Molecular Dynamics Study On Some Problems Of The Water-Carbon Nanotubes Systems Under The Perpendicular Electric Field

The nanoscale materials usually display some unique physical and chemicalproperties, which can not be explained by the marcoscale theory. Water is usuallyregarded as matrix of life. Almost all the activities of lives on earth are closelyrelated to water. The water in nanoscale comfined space and the water near interfacehave become the very important research subjects in recent years because it isessential to understand some activities of lives and some surface physical chemistryproperties. In addition, as the nanoscale tubes, the carbon nanotubes (CNTs) could beused to transport and encapsulate various kinds of small molecules andbiomacromolecules. And there are broad application prospects for the CNTs intransporting the mass at the nanoscale. Therefore, much attention have focused onthe systems of water-CNTs in the past ten years, and lots of remarkable works havebeen reported. The most important issue is how to control the behaviors of waterconfined inside CNTs and outside CNTs but close to the solid surface. Specially, theelectrical control is particular attractive due to its simple and straightforward way ofpractice. Even so, there are still some unsolved problems concerned with the effectsof electric field on the confined water, such as the direction of the electric field andthe gradient of the field strength, etc. Also, the effects of the electric field on theconfined water molecules could affect the behaviors of CNTs is worth consideringand there are few studies on this area.In this thesis, by using molecular dynamics (MD) simulations, we studied theeffects of the perpendicular electric field on the water molecules by confined CNTs,including the changes of the water structure, the transport of single-file water and theapplication of the water mediation effects. The details and some results are presentedas follows.(1) The structures of water inside and outside the (6,6),(8,8) and (10,10) single-walled carbon nanotubes (SWCNTs) under electric field perpendicular to thetube axis are investigated. The results show that dipole reorientations induced byelectric field plays a significant role on the structures of confined water inside andoutside SWCNTs. Inside the SWCNTs, when the field intensity is sufficiently strong,the initial water structures are destroyed and the isolated water clusters could be found.While outside the SWCNTs, the azimuthal distributions of dipole orientations, densityand average number of hydrogen bonds per water molecule around solid wallsbecome more and more nonaxisymmetric as the electric intensity increases.(2) Under the perpendicular electric field with linear gradient, the unidirectionalwater flux is found through the (6,6) SWCNTs, i.e., the water molecules could beinduced to transport from the area with low field intensity to the area with highintensity. To seek the physical mechanism the interaction energies between the watermolecules are computed along the SWCNTs axis. Also, the effects of the length of theCNTs are studied and the longer the CNTs, the faster the water transportation will be.(3) In aqueous environment, a short SWCNT guided by a long SWCNT, eitherinside or outside the longer tube, is capable of moving along the nanotube axisunidirectionally under the electric field perpendicular to the CNT axis with lineargradient due to the water mediation effects. The design suggests a new way ofmolecules transportation or mass delivery. To reveal the physics behind thisphenomenon, the free energy profiles of system are calculated by the method ofpotential of mean force. In addition, the behaviors of water inside (10,10) SWCNTsare studied to reveal the water mediation effects on the short SWCNT.