| The novel nano-confined channels such as carbon tube(CNT),nanoporous graphene,and interlayer channel of graphene have shown great potential in desalination,ion separation,drug delivery and energy harvesting(blue energy).Water confined in a nanoscale channel exhibits very different properties compared to the bulk water,such as the high transport rate and the ordered water structure.Hydrophobic and frictionless surface at the channel wall leads to a high slip length on the liquid-solid intersurface,which determines the fast flow rate of water in confined channel.Numerous experimental and simulation studies have reported that the water flow in CNT is several orders of magnitude higher than those predicted by the classical no-slip Hagen-Poiseuille law.As we all know that the radii of hydrated ions are larger than that of the water molecule in solution,the nano-channcl with appropriate pore size can be used in water desalination.Different ions own distinct hydration shell radii,coordination number,and ionic hydration energy.Thus,designing functionalized and confined channel can effectively separate ions.In addition,the dynamics,flow rate,and selectivity of the water molecule and hydrated ions in the confined channel are mainly affected by steric exclusion,Coulomb interaction,and external driving force.Understanding the physical mechanism of transport and separation of water and ions at the nanoscale is critical for the industrial application of nanofluidic devices.In Chapter one,the water dynamics,water transport mechanism,the ion dynamics,ion transport mechanism,and the dehydration process in the nano-channel are elaborated in detail.Then several specific materials of nano-channels are introduced.Finally,we discuss some important applications in the environment and energy field,namely water desalination,ion separation,and energy harvesting.In Chapter two,we briefly introduce the fundamental theory and calculation methods about molecular dynamics(MD),including equations of motion,force fields,periodic boundary conditions,temperature and pressure control methods,etc.An umbrella sampling method is also introduced to calculate the potential of mean forceof ions passing through the confined channel.Several methods for analyzing data and calculating fluid flow properties as well as the principle of designing nanoscale water pump based on the Venturi effect are also introduced.In Chapter three,the potential applications of h-BN for water desalination are explored.The simulation results find that the h-BN nano-channel with suitable pores can achieve a100%ion rejection meanwhile own ultra-high water permeability.Under the same conditions,the water permeability of the h-BN membrane with N4 pores is higher than that of nanporous graphene.The unique electronic structure of the nanoporous h-BN membrane makes it exhibit better water permeability than nanoporous graphene.Furthermore,using MD simulations,we study the water transport through nanopores with highly polarised rims,and reveal its physical mechanism.It is found that the water flux through the negatively charged N5 pores first grows with increasing the atomic charge,reaches a maximum at charge of the nitrogen atom|q|=-0.4 e/atom for the fixed membrane,and then drops.The maximum flux through the negatively charged N5 pores is about twice larger than that across the neutral pores.In contrast,for positively charged B5 pores,the flux increases only at high charging in flexible pores.Due to the presence of negatively charged neighboring N atoms,the blocking is not so obvious.The observed phenomena would help us to design more efficient monolayer membranes in desalination field.In Chapter four,ion transport through angstrom-scale CNT driven by electric field and pressure is systematically studied.We mainly focus on the dehydration,transport and separation mechanisms of hydrated Na~+/K~+ions,and their dynamics during transport in the confined channel.We propose two dehydration and transport modes:(i)ions tend to escape from the hydration shell to pass through the channel under electric field,(ii)ions are dragged(or wrapped)by water shell to enter the confined channel by pressure driving.For the first mode,larger radii and weaker hydration shells make K~+ions easily shed their hydrated water to transport through the CNT.For the second mode,however,Na~+ions with the small radii and tight hydration shell shed less water from the hydration shells and are strongly dragged by tightly-hydrated water to exit from the angstrom-scale channel.Thus,more Na~+ions will pass through the angstrom-scale channel than K~+driven by pressure.In Chapter five,based on the Venturi effect,a simple but effective nanoscale water pump driven by lateral water flow is designed.According to Bernoulli's principle,the lateral water flow on one side will lead to the pressure decrease,generating a pressure drop between the two chambers.More water molecules in the relatively static solution chamber are sucked into the flowing solution chamber.This is the first time to study the Venturi effect at the nanoscale.Moreover,we investigate the structure and dynamics of water confined between two parallel graphene layers using MD simulation.The results show that the diffusion coefficient of water confined between two graphene sheets with an interlayer distance less than 0.78 nm is much weak compared to the self-diffusion coefficient of water in the bulk solution.The diffusion coefficient of water is proportional to the capillary size.We also find that the viscosity of water is not only greatly enhanced for subnanometer capillaries,but also inversely related to the capillary size.Although the viscosity of water is enhanced,the water can still fast permeate through the strongly confined lamellar graphene membranes. |
PDF Full Text Request