Molecular Simulation Study On The Interaction Of Small Molecules With Carbon Nanomaterials:Entrance Effect And Structuring

The significance of water in nature is self-evident.Almost all living creatures cannot live without it.Although water is a compound with a relatively simple structure,there are so many mysteries on the unusual properties of water.Especially in the confined environment of nanometer scale,such as single-walled carbon nanotubes,the confined spaces between two solid plates,biological protein channels,etc,water molecules show a much more complex properties than those in macroscopic environments.The study of carbon nanotubes and other forms of nanopore channel structure is of importance,for both practical applications and theoretical significance.A particular example is the high-flux transport and selective passage of water molecules through carbon nanotubes.The microscopic movements and underlying mechanisms have caused widespread interest.This kind of study not only plays an invaluable role in the development and design of pore channels in areas such as gas storage,ion screening,water purification,and drug delivery,but also provides insights for the substance transport via transmembrane proteins and other paths across cell membranes.However,there often exist energy barriers at the nozzle,so-called port effect of the nanopores.In addition,water in a confined space often exhibits structures and properties different from that of the bulk.Therefore,understanding the mechanism of water arrangements in these environments has important guiding significance for the development of nanofluids.This article uses molecular dynamics simulation methods to study the state and flow behavior of water molecules at the nanoscale scale,and the microscopic mechanism.Our studiess include the following aspects:(1)Molecular dynamics simulation of the port effect for water crossing carbon nanotubes.Based on the piston model designed in this thesis,the flow behaviors of water molecules across carbon nanotubes are first thoroughly examined.Then,the required external driving force for the influx of water into carbon nanotubes of different diameterswas investigated.Compared with macroscopic flows that are well described by the Hagen-Poiseuille equation,water diffusion and transport with carbon nanotubes with a diameter<1 nm shows a significant deviation.We ascribe it to the entrance effect and exit effect,that is,there exist an energy barrier that prevents water from entering or exiting the carbon nanotubes.Finally,we identify two different mechanisms of the entrance effect,depending on pore size.(2)Molecular dynamics simulation of water constrained into the confined spaces between graphene and mica.Water in confined spaces exhibit distinctive structural and dynamic properties compared to the water in the bulk phase.We explore the flow and state of water at different constrained spacings,and examine the effect of the number of free potassium atoms in mica on the state of constrained water molecules.(3)Molecular dynamics simulation of the formation of graphene nanobubbles.Due to the wrinkling and elasticity(ripple and flexibility)of graphenes,gas molecules or liquids are found to have the ability to enter the interlayer of graphene to form bulges,so-called graphene nanobubbles or blister.Our molecular dynamics simulations give the mechanism of how gas enters interlayer of graphne as well as the effect of the number of layers.