Orientations of special water dipoles that accelerate water molecules exiting from carbon nanotube

One-dimensional ordered water molecules entering and exiting from a carbon nanotube with an appropriate radius are studied with molecular dynamics simulations.It can be found that a water molecule near the nanotube end is more likely to be expelled from the nanotube if its dipole is almost perpendicular to the nanotube axis.The key to this observation is that those water molecules axe closer to the wall of the nanotube away from the equilibrium position of the Lennar-Jones (LJ) potential.Thus,the interaction energy for those water molecules is relatively high.There are two particular structures of the perpendicular water molecule depending on the dipole direction of the adjacent water molecule in the nanotube.Although the probabilities of these structures are quite small,their contributions to the net flux across the nanotube end are approximately equal to the predominant structures.The present findings further show the possibility of controlling the water flow by regulating the dipole directions of the water molecules inside the nanochannels.     

Orientations of special water dipoles that accelerate water molecules exiting from carbon nanotube

One-dimensional ordered water molecules entering and exiting from a carbon nanotube with an appropriate radius are studied with molecular dynamics simulations. It can be found that a water molecule near the nanotube end is more likely to be expelled from the nanotube if its dipole is almost perpendicular to the nanotube axis. The key to this observation is that those water molecules are closer to the wall of the nanotube away from the equilibrium position of the Lennar-Jones (LJ) potential. Thus, the interaction energy for those water molecules is relatively high. There are two particular structures of the perpendicular water molecule depending on the dipole direction of the adjacent water molecule in the nanotube. Although the probabilities of these structures are quite small, their contributions to the net flux across the nanotube end are approximately equal to the predominant structures. The present findings further show the possibility of controlling the water flow by regulating the dipole directions of the water molecules inside the nanochannels.     

LATTICE BOLTZMANN METHOD SIMULATION ON THE FLOW OF TWO IMMISCIBLE FLUIDS IN COMPLEX GEOMETRY

The multicomponent nonideal gas lattice Boltzmann model by Shan and Chen (S-C) can be used to simulate the immiscible fluid flow. In this paper, we show that the relaxation constant τ≤1 is a necessary condition for the immiscible fluid flow in the S-C model. In a system with very complex boundary geometry, for 0.8≤τ≤1, the S-C model describes the immiscible flow quite well, and τ=1 is the best.     

Water and Ion Permeation through Electrically Charged Nanopore

The behaviour of water and small solutes in confined geometries is important to a variety of chemical and nanofluidic applications. Here we investigate the permeation and distribution of water and ions in electrically charged carbon cylindrical nanopore during the osmotic process using molecular dynamics simulations. In the simulations, charges are distributed uniformly on the pores with diameter of 0.9 nm. For nanopores with no charge or a low charge, ions are difficult to enter. With the increasing of charge densities on the pores, ions will appear inside the nanopores because of the large electronic forces between the ions and the charged pores. Different ion entries induce varying effects on osmotic water flow. Our simulations reveal that the osmotic water can flow through the negatively charged pore occupied by K^＋ ions, while water flux through the positively charged pores will be disrupted by Cl^- ions inside the pores. This may be explained by the different radial distributions of K^＋ ions and Cl^- ions inside the charged nanopores.     

A Quasi-One-Dimensional Model for a Chain of Water Molecules on the Nanometer Scale

Water confined into the interior channels of narrow carbon nanotubes or transmembrane proteins can form collectively oriented molecular chains held together by tight hydrogen bonds. We develop a quasi-one-dimensional model for a chain of water molecules which interact with each other via the Coulomb and power-like repulsive interactions. We explore the equilibrium property of the water chain and derive an exact analytical expression for the total interaction energy of the water chain, denoted by W（0）int. It is found that W（0）int is minimal when the distance between the two neighboring water molecules in a hydrogen-bonded chain is equal to 0.265 nm. The model is expected to be useful for studying analytically the properties of single-file water molecules inside water channels, such as the concerted motion of water molecules.     

Effect of Center-of-Mass Motion Removal in the Molecular Dynamics Simulations

Molecule dynamics simulation is now widely used in the study of nano pores, proteins and nano-scale devices. The limited friction in such a system requires the method of center-of-mass motion removal to be applied. We test the effect of different time period T of this method under osmotic pressure difference, and find that the impact on the net flux is very small together with the effective reduction of the accumulated numerical error when the period T is above 0.1 ps. The simulation results also show that the change of this time period of method has very little effect on the potential of mean force of the water inside the carbon nanotubes.     

提高水分子流出纳米碳管速度的特殊水分子偶极排布研究

使用分子动力学的方法,研究了水分子进出狭窄碳纳米管的过程．发现管口处水分子的偶极垂直于碳管时容易流出碳管．根据碳管中与之相邻的水分子的偶极方向可以把这种特殊构型分为2类．虽然,这2类特殊结构的出现概率非常小,但是它们对净流过碳管水分子的贡献与其它结构的贡献基本相同．这2种偶极排布中水分子比较接近管壁、远离Lennard-Jones势的平衡位置,导致这2种偶极排布中水分子能量升高,处于相对不稳定的状态,容易流出碳管．这个发现表明可以通过调控碳纳米管内的水分偶极方向控制管内的水分子流动．     

分子尺度不对称体系中热噪声引发的单向输运

不对称体系中的单向输运现象广泛存在于自然界中,宏观理论认为,体系的空间反演不对称性和有一定自相关时间的外加振动是引发单向运动的两个必要条件,因而一般认为,在分子尺度的不对称体系中,仅有热噪声也还是不会引发单向运动的。随着实验技术和计算手段的发展,有迹象表明,在分子尺度的不对称体系中,即使没有外加振动也会有单向运动产生。文章介绍在分子尺度由体系热噪声引发的不对称体系中,单向输运的理论进展,以及产生这种单向输运的条件。     

Water Transport through Multinanopores Membranes

We investigate the influence of correlation between water molecules transport through the neighbouring nanopores, whose centres are at a distance of only 6.2A, using the molecular dynamics simulations. Water molecule distribution in nanopore and average water flow are obtained. It is found that the average water molecule number and water flow are slightly different between a system made of the neighbouring nanopores and a system of a single pore. This indicates that transport of water chains in neighbouring pores do no show significant influence each other. These findings should be helpful in designing efficient artificial membrane made of nanopores and providing an insight into effects of the biological channel structure on the water permeation.     

Effects of Surface Dipole Lengths on Evaporation of Tiny Water Aggregation

Using molecular dynamics simulation, we compared evaporation behavior of a tiny amount of water molecules adsorbed on solid surfaces with different dipole lengths, including surface dipole lengths of 1 fold, 2 folds, 4 folds, 6 folds and 8 folds of 0.14 nm and different charges from 0.1e to 0.9e. Surfaces with short dipole lengths (1-fold system) can always maintain hydrophobic character and the evaporation speeds are not influenced, whether the surface charges are enhanced or weakened; but when surface dipole lengths get to 8 folds, surfaces become more hydrophilic as the surface charge increases, and the evaporation speeds increase gradually and monotonically. By tuning dipole lengths from 1-fold to 8-fold systems, we confirmed non-monotonic variation of the evaporation flux (first increases, then decreases) in 4 fold system with charges (0.1e-0.7e), reported in our previous paper [S. Wang, et al., J. Phys. Chem. B 116 (2012) 13863], and also show the process from the enhancement of this unexpected non-monotonic variation to its vanishment with surface dipole lengths increasing. Herein, we demonstrated two key factors to influence the evaporation flux of a tiny amount of water molecules adsorbed on solid surfaces: the exposed surficial area of water aggregation from where the water molecules can evaporate directly and the attraction potential from the substrate hindering the evaporation. In addition, more interestingly, we showed extra steric effect of surface dipoles on further increase of evaporation flux for 2-folds, 4-folds, 6-folds and 8-folds systems with charges around larger than 0.7e. (The steric effect is first reported by parts of our authors [C. Wang, et al., Sci. Rep. 2 (2012) 358]). This study presents a complete physical picture of the influence of surface dipole lengths on the evaporation behavior of the adsorbed tiny amount of water.