水通過納米通道傳輸行為及物理機(jī)制的分子動力學(xué)研究
發(fā)布時(shí)間：2018-09-01 20:19
【摘要】：納米管和納米孔等納米通道結(jié)構(gòu)對流體及其內(nèi)部物質(zhì)的高通量選擇性輸運(yùn)特性具有重要的應(yīng)用價(jià)值和研究意義。對納米通道傳輸特性的研究不僅有利于分子篩選、藥物輸運(yùn)以及水凈化等領(lǐng)域高效納米多孔膜的設(shè)計(jì),而且對細(xì)胞膜通道跨膜輸運(yùn)及其調(diào)節(jié)機(jī)制的深入理解也有重要作用。納米通道流體傳輸特性的掌握需要從分子尺度上對流體動力學(xué)行為進(jìn)行研究,并結(jié)合傳熱傳質(zhì)學(xué)、物理化學(xué)、統(tǒng)計(jì)熱力學(xué)等對其物理機(jī)制進(jìn)行揭示。本文以碳納米管作為納米通道簡單模型,基于分子動力學(xué)模擬和理論研究,對水通過納米通道的滲透和擴(kuò)散傳輸特性、納米通道內(nèi)水的結(jié)構(gòu)特征及其對傳輸特性的影響、基于不對稱熱漲落的納米通道熱分子泵效應(yīng)三個(gè)方面展開了系統(tǒng)研究,取得了以下研究成果:1.水通過納米通道的滲透和擴(kuò)散傳輸特性通過分子動力學(xué)模擬研究了溶質(zhì)尺寸和孔隙密度對水通過納米通道的滲透和擴(kuò)散傳輸特性的影響,揭示了溶質(zhì)尺寸和孔隙密度在水通過納米通道滲透傳輸過程中產(chǎn)生的尺度效應(yīng),提出這是由于溶質(zhì)粒子與納米孔的隨機(jī)碰撞干涉引起的。通過系統(tǒng)研究發(fā)現(xiàn)水通過納米多孔膜中單個(gè)納米孔的滲透流速與水化溶質(zhì)投影面積和單孔膜面積(即孔隙密度倒數(shù))之比存在線性關(guān)系�；诜肿觿恿W(xué)模擬結(jié)果建立了描述該尺度效應(yīng)影響下水通過納米通道滲透流速的無量綱準(zhǔn)則方程。引入溶質(zhì)水化理論修正了連續(xù)性時(shí)間隨機(jī)行走模型和集體擴(kuò)散模型,使其可以準(zhǔn)確地描述復(fù)雜實(shí)際溶液通過納米通道的滲透和擴(kuò)散傳輸特性。通過對納米通道對離子的選擇性輸運(yùn)分子動力學(xué)模擬研究,發(fā)現(xiàn)只有當(dāng)納米通道內(nèi)徑小于水化離子直徑時(shí),納米通道才會阻礙離子通過,由此得出納米通道對離子的選擇性輸運(yùn)機(jī)制是基于溶質(zhì)水化直徑的尺寸選擇性。2.納米通道內(nèi)水的結(jié)構(gòu)特征及其對傳輸特性的影響通過對不同溫度下(253.15K-373.15 K)水通過不同內(nèi)徑(0.459 nm-1.679 nm)納米通道的傳輸特性的分子動力學(xué)研究,發(fā)現(xiàn)溫度會誘導(dǎo)水通過納米通道的高通量輸運(yùn)和慢速輸運(yùn)之間的轉(zhuǎn)變現(xiàn)象。隨著溫度的降低,其流速可降低3個(gè)數(shù)量級以上,流速的顯著下降是由于納米通道內(nèi)水的結(jié)構(gòu)發(fā)生有序化轉(zhuǎn)變從而產(chǎn)生了具有強(qiáng)大流動阻力的相界面。系統(tǒng)確定水在不同尺寸納米通道內(nèi)的結(jié)構(gòu)特征及轉(zhuǎn)變溫度,揭示了水在納米通道內(nèi)產(chǎn)生有序化結(jié)構(gòu)的物理機(jī)制是由氫鍵穩(wěn)定性與分子隨機(jī)熱運(yùn)動之間的競爭關(guān)系所主導(dǎo)。研究了納米通道內(nèi)水的結(jié)構(gòu)轉(zhuǎn)變對水和質(zhì)子傳輸特性的影響,提出了溫度調(diào)控納米通道對水和質(zhì)子高通量選擇性輸運(yùn)的可行方案。建立了描述水通過納米通道傳輸特性與流體自擴(kuò)散系數(shù)、通道尺寸之間定量關(guān)系的理論公式。研究了納米通道內(nèi)的水分子在有序結(jié)構(gòu)和無序自由態(tài)時(shí)的自擴(kuò)散行為,發(fā)現(xiàn)任何結(jié)構(gòu)狀態(tài)下,納米通道內(nèi)水分子的自擴(kuò)散行為均符合Fickian擴(kuò)散機(jī)制,但擴(kuò)散系數(shù)均低于體相水。3.基于不對稱熱漲落的納米通道熱分子泵效應(yīng)通過對溫差作用下水通過納米通道的傳輸特性的分子動力學(xué)研究發(fā)現(xiàn)了熱分子泵效應(yīng),即:盡管存在強(qiáng)大的反向化學(xué)勢勢壘,水分子依然可以自發(fā)地快速通過納米通道從熱端(低化學(xué)勢)向冷端(高化學(xué)勢)傳輸。熱分子泵效應(yīng)具有強(qiáng)大的分子泵送能力,對于直徑為0.81nm的納米管,15 K的小溫差就可以產(chǎn)生5.3 MPa的當(dāng)量驅(qū)動壓力,而且相同溫差下,熱分子泵效應(yīng)驅(qū)動能力不隨納米通道長度的增加而降低。通過系統(tǒng)的理論分析提出熱分子泵效應(yīng)的物理機(jī)制是納米通道進(jìn)出口處水分子的不對稱熱漲落誘導(dǎo)產(chǎn)生的不對稱傳輸現(xiàn)象�；谏鲜霭l(fā)現(xiàn),提出了溫差驅(qū)動反滲透海水淡化的方法。研究表明,在15 K的小溫差下,10 cm2孔隙密度為1.5×1013 pores/cm2的納米多孔膜的淡水產(chǎn)量高達(dá)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.
【學(xué)位授予單位】：上海交通大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：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.
【學(xué)位授予單位】：上海交通大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：TB383.1
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