EAN离子液体及其水混合溶液中氢键行为的分子模拟研究

发布时间：2018-06-16 02:43

本文选题：质子化离子液体 + 离子液体/水混合物　；参考：《江西师范大学》2017年硕士论文

【摘要】：近年来,离子液体因低蒸汽压、高热稳定性、不可燃、高电导率等独特的物理化学性质,备受关注。更为重要的是,研究人员发现通过改变阴阳离子的结构和组成可以获得不同功能型的离子液体。此外,少量水的加入也能够极大地改变离子液体的物理化学性质,这有利于拓展离子液体的实际应用范围。无论是纯离子液体还是其和水形成的混合物,它们的特殊物理化学性质本质上取决于阴离子、阳离子以及水分子之间的相互作用,主要包含静电力、范德华力和氢键这三种作用。相比于静电力和范德华力,氢键具有局域性和取向性的双重特点,使得实验表征手段往往难于直接观察氢键性质。然而,分子动力学模拟能够帮助人们从分子水平上获取复杂溶液体系中的结构、动力学以及氢键行为的微观信息,为现有的实验结果提供深入的解释。因此,本论文主要是采用分子动力学模拟方法研究纯硝酸乙基氨(Ethylammonium nitrate,EAN)质子化离子液体及其水溶液混合物中的结构性质,动力学性质及其氢键行为。首先,本工作采用分子动力学模拟方法研究了:在300 K到400 K范围内,不同温度对纯EAN离子液体的结构性质、动力学性质以及氢键行为的影响。模拟结果表明温度的变化几乎对质子化EAN离子液体的结构性质没有影响,这预示着温度的升高并不会改变阴阳离子之间的静电作用力和范德华力。然而,温度的升高却对离子液体的动力学性质的影响非常强烈。当温度从300 K升高至400K时,计算结果发现阴阳离子之间的氢键强度急剧地减弱,从而导致阴阳离子的扩散系数和转动速率都增大了近一个数量级。与此同时,本工作还发现阴阳离子都会表现出很明显的次扩散行为。相应地,通过计算阴阳离子对的解离动力学性质发现阴阳离子对的解离速率加快了将近1个数量级,这种独特的现象主要是由于温度的升高导致阴阳离子之间的氢键强度减弱。然后,本工作将研究体系从纯EAN离子液体推广到更为复杂的EAN/水混合物体系,并采用分子动力模拟方法系统地研究组成对混合体系的结构性质、动力学性质以及氢键行为的影响。总共计算了19个EAN/水混合体系中NH3+-NO3-,NH3+-H_2O,NO3--H_2O和H_2O-H_2O这四种氢键的时间相关函数,结果表明这四种氢键的强度都是随着水的浓度的增加而减弱,以及阳离子、阴离子和水分子周围的氢键结构也发生显著的变化。与此同时,随着水的浓度增加,阳离子、阴离子和水分子的扩散速率和转动速率都加快。另一方面,当水的质量分数小于10wt%时,这四种HB强度的顺序为NH3+-NO3-NO3--H_2ONH3+-H_2OH_2O-H_2O,而当水的质量分数大于10wt%时,相应的顺序为NH3+-NO3-NH3+-H_2ONO3--H_2OH_2O-H_2O。由于NO3--H_2O和NH3+-H_2O的氢键在低的和高的水浓度下的相反排列顺序,以及阳离子和阴离子周围的氢键结构的变化不同,从而导致阳离子和阴离子与水的扩散系数的偏差随着水的浓度增加而越来越大,这非常有利于阳离子和阴离子的离子对分解。此外,离子迁移率和离子浓度之间的竞争效应导致EAN/水混合物的离子电导率最初随着水的质量分数增加而增加,而当水的质量分数超过70wt%时,离子的电导率又急剧地下降。因此,本研究工作从微观角度揭示了,在质子化离子液体中,阴阳离子之间的氢键作用相比于静电作用和范德华作用有着相同的重要性。同时在分子水平上揭示了EAN/水混合中氢键行为随着组成的变化规律,并建立了氢键行为变化与相关结构和动力学性质之间的相互关系,这对实验科学家研究离子液体及其水混合物中独特的物理化学性质变化有着一定的理论指导意义。
[Abstract]:In recent years, ionic liquids have attracted much attention due to their unique physical and chemical properties, such as low vapor pressure, high thermal stability, non combustible and high conductivity, and more importantly, researchers have found that different functional types of ionic liquids can be obtained by changing the structure and composition of the ions and ions. In addition, the addition of a small amount of water can also greatly change the ions. The physical and chemical properties of liquids are beneficial to the expansion of the practical applications of ionic liquids. Their special physical and chemical properties, whether pure ionic liquids or mixtures of water and water, are essentially dependent on the interaction of anions, cations and water molecules, including three kinds of static electricity, Vander Ed Ley and hydrogen bonds. Compared to the static power and Fan Dehua force, the hydrogen bond has the dual characteristics of local and orientation, which makes the experimental characterization difficult to directly observe the hydrogen bond properties. However, the molecular dynamics simulation can help people obtain the microstructure, dynamics and hydrogen bond behavior in the complex solution system from the molecular level. The existing experimental results provide an in-depth explanation. Therefore, this paper mainly uses molecular dynamics simulation to study the structural properties, kinetic properties and hydrogen bond behavior of pure nitric acid ethyl ammonia (Ethylammonium nitrate, EAN) protonated ionic liquids and their aqueous solutions. First, the molecular dynamics simulation method is used in this work. The effects of different temperatures on the structural properties, kinetic properties and hydrogen bond behavior of pure EAN ionic liquids were investigated in the range of 300 K to 400 K. The simulation results show that the temperature changes almost have no effect on the structural properties of the protonated EAN ionic liquids, which indicates that the temperature rises and does not change the electrostatic effect between the Yin and yang ions. Force and Fan Dehua force. However, the increase of temperature has a strong influence on the kinetic properties of ionic liquids. When the temperature rises from 300 K to 400K, it is found that the hydrogen bond strength between the Yin and yang ions decreases sharply, which leads to the increase of the diffusion coefficient and rotation rate of the Yin and yang ions by nearly one order of magnitude. It is also found that both yin and yang can show obvious secondary diffusion behavior. Accordingly, by calculating the dissociation kinetics of yin and yang ions, it is found that the dissociation rate of yin and yang ions accelerates nearly 1 orders of magnitude. This unique phenomenon is mainly due to the decrease of the hydrogen bond strength between the Yin and yang ions because of the increase of temperature. In this work, the research system is extended from pure EAN ionic liquid to a more complex EAN/ water mixture system, and the molecular dynamic simulation method is used to systematically study the structural properties, dynamic properties and hydrogen bond behavior of the mixed system. A total of 19 EAN/ water mixtures are calculated, NH3+-NO3-, NH3+-H_2O, NO3--H_2O and H. _2O-H_2O the time dependent function of these four hydrogen bonds shows that the strength of the four hydrogen bonds is weakened with the increase of water concentration, and the cation, the hydrogen bond structure around the anion and water molecules also changes significantly. At the same time, the diffusion rate and rotation of the cation, anion and water molecules as the concentration of water increases. On the other hand, when the mass fraction of water is less than 10wt%, the order of these four HB intensities is NH3+-NO3-NO3--H_2ONH3+-H_2OH_2O-H_2O, and when the mass fraction of water is greater than 10wt%, the corresponding order is the hydrogen bond of NO3--H_2O and NH3+-H_2O at low and high water concentrations when the water content is greater than that of the NH3+-H_2O. In the opposite arrangement, the changes in the hydrogen bond structure around the cation and anions are different, which leads to the increase in the diffusion coefficient of cations and anions and water as the concentration of water increases, which is very beneficial to the ion pair decomposition of cations and anions. The ionic conductivity of the EAN/ water mixture should be increased initially with the increase of the water mass fraction, and the conductivity of the ions drops sharply when the water mass fraction exceeds 70wt%. Therefore, this study reveals from the microscopic point of view that in the protonated ionic liquid the hydrogen bond between the negative ions is compared to the electrostatic action and the effect of the hydrogen bond in the protonated ionic liquid. The Fan Dehua action has the same importance. At the same time, the variation of hydrogen bond behavior in the EAN/ water mixture is revealed at the molecular level, and the relationship between the change of hydrogen bond behavior and the related structure and the kinetic properties is established. The experimental scientists study the unique physical and chemical properties of the ionic liquid and its water mixture. The qualitative change has a certain theoretical guiding significance.
【学位授予单位】：江西师范大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O645.1

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