CAS OpenIR  > 中科院上海应用物理研究所2004-2010年
水在纳米通道中的动力学行为研究
Alternative Titlemolecular dynamics studies on the behaviour of water confined in nanotube
陆杭军
Subtype博士
Thesis Advisor方海平
2008-05-30
Degree Grantor中国科学院上海应用物理研究所
Place of Conferral上海应用物理研究所
Keyword分子动力学 纳米水通道 Fokker-planck方程 碳纳米管 一维水链
Abstract水受限在纳米尺度的孔道中表现出与宏观体相水(bulk water)不一样的性质,其具体微观运动规律与机制引起了广泛的兴趣。在特定半径的纳米通道中,水分子是以单分子链的形式排列,它们通过氢键网络联系在一起,做协调一致的运动。如果改变纳米通道的半径或纳米通道与水之间的相互作用,将引起纳米通道内的液汽相变和开状态与关状态的转换。另外,在纳米通道中水的密度分布常呈现出驻波状分布,发生在纳米尺度下的这些有趣现象背后的规律与机制至今还不是很清楚。研究这些水在纳米尺度通道中发生的有趣现象背后的规律与机制,对人们理解许多重要的纳米孔道中溶液输运过程非常有帮助,也有助于对某些生物水通道的结构和功能的理解。 然而,许多纳米通道系统是极其复杂的,比如生物水通道。直接研究纳米尺度的生物水通道中水的特殊行为的分子机制很不容易。近年来,基于纳米碳管(CNT)的纳米通道,被广泛用于生物水通道蛋白的模型系统,来研究生物水通道蛋白的一些基本规律。我们以直径为8.1Å的(6,6)碳纳米管为模型,主要研究了一维水链在纳米通道中的性质。生物水通道对水通透的调控主要通过两种手段: 改变通道几何形状和电荷的分布(带电残基的位置)。本博士论文主要通过挤压改变碳管几何形状和通过外加电荷并改变其位置等手段,集中研究了通道几何形状和电荷位置的改变对水行为的影响。 跟我们课题组万荣正等人合作,我们用分子动力学模拟对碳纳米管受不同形变情况下管内水分子的性质进行研究,发现了良好开关特征。而开关性质与抗干扰能力发现与水分子的驻波状密度分布有关联,但是,其具体物理机制还不是很清楚。在此基础上,我们对其进行了更深入和系统地研究。 对宏观上的管道来说,如果两边没有压力差,水在里面的分布是均匀的,而在受限的直径为8.1 Å,长为13.4 Å的纳米管道中水的密度分布却表现出驻波状分布。当我们加长碳纳米管1.2 Å,驻波状又变弱,特别在中部几乎消失,而当碳纳米管受压变形后驻波状又重新出现。这就引起对水在碳纳米管中的波形分布产生机制的更大兴趣。当碳管长度增加水的密度分布会怎样变化?其良好的开关机制还存在吗?计算机模拟发现开关机制并没有受长度的变化受明显的影响,同时,我们提出了一个简单的理论模型探讨了驻波状密度分布的物理根源,解释了这些有趣的密度变化现象。通过计算机模拟与理论分析得出结论是:水分子在纳米孔道中呈驻波状分布的原因主要是氢键与纳米管的约束。我们的研究结果有助于理解在生物水通道和其他纳米通道中类似的现象。 为了考察这种开关机制的抗干扰能力,我们用分子动力学模拟了外部周期性扰动对纳米管中一维水链的运动的影响。结果发现存在一个临界频率fc (大约为1333 GHz),它对水在碳纳米管中输运性质起了比较重要的作用。当外部扰动频率小于1333GHz时,其水流通量flow、净流量flux、氢键数目、水链的翻转频率几乎不受外部扰动影响,说明碳纳米管中的一维水链的运动具有很好的抗干扰能力。对外部扰动有如此好的屏蔽效果,主要是因为纳米管中的一维水链通过较强的氢键相连。我们的研究讨论可能对理解生物系统为什么能够在充满噪音的环境中精确实现信号传输有帮助。 我们还用Fokker-Planck方程对(6,6)碳纳米管中一维水链的输运性质进行了研究。探讨了Fokker-Planck方程在纳米尺度的有效性。通过与模拟结果比较,发现Fokker-Planck方程在一定程度上能基本刻画碳管形变对一维水链运动的调控,同时从另外方面进一步说明了我们的模拟得到的良好开关行为规律是可靠的。另外,还探讨了流量随外部渗透压变化的规律。 为了定量研究外部电荷对水链在纳米管中的运动行为影响,与李敬源等人合作我们一起构造了一个由单个纳米碳管(CNT)和单个外加电荷组成的模型系统。研究了通过调节外加电荷和碳管的不同距离对纳米通道中的水分子行为进行调控。
Other AbstractWater confined in the nanoscale channels usually has novel behaviors different from bulk water, and its micro dynamics properties and mechanism behind attract considerable attention. Water molecules confined in the nanoscale channels with proper radius are found to be in the single-file arrangement, and move concertedly. A liquid-vapor transition (switch between open state and closed state) can occur when a change in radius of the channel or the water-channel interaction is triggered. In addition, the wavelike patterns of the water density distribution along the nanochannel axis have been found in computer simulation. The physical mechanism behind the interesting phenomenon are clouded with doubts and suspicions. Investigation of the underlying molecular mechanisms might be helpful to understand liquid transport through nanoscale channels, together with the structure and function of biological. However, it is quite difficult to investigate many important nanoscale systems directly, such as biological water channels, considering the complicated structure of membranes and membrane-water interactions. Recently, carbon nanotubes (CNTs) have been used widely as model systems to exploit some of the primary behaviors of the complex biological water channels. We use (6, 6) carbon nanotube with 8.1 Å diameter as a simple model to study the properties of the water chain inside. The two typical ways of biological water channels to control the water permeation are by changing the geometry of the channel or changing the charge distribution (the position of the charged residue). The thesis focused the effects of changes in geometry of the channel or the external charge position on the water behavior. By co-working with Rongzheng Wan et al, the dynamics of water molecules in a single-walled carbon nanotube under continuous deformations are studied with molecular dynamics simulations. The effective gating of water permeation across this nanotube was found. The gating mechanism and capability of noise resistant are related to the wavelike pattern of water density inside the channel. However, the physical mechanisms behind them are not clear. Based on them, we go further in studying water behavior inside the nano channel and mechanism. It is well-known that the density distribution is a constant along a macroscopic channel with a uniform radius if there is no pressure difference between the ends of the channel. However, wavelike patterns of the water density distributions have been found in a carbon nanotube with 8.1 Å in diameter and 13.4 Å in length, where the water molecules inside the channel form single-file structure. Moreover, it is found that, if the length of the carbon nanotube is a little larger, say 14.6 Å, the wavelike pattern of water density distribution near the center of this channel becomes much weaker. This raises more interests on the origin of the wavelike pattern of water density distribution inside the channel, how the water density distribution changes with respect to the length of the channel, whether the gating still exists for longer CNTs. Simulation results indicate that the gating behavior is almost not affected by the length of CNTs. A simple theoretical model is proposed to exploit the physical origin of the wavelike patterns. We find that the potential barriers at both ends together with the tight hydrogen-bonding network are the main responsibility. These findings are helpful to understand similar phenomena in biological channels and other nanoscale pores. In order to investigate the capability of noise resistant, we studied the response of water permeation properties through a carbon nanotube on the time-dependent mechanical signals. It is found that there is a critical frequency of vibrating fc (about 1333 GHz) which plays a significant role in the water permeation properties. The total water flow, the net flux, the number of hydrogen bonds and the dipole flipping frequency of the single-file water chain inside the nanotube are almost unchanged for the frequency of vibrating f< fc. Simulation results show that the nanotube can effectively resistant to the mechanical noise. Such excellent effect of noise screening attributes to the exceptive property of water molecules connected by strong hydrogen bonds with each other and formed a one-dimensional water chain inside the nanotube. Our findings are important for the understanding of why biological systems can achieve accurate information transfer in an environment full of fluctuations. The water transport through (6, 6) carbon nanotube is investigated with Fokker-Planck equation. By comparing the theoretical results from Fokker-Planck equation and simulation results, we find that Fokker-Planck equation is suitable to describe the water transport through carbon nanotube in certain extent and our simulation results are believable. We also investigate the effect of osmotic pressure on water flux. By co-working with Jingyuan Li, we devised a model system using a CNT with an external positive charge located at multiple positions (with various radial distances from the CNT) to study the effect of external charges on the water behavior in the nanochannel. It is interesting to find that this nanotube shows an excellent on-off gating behavior.
Pages100
Language中文
Document Type学位论文
Identifierhttp://ir.sinap.ac.cn/handle/331007/7233
Collection中科院上海应用物理研究所2004-2010年
Recommended Citation
GB/T 7714
陆杭军. 水在纳米通道中的动力学行为研究[D]. 上海应用物理研究所. 中国科学院上海应用物理研究所,2008.
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