Transport Of Water Molecules In The Carbon Nanotube Molecular Dynamics Study

The content of the research presented in this paper contains two major parts:the first part is the design of water pump based on double-walled carbon nanotubes (DWCNTs); the second part is about the molecular dynamics (MD) simulation of water penetration through the single-walled carbon nanotubes (SWCNTs) array. Despite progress in this direction has been made, a unidirectional water flow is still difficult to achieve. The behavior of water transportation across nanochannel is crucial for the design of nanofluidic devices. The artificial water channels of CNTs have drawn extensive attention. The morphology of their hollow tubes makes them to be promising candidates for nanochannel of water transportation.We propose a design for water molecular pump using (5,5)@(10,10)&(6,6)@(11,11) DWCNTs (see Figure5.6). The inner tube that acts as water channel is fixed, while the outer shell is extracted along axial direction to stimulate high flux water flow. We studied the water transport properties of two water pump by MD simulation. The simulation results reveal that:(1) The water flux is sensitive to the rate v of outer tube movement. The water flux of both pump will increase and then decrease with the increase of drawing speed. The flux of (6,6)@(11,11) pump is higher than (5,5)@(10,10) pump at the same rate v.(2) The water destiny along the tube axis is distinctly affected due to the movement of outer rube along z axis. Peaks in the the axial density distribution curve increased. It illustrated that the density distribution is more uniform and the motion of water molecules is more smooth.(3) The flipping of water dipole is observed during the simulation. The distribution curve of water molecular dipole have two peaks. And we found that water flux increased with the increase of the two peaks height difference. These findings may provide possibilities for developing water transport devices that functions without osmotic pressure or a hydrostatic pressure gradient.In addition, we employed the MD simulation to study water penetration through CNT array which is formed by4hexagonally packing CNTs (see Figure5.6). We refer to the distance between two neighboring CNTs as d. Three simulations with different d value is carried out. We computed the destiny, distribution, and orientation of water molecules inside and outside CNTs. It is found that when the distance d is small (3.0A), water molecules cannot penetrate through the space between the CNTs. When the distance is increased to a certain value (3.4A), a water chain is observed in the centre of space between three neighboring CNTs. As the distance is increased to greater value (7.0A), each CNT is surrounded by water layer. With increase of distance d, the radial position of the fist water layer inside the CNTs will move toward CNTs wall. The distribution of water varied with the distance, which results in the difference of water orientation. Particularly, when the distance is3.0A, the orientation of water in the space between CNTs is similar with the water in (5,5)&(6,6) CNTs. In addition, we computed the axial average speed of water molecules in CNT arrays and single CNT model. The results shows that:The average speed decreased with the increase of the distance d. The speed of water molecules inside CNTs is large than those outside. But the difference decreased with the increase of distance d.