节点文献
含CO2复杂体系热力学性质的分子动力学模拟
Molecular Dynamics Simulation of the Thermodynamic Properties of the Complex Systems with CO2
【作者】 王松；
【导师】 银建中；
【作者基本信息】 大连理工大学 ， 化工过程机械， 2012， 硕士
【摘要】 绿色化学是实现化学工业可持续发展的必然选择,超临界CO2作为一种高效的绿色溶剂,其应用和发展已获得越来越多的关注。超临界CO2限于自身无极性的特性,对极性化合物、生物大分子等的溶解能力较低,故将超临界CO2与极性溶剂联合使用便成为拓展超临界CO2应用范围的重要手段。本文采用分子动力学方法,重点研究CO2与极性物质(短链醇、离子液体、水等)构成的多元复杂体系的输运特性、体积变化及微观结构等通过常规实验方法和模型公式计算难于获得或不能获得的信息,为拓展超临界CO2的应用范围提供理论依据,全文共分三部分进行,主要工作如下：(1)CO2/共溶剂体系的热力学性质的模拟。以超临界流体沉积法制备Ag介孔氧化硅纳米复合材料为背景,首次通过分子动力学方法模拟323.15K时乙醇/乙二醇/超临界CO2三元体系,并分析乙二醇用量、乙醇用量及压力等因素对体系粘度、扩散系数、溶解度参数等造成的影响,试图解释三元溶剂体系的组成和压力等因素对担载效果的影响。分析发现加入共溶剂能够明显增加CO2对前驱物AgNO3的溶解能力,同时降低体系扩散性,提高混合物临界条件。乙二醇对提高溶解能力的作用大于乙醇,但乙二醇对体系粘度增大的贡献也明显大于乙醇,并且过量的乙二醇将导致体系分相产生。同等温度条件下,提高压力有助于提高体系的溶解能力,但同时也降低了体系的扩散性。(2)离子液体吸收CO2和SO2的分子动力学模拟。以离子液体与气体小分子混合物的工业应用为背景,采用分子动力学方法模拟313.15K时高压条件下[bmim][PF6]/CO2和[bmim][NO3]/CO2混合物及293.15K时常压条件下[bmim][PF6]/SO2和[bmim][BF4]/SO2混合物。重点考察了CO2和SO2对离子液体的扩散系数、粘度、体积变化和微观结构的影响。结果表明,SO2和CO2均能有效降低离子液体的粘度,提高其扩散系数,但对离子液体体积产生极小的影响,当气体溶质分子摩尔分数达到约0.5时,混合物体积膨胀度小于35%。微观结构分析结果表明,气体溶质对离子液体离子间相互作用的影响微小,证实普通离子液体对CO2和SO2的吸收是气体溶质占据离子间自由空间的过程。(3)含AOT表面活性剂的超临界CO2反胶束体系模拟。以超临界二氧化碳反胶束耦合膜分离提取发酵液中1,3-丙二醇为背景,首次采用分子动力学方法研究水/AOT/乙醇/超临界CO2反胶束体系并试图分析其对1,3-丙二醇的增溶特性。证明水/AOT/乙醇/scCO2体系中的聚团分为三层结构：水核中心区；水、AOT头基(含Na+)、部分AOT尾端链层共存层；AOT尾端链、CO2分子共存层。乙醇分子部分嵌在AOT尾端链,部分进入水核,只有少量乙醇分子游离于CO2主体相中。对1,3-丙二醇/水/AOT/乙醇/超临界CO2体系的模拟表明选择性增溶1,3-丙二醇的条件较为苛刻,但仍有少量1,3-丙二醇能够进入聚团水核中。
【Abstract】 Green Chemistry is the inevitable choice to realize the sustainable development of chemical industry. As one of the high efficient green solvents, supercritical CO2’s application and development are gaining more and more attention. However, CO2is a poor solvent for polar compounds and biological macromolecules for it is a non-polar molecule. Utilizing CO2combined with polar solvent is one of the important methods to expand its application. In order to provide a theoretical basis to the application range expansion of supercritical CO2, molecular dynamics simulations are carried on the complex systems with CO2and polar substances (short-chain alcohols, ionic liquids, water, etc.) to analyze the transport properties, volume change and micro-structure which are difficult or can’t be obtained by conventional experimental methods and model formula. The main work and results are as follows:(1) Simulation on thermodynamic properties of CO2with co-solvent. Based on the experiments of preparing Ag/SiO2nanocomposition using the method of supercritical fluid deposition, molecular dynamics simulations are carried on ethanol/ethylene glycol/supercritical CO2systems for the first time. The effect of glycol dosage, alcohol dosage and pressure on viscosity, diffusion coefficient, solubility parameters of the system are analyzed, trying to explain the effect of composition and pressure and other factors of the ternary solvent system on the deposition effect. The results show that adding co-solvents can significantly increase the solubility of AgNO3in CO2, while reducing the system’s diffusion and increasing the mixture’s critical condition. The ability of glycol to improve the solubility of CO2is greater than ethanol though the contribution to system’s viscosity of glycol is also significantly greater than ethanol. Moreover, excessive glycol will result in phase separation. Raising pressure will improve the solubility of the system, but also reduce the diffusion of the system at the same temperature.(2) Molecular dynamics simulations on ionic liquids absorbing CO2and SO2. Based on the industrial applications of the mixtures with ionic liquids and gas, molecular dynamics simulations are carried on [bmim][PF6]/CO2and [bmim][NO3]/CO2mixture at313.15K and high pressure as well as [bmim][PF6]/SO2and [bmim][BF4]/SO2mixtures at293.15K and low pressure. The effect of CO2and SO2on the ionic liquids’diffusion coefficient, viscosity, volume change and micro-structure are mainly investigated. The results show that the SO2and CO2can effectively reduce the viscosity of the ionic liquids and improve its diffusion coefficient, but have little effects on the ionic liquid volume. When the gas solute molecules fraction is about0.5, volume expansion of the mixtures is less than30%. The results of microstructure analysis indicate that gas solutes have no significant effect on the interaction between the ionic liquids, and confirm that the absorption of ordinary ionic liquids for CO2and SO2is the gas solutes occupying the void spaces in the ionic liquids.(3) Simulation on the supercritical CO2reverse micelles with surfactant of AOT. Based on the background that separate1,3-propanediol from fermented liquid using supercritical CO2reverse micelles with membrane, this article investigates the water/AOT/ethanol/supercritical CO2reverse micelles system by molecular dynamics method and makes an attempt to analyze its solubilization properties of1,3-propanediol for the first time. The micro-structure of reverse micelles demonstrate that water/AOT/ethanol/supercritical CO2reverse micelles exist three layers:an aqueous core phase; an interfacial region among the water, the headgroups of AOT (including Na+) and part of the AOT tails; an interfacial region between the AOT tails and the CO2phase. Some ethanol molecules embed in the AOT tails and some enter into the water core region, while only a small amount of ethanol molecules dissociate in the bulk CO2phase. For1,3-propanediol/water/AOT/ethanol/supercritical CO2system, the simulation results indicate that the conditions of the selective solubilization are rigorous, but there are still a few1,3-propanediol molecules can enter into the water core of reverse micelles.