双壁碳纳米管通水阻盐性能的分子动力学模拟

发布时间：2018-02-16 16:47

本文关键词： 双壁碳纳米管分子动力学模拟反渗透海水淡化管间距　出处：《大连理工大学》2015年硕士论文　论文类型：学位论文

【摘要】：反渗透是目前大规模海水淡化推广中的主流技术。开发高效通水阻盐性能与高机械强度的膜材料是目前反渗透技术中急需解决的问题之一。随着碳纳米管的高速通水性质以及对盐离子有较高的截留性质逐渐被人们所熟知,将碳纳米管掺杂在反渗透膜材料中成为解决上述问题的途径之一。而在诸多类型的碳纳米管材料中,双壁碳纳米管的性质优于单壁碳纳米管,故本文以双壁碳纳米管作为研究对象,采用分子动力学模拟方法对非常规双壁碳纳米管在反渗透过程中进行研究。通过调节内外管间距来考察管间距对管道内水分子与盐离子运动特性的影响。首先,本文采用单层反渗透膜模型,以0.5 mol·L-1氯化钠水溶液模拟海水,在盐水一侧施加静水压力模拟反渗透条件。其次,渗透膜的主体为公度双壁碳纳米管,其内外壁均为扶手椅型,且内壁手性保持为(8,8)。本文重点考察模拟过程中不同尺寸双壁碳纳米管的阻盐效率与通水性能,计算管内水分子平均力势,并分析水分子的氢键与偶极矩特性。另外,我们采取了三种不同的水分子势能模型(TP3P、TIP4P与SPC/E)来考察水模型的选取对模拟结果的影响。研究结果表明,管间距不仅会影响传输通道内水分子的纳观构型(平均力势、氢键与偶极矩等),还会使盐离子与水分子在碳纳米管中呈现渗透差异性。在通水阻盐方面,小尺寸双壁碳纳米管可以有效实现盐水分离但水通量较小,大尺寸双壁碳纳米管的通水量较高但阻盐效率有所下降,而中尺寸双壁碳纳米管的截盐效率可达100%,并且在净水流量的数值上与大尺寸双壁碳纳米管相近,这说明中尺寸双壁碳纳米管(即：管间距为0.815nm)具有优异的通水阻盐性能。另外,水分子模型会在一定程度上影响阻盐率、通水量及水分子的氢键特性,例如：TIP3P水分子在反渗透模型中的渗透性较高,氢键寿命较短。然而,在不同的水模型体系中,通水阻盐性能随管间距的变化趋势相近。由模拟结果可知,水分子模型对通水阻盐性能的影响小于双壁碳纳米管几何尺寸的影响。本文采用分子动力学模拟方法,从分子层面阐述双壁纳米碳纳米管的通水阻盐机理,以期为高性能碳纳米管掺杂反渗透膜的制备提供依据,并为人们改进海水淡化渗透技术提供理论支持。
[Abstract]:Reverse osmosis (RO) is the mainstream technology in large-scale seawater desalination. The development of membrane materials with high water and salt resistance and high mechanical strength is one of the most urgent problems in reverse osmosis. With the development of carbon nanotubes (CNTs) at high speed. The water permeability and the high retention of salt ions are becoming more and more familiar. Doping carbon nanotubes into reverse osmosis membrane materials is one of the ways to solve the above problems. In many kinds of carbon nanotube materials, the properties of double-walled carbon nanotubes are better than that of single-walled carbon nanotubes. So this paper takes double walled carbon nanotubes as the research object. In this paper, molecular dynamics simulation was used to study the effect of tube spacing on the movement of water molecules and salt ions during reverse osmosis (RO) process of unconventional double-walled carbon nanotubes. In this paper, a single-layer reverse osmosis membrane model was used to simulate seawater with 0.5 mol 路L ~ (-1) sodium chloride solution, and the hydrostatic pressure was applied on one side of the salt water to simulate the reverse osmosis condition. Secondly, the main body of the membrane was a double-walled carbon nanotube, and the inner and outer walls of the membrane were all armchair. The chirality of the inner wall of the tube is kept at 8%. In this paper, the salt inhibition efficiency and water permeability of different size double walled carbon nanotubes are investigated, the average force potential of water molecules in the tube is calculated, and the characteristics of hydrogen bond and dipole moment of water molecule are analyzed. Three different water molecule potential energy models (TP3PnTIP4P and SPC / E3) are used to investigate the influence of water model selection on the simulation results. The results show that tube spacing not only affects the nanometries (mean force potential) of water molecules in the transport channel. Hydrogen bonds and dipole moments can also make salt ions and water molecules permeate differently in carbon nanotubes. In terms of water retention and salt resistance, small size double-walled carbon nanotubes can effectively separate brine but have less water flux. Large size double-walled carbon nanotubes have higher water flux but lower salt resistance efficiency, while medium size double-walled carbon nanotubes have a salt cutting efficiency of 100, and are similar to large size double-walled carbon nanotubes in the value of water purification flow. This indicates that the medium-size double-walled carbon nanotubes (that is, the spacing of the tubes are 0.815 nm) have excellent water-salt resistance. In addition, the water molecular model can influence the salt resistance, water flux and hydrogen bonding properties of water molecules to some extent. For example, the water molecule of TIP3P has higher permeability and shorter hydrogen bond lifetime in the reverse osmosis model. However, in different water model systems, the variation trend of water resistance and salt resistance with the tube spacing is similar. The effect of water molecular model on the salt resistance of double-walled carbon nanotubes is less than that on the geometry of double-walled carbon nanotubes. In this paper, the molecular dynamics simulation method is used to explain the mechanism of water-permeation and salt resistance of double-walled carbon nanotubes at the molecular level. In order to provide the basis for the preparation of high performance carbon nanotube doped reverse osmosis membrane and provide theoretical support for improving seawater desalination osmosis technology.
【学位授予单位】：大连理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TB383.1;O613.71

【参考文献】