【作者】 仲秀娟；

【导师】 曹达鹏； 刘志平；

【作者基本信息】 北京化工大学 ， 化学工程与技术， 2012， 博士

【摘要】 离子液体是在绿色化学框架下发展起来的一种新型绿色溶剂,具有挥发性小、热稳定和化学稳定性好、可多次循环使用等优点,有望显著降低在传统化工过程中大量使用的有机挥发性溶剂,从而构建环境友好的新型绿色化学工艺。因此近年来离子液体受到广泛关注,在环境、化工、生物等领域的应用不断被拓展。离子液体由阴、阳离子构成,通过离子配对和引入官能团修饰,可以有效调控其物理化学性质,满足工业上特定需求。这种“可设计性”是离子液体的最大优势,目前见诸报道的阴、阳离子种类都已超过数百种,所以理论上离子液体的种类远远超过有机溶剂。若从实验上逐一合成并研究其性质,研究成本高,研究周期长；若能建立起可靠定量的离子液体结构-性质的关系,对于从数量巨大的候选物筛选满足特定需求的功能化离子液体具有重要的科学意义。分子模拟是从分子间相互作用出发研究离子液体的各种性质-微观结构之间的联系,真正贯彻“自下而上”(bottom-up)的分子设计思路。本文用分子模拟方法针对几种离子液体及其与水的混合体系开展系统研究,并分析了宏观性质-微观分布-分子结构之间的关系,对满足特定需求的离子液体的分子设计具有重要的参考意义。主要工作如下：1、在AMBER力场的基础上,开发了一套由1-烷基-3-甲基咪唑阳离子[Cnmim]+(n=1,2,4,6,8,10)和[Tf2N]-、[CF3SO3]-、[CF3CO2]-、[CH3CO2]-、[CH3SO4]-、[BF4]-、[PF6]-7种阴离子的联合原子力场(UnitedAtom, UA)。针对目前大多数离子液体力场预测动力学性质与实验数据相差一个数量级的问题,提出在该分子力场中考虑平均极化效应的电荷拟合方案,即用双离子对的从头计算结果拟合离子液体分子力场中的原子电荷,从而间接考虑了阴、阳离子间较强极化作用的平均效应,大大改善了分子模拟的预测结果。此外,在该联合原子的分子力场中,提出了将UA的质心作为UA基团的位置,在此基础上拟合了力场中其它参数,从而使联合原子力场更接近真实的分子结构。2、用分子动力学模拟的方法对纯离子液体联合原子力场(UA)进行验证。7种阴离子的纯离子液体在298～373K温度范围内的模拟密度与实验值吻合得非常好,其平均相对偏差约为1%。蒸发焓(△Hvap)的模拟结果与最近实验上不同方法测定的△Hvap都比较吻合,且重现了实验上蒸发焓随咪唑烷基侧链增长的变化趋势。对于动力学性质(如扩散系数和粘度),目前非极化力场预测的结果与实验数据相比大多有数量级的差别,采用本文提出UA力场模拟的结果与实验数据符合很好,并且正确描述了温度和烷基链长对扩散系数和粘度的影响趋势。结果表明,本工作在预测性质的准确度和计算复杂度以及力场可移植性之间寻找平衡,构建了一个模拟结果可靠、计算量适中的离子液体力场。3、在不引入任何额外参数调整的情况下,通过常规的Lorentz-Berthelot (LB)混合规则获得不同原子间的交叉作用参数。用于[C4mim][BF4]-H2O混合体系的模拟,本文提出的联合原子力场能准确预测该体系正的过量体积和过量焓以及溶液粘度随浓度的变化,反映出该力场的良好扩展性。通过研究该体系的微观结构,发现随着水浓度增加,水在混合物中的形态发生很大变化,可明显定位出两个转折点：在x2<0.2的低水浓度溶液中,大多数水分子孤立存在于由阴、阳离子作用形成的极化网络结构中；在0.2<x2<0.8的溶液中,水分子通过自聚形成水分子簇；x2≥0.8的溶液中,几乎所有水分子聚集成一个大的水分子簇,并且发现有离子液体的自聚,在0.9<x2<0.95时,自聚程度最大。4、采用前述建构力场的方法,模拟很好重现了C2甲基取代后密度减小、蒸发焓和粘度增大以及扩散速度减小等实验现象。针对文献提出的粘度增加来源于烷基链转动能垒增加的观点,本工作通过假想物质的模拟研究,表明仅增加转动能垒不会导致粘度的大幅变化,仅占粘度总增加量的13%。粘度增加主要来源于电荷的重新分布(占粘度总增加量的41%)和范德华参数的改变(占粘度总增加量的46%)。更多还原

【Abstract】 Ionic liquids （ILs） are a class of novel environmental benign “greensolvents”, which is developed in the framework of green chemistry. ILs arenon-flammable, pollution-free, easy separation and easy recovery, which makethem are regarded as environment-friendly green solvent. In recent years, thestudies of ionic liquid mainly focus on the chemistry and separation process.Moreover, their properties can be tailored for specific applications by differentcombinations of cations and anions or altering their structure, which makethem as “designer green solvents”. However, the traditional trial-and-errorway to find new ILs or optimize their combinations leads to the large cost onthe syntheses and experiments. The understanding of how the structure and thechemical constitute of ILs effect their properties is very important. Molecularsimulations on the atomistic level provide an effective way in screening alarge number of candidate materials. In this work, several ILs was studied bymolecular dynamics simulations. The main research results are summarized asfollows:1. We proposed a cost-effective united-atom （UA） force field for ionicliquids （I Ls） composed by1-alkyl-3-methyl-imidazolium cations （[C_nmim]⁺, n=1¹0） and seven kinds of anions, including tetrafluoroborate （[BF₄]）,hexafluorophosphate （[PF6]）, methylsulfate （[CH₃SO₄]-）,trifluoromethylsulfonate （[CF₃SO₃]-）, acetate （[CH₃CO₂]）, trifluoroacetate（[CF₃CO₂]-）, and bis（trifluoromethylsulfonyl）amide （[NTf2]-）. The chargedistribution was derived from the RESP fitting to the ab initio calculations ofion pair dimers in this work. Thus, the charge transfer and polarizability isaccounted for in an effective way. In addition, take the COM of the UA groupas the center of UA, and then fitting other parameters. It was found wellreproduce the liquid densities of all the ILs studied in this work.2. Molecular dynamics （MD） simulations were performed over a widerange of temperatures to validate the force field. The liquid densities werepredicted very well for all of the ILs, with typical deviations less than1%. Thesimulated enthalpies of vaporization, Hvap, are also in good agreement withexperimental values, with slight overestimation about5kJ/mol. More criticalvalidations were made by comparing the transport properties, including theself-diffusion coefficient and shear viscosity, between simulation andexperiments. The simulated self-diffusion coefficients and viscosities for allthe ILs studied in this work are in good agreement with experiments. Theforce field also showed good performance on the temperature and alkyl chainlength dependence of these properties. The viscosities of ILs is graduallyincreased with the alkyl length of cations, with a minimum of the viscosity atn=2, which good agreement with experiments. 3.Extensive molecular dynamics simulations for [C₄mim][BF₄]/water mixtures are performed using above force field. Cross parametersare obtained by the Lorentz Berthelot （LB） combining rules withoutfurther optimization. The positive excess molar volume and excess molarenthalpy, as well as their dependence on temperature, are well reproducedin our simulations.个In addition, the local structure, as characterized byradial distribution functions （RDFs）, spatial distribution functions （SDFs）and water clustering analysis, is used to elucidate the connection betweenstructure evolution and the macroscopic observations in simulations. Whenx₂（mole fraction of water）<0.2, most of water molecules are isolated eachother and experience a local environment in the polar network nearly thesame as that in neat IL. In the concentration range of0.2<x₂<0.8, watermolecules tend to form clusters by self-aggregation. When x₂≥0.8, ILis percolated by water molecules.4.Using the above method of developing force field molecularsimulation well reproduce the decreased densities, self-diffusioncoefficients and increased heats of vaporization and viscosities with themethylation at position of C2. In addition, it is pointed that the increasedviscosity is caused by the flexibility of alkyl chains of cation. In order tounderstanding the mechanism of the increased viscosity of the C2methylation, the author assume that there exists another two aritificial ILs.It was found that, only increase rotation energy barrier will not lead to an obvious increase of viscosity, which account for only13%of the totalincrement of viscosity. The increased viscosity mainly caused by theredistribution of charges （account for41%） and the Van der Waalsinteraction （account for46%）.更多还原