Fluid structure and transport properties of water inside carbon nanotubes

The fluid structure and transport properties of water confined in single-walled carbon nanotubes (CNTs) with different diameters have been investigated by molecular-dynamics simulation. The effects of CNT diameter, density of water, and temperature on the molecular distributions and transport behaviors of water were analyzed. It is interesting that the water molecules ordered in helix inside the (10, 10) CNT, and the layered distribution was clearly observed. It was found that the axial and radial diffusivities in CNTs were much lower than that of the bulk, and it ever decreased as the diameter of CNT decreases. The axial thermal conductivity and shear viscosity in CNTs are obviously larger than that of the bulk and those in the radial direction, they increase sharply as the diameter of CNT decreases, which is clearly in contrast to the diffusivity. The inner space of CNT and the interactions between water molecules and the confining walls play a key role in the structure and transport properties of water confined in the CNTs.     

径向电场对纳米管中水分子通量的影响

水分子在纳米通道中的运动对于生命活动、纳米器件的设计等都有着重要的意义. 现在已经证实, 在(6,6)的碳纳米管中, 水分子会以单分子水链的形式协同通过碳纳米管. 但是如何控制水分子的流量仍然是一个困难的课题. 本文研究了在径向电场作用下, 碳纳米管中水分子通量的变化趋势和碳纳米管的开关行为.发现在碳纳米管两端存在200 MPa的压力差时, 电场强度从1 V·nm^-1增加到3 V·nm^-1, 水分子通量线性减小. 当径向电场强度增加到3 V·nm^-1时, 碳纳米管处于关闭状态, 水分子无法通过碳纳米管. 进一步, 我们发现水偶极与碳纳米管管轴夹角的平均值的概率分布和翻转频率都与水分子在纳米管中的个数有很大关系.     

分子动力学模拟研究纳米碳管中甲醇-水混合溶液的结构与输运性质

利用分子动力学模拟方法,对比考察了平衡条件、外压作用、梯度电场作用下,摩尔比为1:1的甲醇-水混合溶液在纳米碳管(CNT)中的静态结构以及输运行为.研究发现:在平衡体系与外压作用下,纳米碳管内甲醇与水呈现出明显的不混溶现象,甲醇主要分布于管壁附近,水分子主要分布于纳米碳管轴心附近;而在梯度电场作用下,纳米碳管由疏水性向亲水性转变,更多的水分子分布于管壁,导致纳米碳管内甲醇-水的不混溶现象消失.另一方面,在外压作用下,纳米碳管内甲醇与水呈现单向移动;而在梯度电场下,甲醇与水呈现快速的双向移动,其流通量较相应外压作用体系高出近一个数量级,但由于双向的流通量大小相近,导致净流通量与外压作用下的净流通量差异不大.     

分子动力学模拟研究纳米碳管中甲醇-水混合溶液的结构与输运性质

利用分子动力学模拟方法,对比考察了平衡条件、外压作用、梯度电场作用下,摩尔比为1：1 的甲醇-水混合溶液在纳米碳管（CNT）中的静态结构以及输运行为. 研究发现：在平衡体系与外压作用下,纳米碳管内甲醇与水呈现出明显的不混溶现象,甲醇主要分布于管壁附近,水分子主要分布于纳米碳管轴心附近;而在梯度电场作用下,纳米碳管由疏水性向亲水性转变,更多的水分子分布于管壁,导致纳米碳管内甲醇-水的不混溶现象消失. 另一方面,在外压作用下,纳米碳管内甲醇与水呈现单向移动;而在梯度电场下,甲醇与水呈现快速的双向移动,其流通量较相应外压作用体系高出近一个数量级,但由于双向的流通量大小相近,导致净流通量与外压作用下的净流通量差异不大.     

Effect of Size and Temperature on Water Dynamics inside Carbon Nano-Tubes Studied by Molecular Dynamics Simulation

Water transport inside carbon nano-tubes (CNTs) has attracted considerable attention due to its nano-fluidic properties, its importance in nonporous systems, and the wide range of applications in membrane desalination and biological medicine. Recent studies show an enhancement of water diffusion inside nano-channels depending on the size of the nano-confinement. However, the underlying mechanism of this enhancement is not well understood yet. In this study, we performed Molecular Dynamics (MD) simulations to study water flow inside CNT systems. The length of CNTs considered in this study is 20 nm, but their diameters vary from 1 to 10 nm. The simulations are conducted at temperatures ranging from 260 K to 320 K. We observe that water molecules are arranged into coaxial water tubular sheets. The number of these tubular sheets depends on the CNT size. Further analysis reveals that the diffusion of water molecules along the CNT axis deviates from the Arrhenius temperature dependence. The non-Arrhenius relationship results from a fragile liquid-like water component persisting at low temperatures with fragility higher than that of the bulk water.     

Molecular dynamics simulation study of the structural characteristics of water molecules confined in functionalized carbon nanotubes

Molecular dynamics (MD) simulations were performed to study the structural properties of water molecules confined in functionalized carbon nanotubes (CNTs). Four CNTs, two armchair-type (6, 6), (7, 7) and two zigzag-type (10, 0), (12, 0) CNTs, representing different helicities and different diameters, were chosen and functionalized at their open ends by the hydrophilic -COOH and the hydrophobic -CH3 groups. The structural properties of water molecules inside the functionalized CNTs, including the orientation distributions of dipole moment and O-H bonds, the length of the single-file water chain, and the average number of hydrogen bonds, were analyzed during a process of simulations. MD simulation results in this work showed that the -CH3 functional groups exert little special effects on the structural properties of water molecules. It is mainly due to the relatively small size of the -CH3 group and its hydrophobic nature, which is consistent with hydrophobic CNTs. For CNTs functionalized by -COOH groups, the configurations of -COOH groups, incurvature or excurvature, determine whether water molecules can enter the CNTs. The incurvature or excurvature configurations of -COOH groups are the results of synergy effects of the CNTs' helicity and diameter and control the flow direction of water molecules in CNTs.     

On the Origin of Water Flow through Carbon Nanotubes

The transport of water molecules through carbon nanotubes (CNTs) is of primary importance for understanding water‐mediated biological activities as well as for the design of novel nanoporous materials. Herein, we analyze the water flow through CNTs by using molecular dynamics simulations with the hope of finding basic parameters determining the flow value. Of particular interest is that a simple equation as a function of water diffusion, occupancy and CNT size, can well describe the water flow through CNTs with different sizes. Specifically, both the simulation and equation flow exhibit power law relations with the CNT diameter and length, where the two exponents are close to each other. The water occupancy and translocation time also demonstrate interesting relations with the CNT size. The water dipole orientations and density profiles are also sensitive to the change of CNT size. These results greatly enhance our knowledge on the nature of water flow through CNTs and are helpful in predicting the water flow of CNTs up to the experimental length scale.     

Flow structure of water in carbon nanotubes: poiseuille type or plug-like?

We have conducted molecular dynamics simulations of water flow in carbon nanotubes (CNTs) for (6,6) to (20,20) CNTs at a streaming velocity of 100 ms. The fluidized piston model (FPM) and the ice piston model (IPM) are employed to drive flow through the CNTs. The results show that the single-file water flow inside (6,6) CNT has a convex upward streaming velocity profile, whereas the velocity profiles in (10,10) to (20,20) CNTs are flat except near the tube wall. The flow structure of cylindrical water in the (8,8) CNT is intermediate between that for the (6,6) CNT and the larger CNTs. The flow parameters are found not to exhibit any dependence on streaming velocity at up to 300 ms in the (12,12) CNT. The hydrogen bond lifetimes of water flowing in CNTs tend to be longer than for the corresponding equilibrium states, and nonzero flow does not reduce the microscopic structure or structural robustness (hydrogen bond lifetime). Although the atomic density profile varies with tube diameter, reflecting the change in static microscopic structure of flow from single file to cylindrical, tube diameter does not induce a clear transition in streaming velocity, temperature, or hydrogen bond lifetime over this diameter range. The results suggest that water flow in CNTs of this size is more pluglike than Poiseuille type, although the flow structure does not strictly accord with either definition.     

以荷电化碳纳米管构筑正渗透膜用于海水淡化的分子动力学模拟

受传统膜科学中分离膜的荷电化可提升膜盐水分离效能的启发,在前期工作基础上尝试以荷电化碳纳米管CNT(8,8)为水通道仿生构筑正渗透膜,利用分子动力学模拟的方法研究水分子在膜中的传递行为.模拟中,以0.5mo·lL-1氯化钠溶液模拟海水,1mo·lL-1的氯化镁溶液为汲取液,考察不同电量电荷修饰对碳纳米管正渗透膜中水分子密度分布、扩散系数以及水通量的影响.结果显示,电荷修饰对碳纳米管中水分子的密度分布和扩散速率以及水通量影响较显著,当碳纳米管管口荷电量为-0.3e时,碳纳米管膜可获得最大水通量.     

Density, distribution, and orientation of water molecules inside and outside carbon nanotubes

The behavior of water molecules inside and outside 1.1, 2.8, 6.9, and 10.4 nm diameter armchair carbon nanotubes (CNTs) is predicted using molecular dynamics simulations. The effects of CNT diameter on mass density, molecular distribution, and molecular orientation are identified for both the confined and unconfined fluids. Within 1 nm of the CNT surface, unconfined water molecules assume a spatially varying density profile. The molecules distribute nonuniformly around the carbon surface and have preferred orientations. The behavior of the unconfined water molecules is invariant with CNT diameter. The behavior of the confined water, however, can be correlated to tube diameter. Inside the 10.4 nm CNT, the molecular behavior is indistinguishable from that of the unconfined fluid. Within the smaller CNTs, surface curvature effects reduce the equilibrium water density and force water molecules away from the surface. This effect changes both the molecular distribution and preferred molecular orientations.     

Highly selective adsorption of methanol in carbon nanotubes immersed in methanol-water solution

The systems of open-ended carbon nanotubes (CNTs) immersed in methanol-water solution are studied by molecular dynamics simulations. For the (6,6) CNT, nearly pure methanol is found to preferentially occupy interior space of the CNT. Even when the mass fraction (MF) of methanol in bulk solution is as low as 1%, the methanol MF within the CNT is still more than 90%. For CNTs with larger diameters, the methanol concentrations within CNTs are also much higher than those outside CNTs. The methanol selectivity decreases with increasing CNT diameter, but not monotonically. From microscopic structural analyses, we find that the primary reason for the high selectivity of methanol by CNTs lies on high preference of methanol in the first solvation shell near the inner wall of CNT, which stems from a synergy effect of the van der Waals interaction between CNT and the methyl groups of methanol, together with the hydrogen bonding interaction among the liquid molecules. This synergy effect may be of general significance and extended to other systems, such as ethanol aqueous solution and methanol/ethanol mixture. The selective adsorption of methanol over water in CNTs may find applications in separation of water and methanol, detection of methanol, and preservation of methanol purity in fuel cells.     

Dynamic behaviors on zadaxin getting into carbon nanotubes

The dynamic behaviors of drug zadaxin getting into carbon nanotubes (CNTs) in different water surroundings were investigated by molecular dynamics simulation. It was found that the diameter (1.9 nm) of (14, 14) CNT is the critical size for inserting zadaxin into CNT at the present conditions. In addition, the length of CNTs is another factor for inserting. A certain length is needed. It implies that interactions of zadaxin with both the CNT and the water molecules are competitive in the insertion process. The CNT-zadaxin attractive interaction is found to be the main driving force with the lower density of water molecules in the surroundings, while the zadaxin-water interaction becomes dominant with the higher density. The study of the authors suggests that biomolecules-CNT systems can be further exploited for the potential applications to drugs, vaccines, and gene delivery.     

Effects of electric fields on proton transport through water chains

Molecular dynamics simulations on quantum energy surfaces are carried out to study the effects of perturbing electric fields on proton transport (PT) in protonated water chains. As an idealized model of a hydrophobic cavity in the interior of a protein the water molecules are confined into a carbon nanotube (CNT). The water chain connects a hydrated hydronium ion (H3O+) at one end of the CNT and an imidazole molecule at the other end. Without perturbing electric fields PT from the hydronium proton donor to the imidazole acceptor occurs on a picosecond time scale. External perturbations to PT are created by electric fields of varying intensities, normal to the CNT axis, generated by a neutral pair of charges on the nanotube wall. For fields above approximately 0.5 VA, the hydronium ion is effectively trapped at the CNT center, and PT blocked. Fields of comparable strength are generated inside proteins by nearby polar/charged amino acids. At lower fields the system displays a rich dynamic behavior, where the excess charge shuttles back and forth along the water chain before reaching the acceptor group on the picosecond time scale. The effects of the perturbing field on the proton movement are analyzed in terms of structural and dynamic properties of the water chain. The implications of these observations on PT in biomolecular systems and its control by external perturbing fields are discussed.     

Bulk synthesis of linear carbon chains confined inside single-wall carbon nanotubes by vacuum discharge

Carbyne, an infinite carbon chain, has attracted much interest and induced significant controversy for many decades. Recently, the presence of linear carbon chains (LCCs), which were confined stably inside double-wall carbon nanotubes (DWCNTs) and multiwall carbon nanotubes (MWCNTs), has been reported. In this study, we present a novel method to produce LCCs in a film of carbon nanotubes (CNTs). Our transmission electron microscopy and Raman spectroscopy revealed the formation of a bulk amount of LCCs after electric discharge of CNT films, which were used as field emission cathodes. The LCCs were confined inside single-wall CNTs as well as DWCNTs. Furthermore, two or three LCCs in parallel with each other are encapsulated when the inner diameter of CNT is larger than approximately 1.1 nm.     

水促进Co/Mo/Al2O3催化剂上碳纳米管的生长

对水促进Co/Mo/Al2O3催化剂裂解乙烯生长碳纳米管(CNTs)的研究发现,通入体积分数(φ)为0.6％的水蒸汽在1h内可将CNTs的生长倍率从3.7 g·g-1提高至70 g·g-1.水的作用在于恢复被无定形碳包覆的催化剂颗粒的活性,水的加入量由于其积碳(促进同体碳生成)和消碳(去除固体碳)的竞争作用而存在最佳值.不同反应时间下乙烯的转化率与有效催化剂含量的分析表明,在CNTs生长后期,水的催化促进作用减弱.将催化剂的相对活性与CNT聚团的相对密度关联发现,反应后期的CNTs主要在聚团内部缠绕生长,催化剂被包覆失活.拉曼测试与差热热重分析表明,生长阻力导致所得CNTs缺陷增多,CNT聚团密度变化与CNT缺陷间存在对应关系.聚团内外CNTs的生长阻力不同,生长倍率不同,导致产品纯度不均匀.     

Curvature‐dependent adsorption of water inside and outside armchair carbon nanotubes

The curvature dependence of the physisorption properties of a water molecule inside and outside an armchair carbon nanotube (CNT) is investigated by an incremental density‐fitting local coupled cluster treatment with single and double excitations and perturbative triples (DF‐LCCSD(T)) study. Our results show that a water molecule outside and inside (n, n) CNTs (n = 4, 5, 6, 7, 8, 10) is stabilized by electron correlation. The adsorption energy of water inside CNTs decreases quickly with the decrease of curvature (increase of radius) and the configuration with the oxygen pointing toward the CNT wall is the most stable one. However, when the water molecule is adsorbed outside the CNT, the adsorption energy varies only slightly with the curvature and the configuration with hydrogens pointing toward the CNT wall is the most stable one. We also use the DF‐LCCSD(T) results to parameterize Lennard‐Jones (LJ) force fields for the interaction of water both with the inner and outer sides of CNTs and with graphene representing the zero curvature limit. It is not possible to reproduce all DF‐LCCSD(T) results for water inside and outside CNTs of different curvature by a single set of LJ parameters, but two sets have to be used instead. Each of the two resulting sets can reproduce three out of four minima of the effective potential curves reasonably well. These LJ models are then used to calculate the water adsorption energies of larger CNTs, approaching the graphene limit, thus bridging the gap between CNTs of increasing radius and flat graphene sheets. © 2016 Wiley Periodicals, Inc.     

Single-file diffusion near channel boundaries

Molecular transport under the conditions of single-file diffusion was investigated near the channel boundaries by using dynamic Monte Carlo and molecular dynamics simulations of tracer exchange between single-file channels and their surroundings. The boundary effect reported in our recent papers (Vasenkov S.; K?rger, J. Phys. Rev. E 2002, 66, 052601. Schüring, A.; Vasenkov S.; Fritzsche, S. J. Phys. Chem. B 2005, 109, 16711) was studied in detail. This boundary effect is characterized by deviations of the intrachannel concentration profiles of tracer molecules observed in the case of single-file diffusion near the channel boundaries from the corresponding profiles typical for normal diffusion. It has been shown in our previous studies that these deviations occur under the conditions when the potential-energy difference inside and outside of single-file channels was both comparable and much larger than the activation energy of intrachannel diffusion. Here, we report a quantitative model describing the boundary effect. According to this model, an occurrence of the boundary effect is related to a complex character of diffusion in finite single-file systems. Such diffusion can be described by the following two types of movements occurring in parallel: (i) correlated displacements of all molecules in any particular channel and (ii) fast displacements of single molecules, which are uncorrelated with the displacements of all other molecules in the same channel. The latter displacements are restricted to a certain length interval that depends on the channel length and the channel occupancy. This length interval is shown to determine the extensions of the channel margins where the boundary effect is observed.     

Effects of size constraint on water filling process in nanotube

Molecular dynamics (MD) simulation and the potential of mean force (PMF) analysis are used to investigate the structural properties of water molecules near the end of nanotube for the whole process from the initial water filling up to the configuration stabilization inside the carbon nanotubes (CNTs). Numerical simulations showed that when a small-sized nanotube is immersed into the water bath, the size constraint will induce a prevailing orientation for the water molecule to diffuse into the tube and this effect can persist approximately 3.3 angstroms from the end of CNT. As the structure within the CNTs stabilizes, the ambient structural properties can indirectly reflect their corresponding properties inside the nanotube. Our results also showed that there exists a close correlation between the PMF analysis and the results of MD simulations, and the properties at nanometer scale are closely related to the size-constraint effect.     

自组装半导体碳纳米管薄膜的光电特性

采用自组装的方法制备99%高纯度半导体碳纳米管平行阵列条带,以金属钯和钪为非对称接触电极制备碳纳米管(CNT)薄膜晶体管(TFTs)器件.主要研究不同沟道长度碳纳米管薄膜晶体管器件的电输运特性和红外光电响应特性,分析了其中的载流子输运和光生载流子分离的物理机制.我们发现薄膜晶体管器件的电学性能和光电性能依赖于器件沟道长度(L)和碳纳米管的平均长度(LCNT).当沟道长度小于碳纳米管的平均长度时,器件开关比最低;当沟道长度超过碳纳米管平均长度时,随着沟道长度的增加,器件开关比增加,光电流减小.相关研究结果为高纯碳纳米管薄膜晶体管器件在红外光探测器方面的进一步应用提供参考依据.