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离子液体的分子动力学模拟
Alternative TitleMolecular Dynamics Simulation Studies on Ionic Liquids
刘晓敏
Subtype博士
Thesis Advisor张锁江
2008-05-28
Degree Grantor中国科学院过程工程研究所
Place of Conferral过程工程研究所
Degree Discipline物理化学(含:化学物理)
Keyword离子液体 分子动力学模拟 分子力学力场
Abstract近年来室温离子液体引起了国内外的广泛关注,离子液体作为传统有机溶剂的替代品在化学反应、分离,催化等方面得到了广泛的应用。作为一种可设计的溶剂,数目繁多的新型离子液体不断涌现,然而结构性质关系暂不明确。计算机模拟作为分子设计不可或缺的手段之一,成为研究体系微观结构及宏观物性的有效途径。本文第2、3章主要针对两类胍类离子液体进行分子动力学模拟,第4章对比了全原子力场及联合原子力场模拟结果的异同,第5章对长链磷类离子液体中可能存在的团簇现象做了探索性研究。具体内容如下: (1) 在AMBER力场的基础上,通过从头计算优化体系构型,利用力场构建的系统方法得到了离子液体的分子力学力场。在不同温度下对5种脂肪族胍类离子液体进行了分子动力学模拟,模拟结果表明,相对于普通溶剂,离子液体具有非常高的内聚能和蒸发焓;通过研究离子液体的径向分布函数及空间分布函数,发现阳离子上的氨基H原子与阴离子上的O原子形成氢键,阴阳离子间形成溶剂化层结构,在第一溶剂化层中,两个阴离子分布在氨基周围,其余分布在阳离子胍平面的上下两侧或甲基周围。 (2) 在AMBER力场的基础上建立了11种环状胍类离子液体的分子力学力场,进行了分子动力学模拟。系统地研究并比较了离子液体的扩散性质,包括体系阴阳离子的自扩散系数、粘度及电导率。结果表明,胍类离子液体相对于普通溶剂扩散很慢,同体系离子液体的活动性随着烷基侧链的缩短而减弱。为了研究体系的构效关系,文中对10余种离子液体的微观结构进行了深入的探索,研究了体系的径向分布函数及空间分布函数。模拟结果表明,体系中存在氢键相互作用,而这种阴阳离子对之间的氢键相互作用对体系的动力学性质影响显著,在很大程度上限制了离子液体的自由运动,阴阳离子之间氢键作用越强,自扩散越慢,粘度越高。通过对离子液体微观结构的研究,发现阴阳离子呈现了溶剂化层结构,每个阳离子周围存在5~7个阴离子。 (3) 在模拟计算过程中,发现对于较为复杂的离子液体体系,全原子(All-atom, AA)力场基础上得到的结果虽然准确性高,但是计算资源消耗非常大。本章在AMBER力场的基础上,建立了4种胍类离子液体的全原子及联合原子(United-atom, UA)力场并进行了分子动力学模拟,其中UA力场的计算量仅仅是全原子力场的1/6。在两种力场的基础上分别进行动力学模拟,结果表明两种力场下的模拟密度,分子间相互作用能量值及得到的微观结构非常接近;两种力场基础上得到的扩散性质有较大的差异。因此,如果一个研究体系含有较多的甲基及亚甲基基团,这些基团对体系的相互作用影响不大,而且体系的扩散性质不是研究的重点,则可以选取UA力场代替AA力场,在保证结果精确度的同时大幅度地节省计算资源。 (4) 开发了一种长链磷类离子液体的联合原子力场,并进行了分子动力学模拟,研究体系中的团簇现象。模拟在多个温度下进行,探索温度对体系结构及团簇的影响。研究发现,随着温度的升高,离子液体阴阳离子之间以及阳离子之间的作用强度降低,团簇的趋势逐渐减弱。由于目前对于离子液体的团簇结构没有明确的定义,因此本文提出了一种基于原子数密度的团簇评价标准。
Other AbstractRecently room temperature ionic liquids (RTILs, ILs) have prompted a significant amount of researches. The most attractive property of RTILs is their use as environmentally benign solvents that can replace organic compounds in chemical reactions, separation, catalyzing and etc. As a designable solvent, a number of functional ionic liquids have been synthesized. However, the microstructures and interactions for most of the RTILs have not been clearly understood yet. Molecular simulation provides an effective way for understanding the microstrcutures and interactions of ionic pairs. In this work, molecular simulations were performed for guanidinium-based ILs, and we also studied the aggregations in phosphonium-based ILs. The details are listed as follows. (1) Based on AMBER force field, we optimized five kinds of acyclic guanidinium-based ILs structures by ab inito calculations and developed the molecular mechanics force fields in a systemic way. Molecular dynamics simulations were performed for these ILs at different temperatures. It is found that comparing to the normal solvents, these ILs have higher cohesive energy densities and heats of vaporization. The microstructures are discussed by radial distribution functions (RDFs) and spatial distribution functions (SDFs). Amino groups in the cation form hydrogen bonds with each kind of anion. It is found that the two anions locate next to the hydrogen atoms in the amino-group terminal and the other anions within the first solvation shell most likely distribute above or below the plane or around the methyl. (2) Force field based on AMBER was proposed for eleven cyclic guanidinium-based ionic liquids, and molecular dynamics simulations for these ILs were performed. Transport properties including the coefficients of self-diffusion, viscosities, and conductivities were also studied and compared. They are consistent with the fact that the viscosities for most of the ILs are much higher than water. It is found that the activity reduces with decreasing the length of alkyl chain for these cyclic guanidinium-based ILs. The microstructures are discussed by RDFs and SDFs. It is found that hydrogen-bonding interactions between the molecules will influence the transport properties, such as the diffusion constant and viscosity. Stronger of the hydrogen-bonding interactions, the viscosity will be higher. It can be seen from the integrating the RDFs that each cation is surrounded by five to seven anions. (3) It is found that when we study the complex system like ILs, results based on the all-atoms (AA) force field is precise but the cost is too expensive. Based on the AMBER force field, we developed both the AA and united-atoms (UA) force field for four kinds of guanidinium-based ILs. Molecular simulation were performed and only 1/6 computational time was needed for UA force field. It is found that the liquid densities, iner-molecular energies and micro-structures is exactly the same with each other, however, differences for diffusion properties between the two force fields are non-negligible. Therefore, when there are several methyls/methylenes in the system, and our focus isn’t on the diffusion perpertie, the UA force field is our first choice. (4) UA force field was proposed for phosphonium-based IL, and molecular dynamics simulations were performed to depict the aggregation. To study the temperature influence on the molecular structure and aggregation, molecular simulations were carried on several temperatures. It is found that interactions between ions reduced and the aggregation is less compact with increasing the temperature. Unfortunately, there aren’t any uniform and explicit standards for judging the nano-structure, or in another way, the aggregation in ILs. As a result, a way based on the atomicity density is developed for estimating and understanding the inhomogeneous structure at nanometer scale in the work.
Pages129
Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/1257
Collection研究所(批量导入)
Recommended Citation
GB/T 7714
刘晓敏. 离子液体的分子动力学模拟[D]. 过程工程研究所. 中国科学院过程工程研究所,2008.
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