得到了加强,而丙酮则没有表现出类似的强聚集性。更多还原

【Abstract】 Monte Carlo and molecular dynamics simulations were used to investigatea variety of supercritical fluids: carbon dioxide, ethanol and the classicalcosolvent -supercritical carbon dioxide systems. Many different force fieldswere tested in this work by simulating the physical properties of the fluids. Indetail, the following topics have been discussed: 1. By using the canonical ensemble Monte Carlo simulation with newdeveloped EPM-M serial models, the structural and PVT properties ofsupercritical carbon dioxide (ρ=100-900 kg/m3,T=313,333,353 K) werestudied. In addition, since the new EPM2-M carbon dioxide molecule has asmall dipole moment, the dielectric constant simulation results are inexcellent agreement with experimental data. 2. The NPT ensemble Monte Carlo method was used to simulate puresupercritical ethanol with two different united-atom force fields (OPLS-UAand TraPPE-UA) in range of 473 K～623 K and 10 MPa～70 MPa. Thestructure and hydrogen bond network were studied with the TraPPE-UAmodel. The results showed that even at 623 K, hydrogen bond interactionsbecome rather weak but still exist and the simulation data quantitatively agreewith NMR data. Furthermore, the hydrogen bond networks of threerepresentative self associating pure fluids: water, methanol and ethanol werecompared. 3. Molecular dynamics simulation was used to investigate the structural,dynamic and dielectric properties of the well known supercritical carbondioxide -cosolvent binary mixtures. TraPPE-UA force field was chosen tomodel cosolvent fluids and EPM2 model was used to simulate carbon dioxide.The simulation results of dielectric constant agree with experiments at lowcosolvent concentration, and at high concentration region, the results arehigher than experimental data. The diffusion constants for carbon dioxide arelarger compared with to those obtained for cosolvent. From structural analysis,it can be seen that alcohol molecules aggregated to form clusters and thehydrogen bond interaction was stronger in comparison with the hydrogenbonds in pure alcohols. Meanwhile, acetone-carbon dioxide mixtures did notshow such a strong aggregation phenomenon.更多还原