Molecular Dynamics Studies Of Coupling Transport Of Water And Ion In Nano-confined Space

Nanofluids are defined as the fluid confined in nanospace,including nanotube,nanopores,nanochannels and other complex nanospace structures.According to the scale of the confined space,nanofluids are usually divided into“classical nanofluids”and“extended nanofluids”.Such confined fluids will exhibit distinctive nanoscale effects,including surface effects,small-size effects,quantum effects,and others.These unique effects provide a wide range of applications for nanofluids,such as ion separation,biomedicine,molecular detection and energy utilization.Especially,when water confined in nanospace,they will exhibit dynamics and thermodynamics different from bulk water,such as ultrafast transport,unique water structure,abnormal dielectric properties.These unique properties of confined water molecules depend on the structure,size,geometry and interface atom of nanospace.For example,the regular,hydrophobic and frictionless inner surface of small size carbon nanotubes(CNTs)leads to the rapid transport of water molecules.Meanwhile,there is massive radius-dependent slippage in CNTs but no slippage in boron nitride nanotubes,which also demonstrate that liquid–solid interfaces or surface atom properties should play a nontrivial role in the water conduction.The confined water molecules in CNTs or channels of graphene layers can exhibit unique single-file chain and two-dimension ice structure,repectively.Due to the interaction between water dipole and inner interface of channels,the effective dielectric constant of interfacial water is also thoroughly different from bulk behaviors.In solution,ions will attract some water molecules and form unique hydration shell because of electrostatic interaction.When the hydrated ion sizes become comparable to or smaller than a nanospace,water molecules in their hydration shells can partially dissociate from the ions and thus modify the transport and ion selectivity.Such a dehydration effect should be a universal mechanism for the ion selectivity.Therefore,understanding the physical mechanism and properties of transport and separation of water and ions at nanoconfined space is critical for the design and application of nanofluid devices and future experimental researches.In chapter one,we first give a brief introduction of nanofluid mechanics.Then we introduce some typical nanofinements,including carbon nanotubes,nanopores and nanoslit channels,as well as their application fields,such as membrane science,single-molecule detection and control,energy utilization,and biosensors.Finally,we dicuss the devolpment of transmembrane transport properties of water and ions,the selectivity of nano confinement for ions,the unique structures of confined water and ions,and the driving schemes of nanofluidic transport.In chapter two,we mainly introduce the fundamental theory and calculation methods of molecular dynamics(MD)simulations.The molecular force field,molecular motion equation and interaction,the process of molecular simulation and the processing method of final data are the core.We also provide a detailed description of electrostatic interactions,i.e.,the cutoff with short-range and the Ewald with long-range.In addition,we briefly introduce the temperature coupling,pressure coupling,periodic boundary conditons and the Gromacs software package used in our simulations.In chapter three,we devote to revealing the coupling transport relation between water and ions through a(10,10)carbon nanotube under the drive of pressure difference.The results show that the distinct cation valence will result in different channel blockage that ultimately leads to the dynamics bifurcation of water and ions.The strong coupling relation between water and ions can be uncovered by their dynamical similarity.Furthermore,with the increase of salt concentration,the dynamics of water and ions exhibit unique behaviors.We also find that the ions can travel through the channel in distinct structures that depend on the ionic conditions,such as single,couple,triple,quadruple,and even more complex clusters.These ion structures provide a key to decipher the dynamical behaviors of water and ions.The revealed connection between dynamics and ion structures is promising for future study.In chapter four,we show a novel nanopump,where theflowing counterions on an asymmetric hydrophobic–hydrophilic membrane can significantly drive the single-file water transport through a carbon nanotube.The ion–water coupling motion in electric fields on the charged surface provides an indirect driving force for this pumping phenomenon.The water dynamics and thermal dynamics demonstrate a unique behavior with the change of electric fields,surface charge density,and even charge species.Particularly,due to the ion flux bifurcation for the positive and negative surfaces,the water dynamics such as the water flow,flux,and translocation time also exhibit similar asymmetry.Surprisingly,the positive surface charge induces an abnormal three-peak dipole distribution for the confined water and subsequent high flipping frequency.This can be attributed to the competition between the surface charge and interface water orientation on it.Our results indicate a new strategy to pump water through a nanochannel,making use of the counterion flowing on an asymmetric charged membrane,which are promising for future studies.In chapter five,we systematically analyzed the coupled transport of water and ions through graphene nanochannels.The results show that a universal order of ion flux is found with K~+>Cl~->Na~+≈Li~+,and the K~+flux is twice as large as those of Na~+and Li~+,under the condition of different channel height,external electric field strengths and ion concentrations,indicating the ion selectivity with such graphene channels.The ion structures and transport dynamics within the channels show sensitive dependence on the channel height,forming two-dimensional hydration shells in the low-height limit.The hydration shells of the ions undergo transformation from three-to two-dimensional structures upon entering the narrow channel.A power law relationship between the ion translocation time and electric field is also found and can be well described by the one-dimensional Langevin equation.In addition,the linear relation between the ion flux and concentration agrees well with the one-dimensional Poisson–Nernst–Planck equation.Our results provide insights into the ionic transport through graphene channels and have implications for the design of novel nanofluidic devices for selective ion transport in future applications.