水通过纳米通道传输行为及物理机制的分子动力学研究
发布时间：2018-09-01 20:19
【摘要】：纳米管和纳米孔等纳米通道结构对流体及其内部物质的高通量选择性输运特性具有重要的应用价值和研究意义。对纳米通道传输特性的研究不仅有利于分子筛选、药物输运以及水净化等领域高效纳米多孔膜的设计,而且对细胞膜通道跨膜输运及其调节机制的深入理解也有重要作用。纳米通道流体传输特性的掌握需要从分子尺度上对流体动力学行为进行研究,并结合传热传质学、物理化学、统计热力学等对其物理机制进行揭示。本文以碳纳米管作为纳米通道简单模型,基于分子动力学模拟和理论研究,对水通过纳米通道的渗透和扩散传输特性、纳米通道内水的结构特征及其对传输特性的影响、基于不对称热涨落的纳米通道热分子泵效应三个方面展开了系统研究,取得了以下研究成果:1.水通过纳米通道的渗透和扩散传输特性通过分子动力学模拟研究了溶质尺寸和孔隙密度对水通过纳米通道的渗透和扩散传输特性的影响,揭示了溶质尺寸和孔隙密度在水通过纳米通道渗透传输过程中产生的尺度效应,提出这是由于溶质粒子与纳米孔的随机碰撞干涉引起的。通过系统研究发现水通过纳米多孔膜中单个纳米孔的渗透流速与水化溶质投影面积和单孔膜面积(即孔隙密度倒数)之比存在线性关系。基于分子动力学模拟结果建立了描述该尺度效应影响下水通过纳米通道渗透流速的无量纲准则方程。引入溶质水化理论修正了连续性时间随机行走模型和集体扩散模型,使其可以准确地描述复杂实际溶液通过纳米通道的渗透和扩散传输特性。通过对纳米通道对离子的选择性输运分子动力学模拟研究,发现只有当纳米通道内径小于水化离子直径时,纳米通道才会阻碍离子通过,由此得出纳米通道对离子的选择性输运机制是基于溶质水化直径的尺寸选择性。2.纳米通道内水的结构特征及其对传输特性的影响通过对不同温度下(253.15K-373.15 K)水通过不同内径(0.459 nm-1.679 nm)纳米通道的传输特性的分子动力学研究,发现温度会诱导水通过纳米通道的高通量输运和慢速输运之间的转变现象。随着温度的降低,其流速可降低3个数量级以上,流速的显著下降是由于纳米通道内水的结构发生有序化转变从而产生了具有强大流动阻力的相界面。系统确定水在不同尺寸纳米通道内的结构特征及转变温度,揭示了水在纳米通道内产生有序化结构的物理机制是由氢键稳定性与分子随机热运动之间的竞争关系所主导。研究了纳米通道内水的结构转变对水和质子传输特性的影响,提出了温度调控纳米通道对水和质子高通量选择性输运的可行方案。建立了描述水通过纳米通道传输特性与流体自扩散系数、通道尺寸之间定量关系的理论公式。研究了纳米通道内的水分子在有序结构和无序自由态时的自扩散行为,发现任何结构状态下,纳米通道内水分子的自扩散行为均符合Fickian扩散机制,但扩散系数均低于体相水。3.基于不对称热涨落的纳米通道热分子泵效应通过对温差作用下水通过纳米通道的传输特性的分子动力学研究发现了热分子泵效应,即:尽管存在强大的反向化学势势垒,水分子依然可以自发地快速通过纳米通道从热端(低化学势)向冷端(高化学势)传输。热分子泵效应具有强大的分子泵送能力,对于直径为0.81nm的纳米管,15 K的小温差就可以产生5.3 MPa的当量驱动压力,而且相同温差下,热分子泵效应驱动能力不随纳米通道长度的增加而降低。通过系统的理论分析提出热分子泵效应的物理机制是纳米通道进出口处水分子的不对称热涨落诱导产生的不对称传输现象。基于上述发现,提出了温差驱动反渗透海水淡化的方法。研究表明,在15 K的小温差下,10 cm2孔隙密度为1.5×1013 pores/cm2的纳米多孔膜的淡水产量高达7.77 L/h。
[Abstract]:Nanotube and nanoporous nanochannel structures have important application value and research significance for high throughput selective transport of fluids and their internal materials. The study of nanochannel transport characteristics is not only conducive to the design of highly efficient nanoporous membranes in the fields of molecular screening, drug transport and water purification, but also conducive to the permeation of cell membranes. It is also important to understand the mechanism of transmembrane transport and its regulation. To understand the characteristics of fluid transport in nanochannels, it is necessary to study the hydrodynamic behavior at the molecular scale, and to reveal the physical mechanism by combining heat and mass transfer, physical chemistry and statistical thermodynamics. Based on the molecular dynamics simulation and theoretical study, the permeation and diffusion characteristics of water through nanochannels, the structural characteristics of water in nanochannels and its effects on the transport characteristics were studied systematically. The effects of solute size and pore density on the permeation and diffusion of water through nanochannels were studied by molecular dynamics simulation. The scale effect of solute size and pore density on the permeation and diffusion of water through nanochannels was revealed, which was attributed to the solute particles. Stochastic collision interference between a particle and a nanopore. It is found that there is a linear relationship between the permeation velocity of water through a single nanopore in a nanoporous membrane and the ratio of the projected area of the hydrated solute and the area of the monoporous membrane (i.e. the reciprocal of the pore density). Based on the results of molecular dynamics simulation, a description of the water flow under the influence of the scale effect is established. The continuum time random walk model and the collective diffusion model are modified by introducing the solute hydration theory to describe accurately the permeation and diffusion characteristics of complex real solutions through nanochannels. Molecular dynamics of ion selective transport through nanochannels is studied. It is found that only when the inner diameter of the nano-channel is smaller than the diameter of hydrated ions can the nano-channel obstruct the ion passage. The selective transport mechanism of the nano-channel for ions is based on the size selectivity of the solute hydration diameter. 2. The structure characteristics of the water in the nano-channel and its influence on the transport characteristics through different temperatures. Molecular dynamics studies on the transport properties of water through nanochannels with different inner diameters (0.459 nm-1.679 nm) at 253.15K-373.15K showed that temperature induced the transition between high-throughput and slow-speed transport of water through nanochannels. The system determines the structural characteristics and transition temperatures of water in different sizes of nanochannels, revealing that the physical mechanism of ordered structure of water in nanochannels is between the stability of hydrogen bonds and the random thermal movement of molecules. Competition is dominant. The effects of water structure transition on water and proton transport properties in nanochannels are studied. A feasible scheme for temperature-controlled high-throughput selective transport of water and proton in nanochannels is proposed. A quantitative relationship between water transport characteristics through nanochannels and fluid self-diffusion coefficient and channel size is established. The self-diffusion behavior of water molecules in nanochannels under ordered structure and disordered free state is studied. It is found that the self-diffusion behavior of water molecules in nanochannels conforms to Fickian diffusion mechanism under any structural state, but the diffusion coefficient is lower than that of bulk water. 3. Thermal molecular pump effect passage in nanochannels based on asymmetric thermal fluctuation. Molecular dynamics studies of the transport properties of water through nanochannels have revealed the thermal molecular pumping effect, i.e. despite the existence of strong reverse chemical potential barriers, water molecules can spontaneously and rapidly transfer from the hot end (low chemical potential) to the cold end (high chemical potential) through nanochannels. For nanotubes with a diameter of 0.81 nm, a small temperature difference of 15 K can produce an equivalent driving pressure of 5.3 MPa, and the driving capacity of thermal molecular pump effect does not decrease with the increase of the length of nanochannels under the same temperature difference. Based on the above findings, a method of reverse osmosis desalination driven by temperature difference is proposed. The results show that the freshwater yield of nanoporous membrane with 10 cm 2 pore density of 1.5 *1013 pores/cm 2 is as high as 7.77 L/h at 15 K.
【学位授予单位】：上海交通大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TB383.1
[Abstract]:Nanotube and nanoporous nanochannel structures have important application value and research significance for high throughput selective transport of fluids and their internal materials. The study of nanochannel transport characteristics is not only conducive to the design of highly efficient nanoporous membranes in the fields of molecular screening, drug transport and water purification, but also conducive to the permeation of cell membranes. It is also important to understand the mechanism of transmembrane transport and its regulation. To understand the characteristics of fluid transport in nanochannels, it is necessary to study the hydrodynamic behavior at the molecular scale, and to reveal the physical mechanism by combining heat and mass transfer, physical chemistry and statistical thermodynamics. Based on the molecular dynamics simulation and theoretical study, the permeation and diffusion characteristics of water through nanochannels, the structural characteristics of water in nanochannels and its effects on the transport characteristics were studied systematically. The effects of solute size and pore density on the permeation and diffusion of water through nanochannels were studied by molecular dynamics simulation. The scale effect of solute size and pore density on the permeation and diffusion of water through nanochannels was revealed, which was attributed to the solute particles. Stochastic collision interference between a particle and a nanopore. It is found that there is a linear relationship between the permeation velocity of water through a single nanopore in a nanoporous membrane and the ratio of the projected area of the hydrated solute and the area of the monoporous membrane (i.e. the reciprocal of the pore density). Based on the results of molecular dynamics simulation, a description of the water flow under the influence of the scale effect is established. The continuum time random walk model and the collective diffusion model are modified by introducing the solute hydration theory to describe accurately the permeation and diffusion characteristics of complex real solutions through nanochannels. Molecular dynamics of ion selective transport through nanochannels is studied. It is found that only when the inner diameter of the nano-channel is smaller than the diameter of hydrated ions can the nano-channel obstruct the ion passage. The selective transport mechanism of the nano-channel for ions is based on the size selectivity of the solute hydration diameter. 2. The structure characteristics of the water in the nano-channel and its influence on the transport characteristics through different temperatures. Molecular dynamics studies on the transport properties of water through nanochannels with different inner diameters (0.459 nm-1.679 nm) at 253.15K-373.15K showed that temperature induced the transition between high-throughput and slow-speed transport of water through nanochannels. The system determines the structural characteristics and transition temperatures of water in different sizes of nanochannels, revealing that the physical mechanism of ordered structure of water in nanochannels is between the stability of hydrogen bonds and the random thermal movement of molecules. Competition is dominant. The effects of water structure transition on water and proton transport properties in nanochannels are studied. A feasible scheme for temperature-controlled high-throughput selective transport of water and proton in nanochannels is proposed. A quantitative relationship between water transport characteristics through nanochannels and fluid self-diffusion coefficient and channel size is established. The self-diffusion behavior of water molecules in nanochannels under ordered structure and disordered free state is studied. It is found that the self-diffusion behavior of water molecules in nanochannels conforms to Fickian diffusion mechanism under any structural state, but the diffusion coefficient is lower than that of bulk water. 3. Thermal molecular pump effect passage in nanochannels based on asymmetric thermal fluctuation. Molecular dynamics studies of the transport properties of water through nanochannels have revealed the thermal molecular pumping effect, i.e. despite the existence of strong reverse chemical potential barriers, water molecules can spontaneously and rapidly transfer from the hot end (low chemical potential) to the cold end (high chemical potential) through nanochannels. For nanotubes with a diameter of 0.81 nm, a small temperature difference of 15 K can produce an equivalent driving pressure of 5.3 MPa, and the driving capacity of thermal molecular pump effect does not decrease with the increase of the length of nanochannels under the same temperature difference. Based on the above findings, a method of reverse osmosis desalination driven by temperature difference is proposed. The results show that the freshwater yield of nanoporous membrane with 10 cm 2 pore density of 1.5 *1013 pores/cm 2 is as high as 7.77 L/h at 15 K.
【学位授予单位】：上海交通大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TB383.1
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