【作者】 于海青；

【导师】 李辉；

【作者基本信息】 山东大学 ， 材料加工工程， 2011， 博士

【摘要】 纳米受限空间水的输运行为与物理、化学、化工、环境以及生命科学等领域的许多关键问题密切相关,理解其微观输运机制对微纳器件的设计及制备至关重要,近年来成为多个领域研究的热点。一方面由于受到纳米空间的限制,水的物理化学特性与宏观体相水的差异很大,纳米受限空间的尺寸、结构及材质对水的结构和动力学过程有重要影响,探索纳米尺度下水的结构和输运行为,有助于人们认识水在受限空间内输运的本质特征及动力学规律。另一方面,建立在对水在受限空间的结构和输运行为理解基础上的纳米技术在未来纳米器件的应用中将起着关键的角色,既可以制作纳米流体装置,又可以用来组装生物传感器、微化学反应器,还可以作为微流量计来控制水传输和药物控释。若能从分子的层次深入研究并掌握水在纳米受限空间中的输运机制和微观传递规律,对于理解受限流体在一些纳米通道中的输运具有非常重要的意义,同时也为分子尺度纳米器械的设计与制作(如纳米流体热管、海水净化设备等)提供坚实的理论依据。因此对纳米空间水输运行为的研究与认识就显得非常迫切。然而,经典的理论往往只适用于描述宏观的现象。在纳米通道中,由于受到空间限制,水的输运性质同宏观体相的输运有很大的差异,而且很难通过实验方法直接测定。本论文以水和纳米管为重点研究对象,通过分子动力学模拟(moleculardynamics simulation, MD)的方法对水分子在受限空间内的输运行为进行了深入的研究和探讨,分析外力驱动水在碳纳米管内的输运机制,预测水在异型碳纳米管内的输运方式,探索纳米管的材质对水分子输运行为的影响规律,论文内容主要分以下四个方面：(1)研究甲烷分子驱动水分子在单一直径碳纳米管内的输运行为,探讨了甲烷分子的数量和碳纳米管的尺寸对水分子在管内输运行为的影响。结果发现随着甲烷分子的不断扩散,受限在碳纳米管内的水分子在甲烷分子吸引力的作用下,会跟随甲烷分子一起运动。甲烷分子和水分子之间的吸引力与碳纳米管和水分子之间的吸引力竞争的结果决定了水分子在管内运动的方向。当甲烷分子和水分子的吸引力大于碳纳米管和水分子的吸引力时,水分子会在甲烷分子的牵引下持续运动直至离开碳纳米管。甲烷分子的数量越大,对水分子的吸引力越大,水在碳纳米管内的输运速率随着甲烷分子数量的增加而增大。相反,在甲烷分子数量不变的情况下,碳纳米管直径的增加不利于水分子在管内的输运,水分子在管内的运动速度随着管径的增大而降低。随着甲烷分子持续地扩散导致甲烷分子对水分子吸引力的降低,水分子在管内的位移会出现明显的“回缩”现象。(2)以(5/7)拓扑缺陷对接的多直径碳纳米管为例,研究甲烷分子驱动水分子在不同直径对接的碳纳米管内的输运行为,结果发现碳纳米管的接合区域和接头数量对水分子在管内的输运有很大影响。不同直径对接的碳纳米管会给甲烷分子提供扩散驱动力,水分子在甲烷分子的作用下,跟随甲烷分子在管内运动。当水分子到达接合区域时,接合区域会阻碍水分子在管内的输运。如果甲烷分子对水的吸引力克服了接合区域的阻碍,甲烷分子就能拖动水分子穿过接合区域到达另一段直径的碳纳米管内,否则,水分子将被卡在接合区域。对于只有一个接合区域的瓶状碳纳米管来讲,管的尺寸越小,其接合区域给水分子输运造成的障碍越大,水分子越难穿过接合区域。甲烷分子拖动水分子在瓶状管内的输运速度随着管子尺寸的增加而增大。而对于拥有较多接头的阶梯状碳纳米管,前100 ps,由于甲烷分子在管内刚刚开始扩散,导致水分子在阶梯状碳纳米管内的输运行为类似于水分子在瓶状碳纳米管内的输运行为,甲烷分子驱动水分子在阶梯状碳纳米管内的输运速度随体系尺寸的增加而增大；100 ps以后,水分子在管内的运动速度随阶梯状碳纳米管尺寸的增加而降低。由于甲烷分子在小尺寸阶梯状碳纳米管内的扩散比较慢,甲烷分子达到扩散平衡的时间较长,为水分子在管内运动提供了较持久的驱动力,拖动水分子在管内运动的时间就越长。小尺寸的阶梯状碳纳米管内水分子运动的位移大于在大尺寸碳纳米管内的位移。值得注意的是,甲烷分子驱动水分子在阶梯状碳纳米管内运动的位移远大于在瓶状碳纳米管内的位移。这是因为较多的接合区域会不断地给甲烷分子提供扩散驱动力,甲烷分子不断地扩散,从而导致水分子在甲烷分子的拖动下不断地穿过一个又一个接合区域。(3)建立异型(Y型/T型)单壁碳纳米管的模型,预测水分子在对称异型管内的输运方式。由于异型碳纳米管是对称的,处于其结合区域的水分子受到两个分支的吸引作用相同,第一个水分子进入两个分支的机会是随机的、均等的。而且分支之间的角度θ越小,水分子进入分支R1或R2内需要克服的势垒越大,就越难进入到分支内。第一个水分子进入到分支R1后,随着两分支之间角度θ的增大,水分子产生分流的时间越长,导致了两个分支之间水分子的个数相差越大,输运的不对称性越强烈。在化学势能差的作用下,水分子在两个分支之间进行间歇式前进,即前几个水分子进入R1后会停下来在平衡位置震荡,等待相同数量的水分子进入R2后,才会继续前进。水分子充满分支R1后会等待另一分支R2充满水分子,然后从出口处溢出。但是在压强的作用下,水分子在对称的异型管内会进行对称的输运。(4)分别比较了水在石墨片、碳化硅片和硅片上的接触角大小及水在碳纳米管、碳化硅管和硅管中接触角的大小,得到了不同材质的亲水／厌水性强弱顺序,并研究了水在相同尺寸的碳纳米管,碳化硅管和硅管中的输运行为。结果发现,同种材质的厌水性管子随着曲率的减小厌水性增强；而亲水性管子随着曲率的减小亲水性增强。三种不同材质的片和管的厌水性强弱顺序一致,三种管子厌水性强弱的顺序为碳纳米管>硅管>碳化硅管。水分子以单链形式自发进入并穿过这三种纳米管的先后顺序为碳纳米管,碳化硅管,硅管,这同纳米管的厌水性强弱没有相关性。水团簇以相同的初始速度在三个不同的管内运动,水团簇离开这三种纳米管的先后顺序为碳纳米管,硅管,碳化硅管,这同三个纳米管的厌水性强弱顺序一致。因此纳米管的厌水性越强越利于水团簇以“弹道”方式在纳米管内输运。本文的研究结果对于深入理解外力驱动水在纳米受限空间的输运机制,有效预测水在异型碳纳米管内的输运方式,充分认识纳米管的材质对水分子输运行为的影响规律,进而开发分子尺度的纳米器械具有重要的理论意义。更多还原

【Abstract】 Transport behavior of water in the nanoconfined space is closely related to many key issues in the fields of physics, chemistry, chemical engineering, environment and life sciences et al. It is essential to understand the transport mechanism of water in the nanoconfined space for the design and preparation of the micro-nano device. Therefore, transport behavior of water becomes research spot in many fields in recent years. On the one hand, due to the nano-space constraint, the physical and chemical properties of water are greatly different from that of the bulk water. The size, configuration, and material of the nanoconfined space have significant effect on the water dynamics. Exploring the nano-scale structure and transport behavior of water helps people to understand the essential characteristics of water transport in the confined space and the kinetics law. On the other hand, the nanotechnology based on the understanding of the structure and transport properties of water in the confined space will play key role in the nano-device application. It is can be used not only to fabricate nano-fluid device, but also to assemble bio-sensors, micro-chemical reactor, and to be a micro-flow meter to control water transport and drug release. If we can study and master the transport mechanism and micro-transfer law of water in the nanoconfined space deeply in the molecular level, which is of great significance for understanding the transport of the confined flow in the some nanochannel systems, and simultaneously, providing a solid theoretical basis for the design and production of nano-devices (for example, nano-fluid heat pipe、devices for seawater purification) in the molecular scale. The classical theory is often suitable to describe the macroscopic phenomena. In the nano-channels, due to the space constraint, the transport properties of water in the confined space is of great difference from that of the bulk water, and that is very difficult to direct determination by experimental methods. In this dissertation, we focus on the water and nanotube and study the transport behavior of water molecules in the confined space deeply. We analyze the transport mechanism of water in the carbon nanotube (CNT) driven by external force, predict the mode of water transport in the nonlinear CNTs, and explore the effect law of different materials of the nanotubes on the water transport behavior. The dissertation content is stated for four parts as follows.(1)It is studied that the transport behavior of water molecules are driven by the methane molecules in the single diameter CNT, and the effects of the number of the methane molecules and the size of the CNT on the transport behavior of the water molecules in the tube are investigated. It is found that the water molecules confined in the CNT move with the diffusion of the methane molecules due to the attractive interaction. The move direction of water molecules is decided by the result of competition between the methane-water attractive force and the CNT-water attractive force. When the methane-water attractive force is stronger than CNT-water attractive force, the water molecules could move with the methane molecules and be drawn out successfully. The greater the number of methane molecules, the greater the attraction between the methane molecules and water molecules, the transport velocity of water molecules increases in the CNT with the number of methane molecules. On the contrary, if the number of methane molecules keep constant, a increase in the diameter of the CNT is not in favor of water transport in the tube, the transport velocity of water molecules in the CNT decreases with the number of methane molecules. The continued diffusion of methane molecules results in the decrease of the attraction between the methane molecules and water molecules, which makes displacement of water molecules in the tube appear the "retraction" phenomenon.(2) Taking the (5/7) topological defects in multi-diameter end-to-end joint CNTs, it is studied the transport behavior of water molecules in the CNTs with different diameters and found that the junction region and the number of the joints have significant effect on the water transport in the nanotube. CNTs with different diameters will provide methane diffusion driving force, the water molecules will follow methane molecules to move in the tube under the water-methane interaction. When the water molecules reach the junction region, the junction region would hinder the transport of water molecules in the tube. If attractive force from the methane molecules overcomes the potential barrier caused by the diameter difference, methane molecules could drag water molecules to another compartment, otherwise, they would be jammed in the junction region. For the bottle-like CNT with one junction region, the smaller the size of the tube, the greater the transport barrier for water molecules across the junction region. The transport velocity of water molecules driven by methane molecules increases with the tube size. For the terrace-like CNTs with more junctions, methane molecules just start to diffuse in the tube within the first 100 ps, which results in the transport behavior of water molecules in the terrace-like CNTs similar to that in the bottle-like CNTs. The transport velocity of water molecules driven by the methane molecules increases with the terrace-like tube size. After 100 ps, the transport velocity of water molecules driven by the methane molecules decreases with the increase of the terrace-like tube size. Because diffusion of methane molecules is relatively slow inside the smaller size of terrace-like CNT, resulting in the longer time for diffusion of methane molecules to reach equilibrium, which provides a more durable driving force for the movement of water molecules in the tube and water molecules move for longer time in the tube. The displacement of water molecules transport in the smaller terrace-like CNT is longer than that in the bigger ones. It is worth noting that translocation displacement of water molecules in terrace-like CNTs is obviously longer than that in bottle-like CNTs. This is because more of the junction regions will provide methane with a continued diffusion force. Methane molecules can pull water molecules from one junction region to another one under the diffusion of the methane molecules.(3) Building models of Y-/T-type SWCNT to study the transport behavior of water molecules in the nonlinear CNT. Because the nonlinear CNT is symmetric, the attraction between each branch and the water molecule confined in the junction region is equal, and therefore the first water molecule enters one of the two branches randomly and fairly. Moreover, the smaller the angle 6 between the branches, the larger potential barrier for the water molecule to enter the branch of R1 or R2, and therefore the more difficult for the water molecule to enter the branches. After the first water molecule entering the branch R1, it takes the water molecules longer time to split into two flows with the increase of the angle between the two branches, which results in the greater difference of the number in the two branches and the stronger asymmetry of the water transport. The water molecules intermittently move between the two branches under the chemical potential difference. Namely, after several water molecules entering the single branch R1, they would not move forward but fluctuate in the equilibrium position. Until the same number of the water molecules enters the single branch R2, the water molecules can move in the branch R1. When the branch R1 is full of water molecules, they will wait for the branch R2 filled with water molecules, and then water molecules overflow from the two outlets of the branches. However, the water molecules will conduct symmetrical transport in a symmetrical nonlinear tube under pressure.(4) Compare the contact angle of the water droplet formed on the graphite sheets, silicon carbide (SiC) graphite-like sheets, and silicon (Si) graphite-like sheets, and the contact angle of water confined in the single-walled carbon nanotube (SWCNT), single-walled silicon carbide nanotube (SWSiCNT), and single-walled silicon nanotube (SWSiNT), respectively, and then different materials hydrophilic/ hydrophobic strength is obtained. It is observed that the water transport in the same size of SWCNT, SWSiCNT, and SWSiNT, respectively. It is found that for the same material, the hydrophobic tube becames stronger hydrophobic with the curvature of the tube decreasing, and the hydrophilic tube becames stronger hydrophilic with the curvature of the tube decreasing. The sequence of the hydrophobicity for three different materials of the sheets and the tubes is uniform. The sequence of the hydrophobicity for three tubes, from the strongest to the weakest, is SWCNT> SWSiNT> SWSiCNT. The sequence of the single water chain spontaneously entering and passing through the three nanotubes is SWCNT, SWSiCNT, SWSiNT, which is not related to the sequence of the hydrophobicity of three tubes. The water cluster moves in three tubes with an initial rate. The sequence of the water cluster leaving the three tubes is SWCNT, SWSiNT, SWSiCNT, which is accordant with the hydrophobicity sequence of the three tubes. Therefore, the stronger hydrophobic tube is more beneficial to the water transport through the tube in a "ballistic" way.It is of important theoretical significance of the results in this study for deeply understanding water transport in nanoconfined space under the external driving force, effectively predicting the transport type of water in the nonlinear CNTs, fully realizing the influence rules of different nanotube materials on the transport behavior of water, and further developing nano-molecular scale devices.更多还原