【作者】 田兴玲；

【导师】 唐孝威；

【作者基本信息】 浙江大学 ， 凝聚态物理， 2013， 博士

【摘要】 碳纳米管具有规则的中空结构。因此,它可以被用来封装很多种类的分子,如氢气、水分子,以及包括药物分子在内的有机小分子等。近年来,人们开始注意到,可以利用碳纳米管来分选不同种类的分子。在本文中,借助于分子动力学模拟,我们分别以醇类分子与水分子、乙烷和乙烯分子为例,研究了不同种类的小分子在单壁碳纳米管内的“竞争性吸附”,指出了分子与碳管管壁间的色散相互作用在竞争性吸附中的关键作用,并探讨了我们所观察到的选择性吸附现象对醇水分离技术、乙烷乙烯分选技术的启示意义。我们的研究包括两方面的内容：(一)醇类诱导的碳管“去润湿”及其对醇水分离技术的启示：醇是化学工业中重要的产品；但是由于醇的亲水性,把它们从醇和水的混合物中分离出来非常困难。借助于分子动力学模拟,我们观察到显著的纳米尺度下的“去润湿”现象。我们使用的醇是常见的直链一元醇(包括C1-C6正醇)和丙三醇(甘油),也用苯酚来进行比较。模拟表明,当单壁碳纳米管浸没在醇的水合溶液中的时候,即使醇的浓度不高(1M),各种醇分子也都能够引起碳纳米管的脱水(去润湿),并且在碳管内积聚。特别地,在窄的单壁碳纳米管,即(6,6)和(7,7)碳管内部,大部分醇类都能够形成几乎均一的一维分子链。能量分析表明,碳管对醇类分子显著的选择性,归因于醇分子和碳管间的色散相互作用强于水和碳管间的色散相互作用。有趣的是,宽的单壁碳管,即(13,13)碳管,对醇的选择度随着醇分子的碳原子数的增加呈e指数增加；对于窄的单壁碳纳米管,它对甲醇、乙醇和丙醇的选择度很高,并且当醇中的碳原子数是3的时候达到最大值。我们讨论了这些现象背后的物理机制及其对醇水分离技术的启示意义。(二)乙烷、乙烯分子在碳管中的竞争性吸附：乙烷—乙烯分离是化学工业中一个非常重要的过程。传统上,这个过程通过低温蒸馏法实现,是非常耗能的。吸附性分离是一种节能环保的替代方法。我们运用分子动力学模拟,研究室温下等摩尔的乙烷和乙烯混合气体在不同直径的单壁碳纳米管内的竞争性吸附。我们发现,对窄的碳管,即(6,6)和(7,7)碳管的情况,碳管对乙烷的选择度fselec的值(碳管内乙烷的分子数比上乙烯的分子数)能达到3.1和3.7。这么高的选择度与很多研究人员的直觉相反——很多人认为,由于乙烷和乙烯具有相同的碳原子数,要通过色散相互作用来分离乙烷—乙烯混合物是很困难的。我们所观察到显著的选择性吸附现象,是由于在极端纳米限阈环境下,乙烷比乙烯多出来的两个氢原子与碳管管壁间的色散相互作用发挥了显著作用。有趣的是,(8,8)碳纳米管会选择性的吸附乙烯(fselec=0.6)。对于直径更宽的单壁碳管,选择度fselec趋向于1。我们还讨论了上述现象的物理机制,还有乙烷、乙烯分子填充碳管的动力学过程,以及系统达到平衡后,气体分子在纳米限阈下的动力学。我们的发现表明,乙烷—乙烯的有效分离可以通过使用具有合适直径的单壁碳纳米管组成的膜来实现。本论文的研究将有助于人们更好地理解小分子在碳纳米管内的吸附性质和动力学行为,并为人们利用碳纳米管分选不同种类的小分子,提供有益的理论借鉴。更多还原

【Abstract】 The carbon nanotubes (CNTs) possess well-difined hollow structures and thus serve as desirable materials for molecules, such as hydrogen, water, and small organic molecules including drug molecules, adsorbing into their interiors. In recent years, it has been realized that the CNTs can be used to separate different kinds of molecules. In this thesis, on the basis of molecular dynamics (MD) simulations, we have studied the interesting phenomena of competitive adsorption between different kinds of molecules inside the single-walled carbon nanotubes (SWNTs), using the cases of alcohol/water, and enthan/ethylene systems for illustrations. We find that the dispersion interactions of inner molecules with the SWNT wall play a key role in the competitive adsorption. The implications of our findings for the dehydration of aqueous alcohols and the ethylene/ethane separation are also discussed. This thesis contains two sections:i) Alcohol-induced drying of carbon nanotubes and its implications for alcohol/water separation:Alcohols are important products in chemical industry, but separating them from their aqueous solutions is very difficult due to the hydrophilic nature of alcohols. Based on MD simulations, we observe a striking nanoscale drying phenomenon and suggest an energy-saving and efficient approach toward alcohols/water separation by using SWNTs. We use various common linear alcohols including C1-C61-alcohols and glycerol for demonstration (the phenol is also used as comparison). Our simulations show that when SWNTs are immersed in aqueous alcohols solutions, although the alcohols concentration is low(1M), all kinds of alcohols can induce dehydration (drying) of nanotubes and accumulate inside wide [(13,13)] and narrow [(6,6) or (7,7)] SWNTs. In particular, most kinds of alcohols inside the narrow SWNTs form nearly uniform1D molecular wires. Detailed energetic analyses reveal that the preferential adsorption of alcohols over water inside nanotubes is attributed to the stronger dispersion interactions of alcohols with SWNTs than water. Interestingly, we find that for the wide SWNT, the selectivity for1-alcohols increases with the number of alcohol’s carbon atoms (Ncarbon) and exhibits an exponential law with respect to Ncarbon for C1-C51-alcohols; for narrow SWNTs, the selectivity for1-alcohols is very high for methanol, ethanol, and propanol, and reaches a maximum when Ncarbon=3.The underlying physical mechanisms and the implications of these observations for alcohols/water separation are discussed. Our findings provide the possibility for efficient dehydration of aqueous alcohols (and other hydrophilic organic molecules) by using SWNT bundles/membranes.ii) Competitive adsorption between ethane and ethylene inside SWNTs: Ethylene/ethane separation is a very important process in chemical industry. Traditionally, this process is achieved by cryodistillation which is extremely energy-intensive. The adsorptive separation is an energy-saving and environmentally benign alternative. Herein we employ MD simulations to study the competitive adsorption of equimolar mixture of gaseous ethane and ethylene inside single-walled carbon nanotubes (SWNTs) of different diameters at room temperature. We find that for narrow SWNTs, i.e.,(6,6) and (7,7) SWNTs, the selectivities towards ethane, fselec, can reach values of3.1and3.7, respectively. Such high selectivities are contrary to the opinion of many researchers that the adsorptive separation of ethylene/ethane mixture by means of dispersion interaction is difficult due to the same carbon number of ethane and ethylene. The key for our observation is that the role of dispersion interaction of ethane’s additional two hydrogen atoms with the SWNT becomes significant under extreme confinement. Interestingly, the (8,8) SWNT prefers ethylene to ethane with fselec=0.6. For wider SWNTs,fselec converges to～1. The mechanisms behind these observations, as well as the kinetics of single-file nanopore filling and kinetics of confined gas molecules are discussed. Our findings suggest that efficient ethane/ethylene separation can be achieved by using bundles/membranes of SWNTs with appropriate diameters.We believe that the current study is helpful to better understand the adsorptive and dynamical properties of small molecules confined within carbon nanotubes, and provides a usefully theoretical insight into the experimental realization of adsorptive separation of different small molecules using carbon nanotubes.更多还原